A.C.E. 2/ACE CD 2.iso/FILES/DOCS/AMOSDOC.LHA/AmosPart3.doc

Text File | 1994-11-27 | 130KB | 3,354 lines

11: HARDWARE SPRITES
One of the biggest attractions of the Commodore Amiga is its ability to
produce high quality games which rivial those found on genuine arcade
machines. This can be amply demonstrated by terrific programs such as
Battle Squadron and Eliminator.

  Now, for the first time, all these amazing features are at your
fingertips! AMOS Basic provides you with complete control over the
Amiga's hardware and software sprites. These sprites can be
effortlessly manoeuvred with the built-in AMAL animation language, so
you don't have to be a machine code wizard in order to create your own
stunning arcade games.

  Hardware sprites are searate images which can be automatically
overlayed on the Amiga's screen. The classic example of a hardware
sprite is the mouse pointer. This is completely independent of the
screen, and works equally well in all the Amiga's graphics modes.

  Since sprites don't interfere with the screen background, they are
perfect for the moving objects required by an arcade game. Not only are
they blindingly fast, but they also take up very little memory. So when
you're writing an arcade game, hardware sprites should always be at the
top of your list.

  Each sprite is 16 pixews wise and up to 255 pixels high. The Amiga's
hardware supports a maximum of eight three-colour sprites or four
fifteen-colour sprites. Colour number zero is transparent - that's the
reason for the odd colour ranges.

  At first glance, these features don't seem particulary impressive.
But there are a couple of useful tricks which can increase both the
numbers and sizes of these sprites beyond recognition.

  One solution is to take each hardware sprite and split it into a
number of horizontal segments. These segments can be independently
positioned, allowing you to apparently display dozens of sprites on the
screen at once. Similarly, the width restriction can be exceeded by
building an object out of several individual sprites. Using this
technique it's easy to generate objects up to 128 pixels wide.

  Until recently the only way to exploit these techniques was to delve
into the mysterious wolrd of 68000 assembler language. So you'll be
delighted to discover that AMOS Basic manages the entire process
automatically! Once you've designed your sprites with the AMOS sprite
editor, you can effortlessly manipulate them with just a single Basic
instruction.

The sprite commands
===================
Remember to have a sprite bank loaded into memory when trying out the
various commands in this chapter. We advise you use the file
SPRITES.ABK from the AMOS data disc.



           SPRITE (display a hardware sprite on the screen)

SPRITE n,x,y,i

The SPRITE command displays a hardware sprite on the screen at
coordinates x,y using image number i.

  n is the identification number of the sprite and can range from 0 to
63. Each sprite can be associated with a separate image from the sprite
bank, so the same image can be used for several sprites.

  x and y hold the position of the sprite using special hardware           146
coordinates. All measurements are taken from the *hot spot* of your
images. This serves as a sort of 'handle' on the sprite and is used as
a reference point for the coordinates. Normally the hot spot is set to
the top left hand corner of an image. However it can be changed within
your program using the HOT SPOT command.

  Hardware coordinates are independent of the screen mode and
effectively start from (-129,-45) on the default screen. AMOS provides
you with several built-in functions for conversions between hardware
coordinates and the easier to use screen coordinates. See the X HARD,
Y HARD, X SCREEN and Y SCREEN functions for more details.

  i is the number of a particular image stored in the sprite bank. This
bank can be created using the AMOS sprite editor, and is automatically
saved along with your Basic program. It can also be loaded directly
with the LOAD instruction. In addition you can use the GET SPRITE
command to grab an image straight off the current screen.

  Any of these parameters x,y and i may be optionally omitted, but the
appropriate commas must be included. For example:

        Load "AMOS_DATA:Sprites/Octopus.abk"
        Sprite 8,200,100,1
        Sprite 8,,150,1
        Sprite 8,300,,

For a demonstration of sprites in action, load EXAMPLE 11.1 from the
MANUAL folder on the AMOS data disc.


Computed sprites
================
Although the Amiga only provides you with eight actual sprites, it's
possible to use them to display up to 64 different objects on the
screen at once. These objects are known as -computed sprites- and are
managed antirely by AMOS Basic. Computed sprites can be assigned by
calling the SPRITE command with a number greater than 7. For example,

        Load "AMOS_DATA:Sprites/Octopus.abk"
        Sprite 8,200,100,1

The size of a computed sprite is taken directly from the image data,
and can vary between 16 and 128 pixels wide, and from 1 to 255 pixels
high.

  Before you can make full use of these sprites you need to understand
some of the principles behind them. Each hardware sprite consists of a
thin narrow strip 16 pixels wide and 256 pixels deep. Depending on the
number of colours, you can have either eight or four of these strips on
the screen at a time.

  It should be obvious that most of the area inside these sprites is       147
effectively wasted. That's because few programs need sprites which are
taller than about 40 or 64 pixels. However there is a simple trick
which enables us to borrow this space to generate dozens of extra
objects on the screen. Look at the picture AMOS1.PIC (included in this
manual file packet) which contains the letters A,M,O and S.

           < picture   AMOS1.PIC >

This sprite can be split into four horizontal segments each enclosing a
single letter. The Amiga's hardware allows each section to be freely
positioned anywhere on the current line, making a total of four
computed sprites. Here's a diagram which illustrates this process.

           < picture   AMOS2.PIC >                                         148

As you can see, a computed sprite is really just a small part of a
hardware sprite displayed at a different horizontal screen position.
Notice the line between each object. This is an unavoidable side effect
of the repositioning process, and is generated by the Amiga's hardware.

  Due to the way computed sprites are produced, there are a couple of
restrictions to their use. Firstly, you can't have more than 8 computed
sprites on a single line. In practice the system is complicated by the
need to produce sprites which are larger than the 16 pixel maximum.
AMOS generates these objects by automatically positioning several
computed sprites side by side. This can be seen from the diagram below:

           < picture   AMOS3.PIC >

The maximum of eight hardware sprites therefore imposes a strict limit
to the number of such objects you can display on a horizontal line. The
total width of the objects must not exceed:

        16*8=128 pixels for three-colour sprites                           149
        16*4=64  pixels for fifteen-colour sprites

If you attempt to ignore limitation, you won't get an error message,
but your computer sprite will not be displayed on the screen. So it's
vital to ensure that the above restriction is never broken. This can be
achieved using the following procedure:

  Add together the widths of all your computed sprites, multiplying the
dimensios of any fifteen-colour sprites by two. If the total is
greater than 128, you'll need to space your sprites on the screen so
that their combined width lies below this value. Take particular care
if you are animating your sprites with AMAL as certain combinations
will only come to light after you've experimented with the sequence for
some time. These problems will be manifested by the random
disappearance of one or more sprites on the screen.

  If the worst comes to the worst, you'll need to substitute some of
your larger sprites with Blitter Objects. This will increase the
overall size of your program significantly, but it should have a
negligible effect on the final quality of your game.

  These restrictions are not confined to AMOS Basic of course. They
apply equally well to all games on the Amiga, even if they're written
entirely in machine code! So there's nothing stopping you from
producing your own Xenon II clone using exactly the same tehcniques.

  Note that, normally, hardware sprite number zero is allocated to the
mouse cursor. You can release this sprite with a simple call to the
HIDE command. See EXAMPLE 11.2.


Creating an individual hardware sprite
======================================
The only real problem with computed sprites is that you never know
precisely which hardware sprite is going to be used in a particular
object. Normally the hardware sprites used in an object will change
whenever the object is moved. Occasionally this can be inconvenient,
especially when you are animating objects such as missiles which need
to remain visible in a wide range of possible sprite combinations.

  In these circumstances it's useful to be able to allocate a hardware
sprite directly. Individual hardware sprites can be assigned using the
SPRITE instruction with an identification number between 0 and 7.
Example:

        Sprite 1,100,100,2

This loads a hardware sprite number 1 with image number 2. N now
corresponds to the number of a single hardware sprite, and can range
between 0 and 7. If your image is larger than sixteen pixels wide, AMOS
will automatically grab the required sprites in consecutive order
starting from the sprite you have chosen. For example:

        Sprite 2,200,100,1

Supposing image number 1 contained a 32-bit image with three colours.
This command would allocate hardware spries 2 and 3 to the image.
Nothing would happen if you were now to attempt to display hardware
sprite 3 with a command like SPRITE 3,150,100,1  because this sprite
has already been used. You would only have access to sprites 0,1,4,5,6
and 7, and the maximum numbers and sizes of your computed sprites would
be reduced accordingly.

  Each 15-colour sprite is implemented using a pair of two three-colour    150
sprites. However, it's not possible to combinea ny two sprites in this
way. Only the combinations 0/1,2/3,4/5,6/7 are allowed. One side effect
of this, is that you should always assign your hardware sprites using
even sprite numbers. Otherwise, AMOS will start your sprite from the
next group of two, effectively wasting the first sprite.

  Also note that if you try to create a large fifteen-colour sprite
with this system, you could easily use up all the available sprites in
a single object.

  WARNING! If you are writing a screen scrolling game, you may
encounter problems using sprites in conjunction with the SCREEN OFFSET
and SCREEN DISPLAY commands. These generate a DMA clash between the
sprite system and the screen bit-maps, and can occasionally lead to
unwanted screen effects.

  This problem is only relevant if you are using hardware sprites 6/7.
When the screen is shifted to the left with SCREEN OFFSET, the amount
of time for your sprite updates is reduced, as the screen DMA has
priority over the sprite system. Unfortunately, there isn't enough
processing time to draw sprites 6/7, and they will therefore be
corrupted on your display.

  To clear up this problem, create sprites 6/7 as individual hardware
sprites and position them off the screen using negative coordinates.
This will stop AMOS Basic from using them in your computed sprites.
Providing sprites 6/7 are never displayed on the screen during your
scrolling operations, all will be well.


The sprite palette
==================
The colours required by a hardware sprite are stored in the colour
registers 16 to 31. Providing your current screen mode doesn't make use
of these registers, the sprite colours will be completely separate from
your screen colours. Interestingly enough, this is also the case for
the 4096-colour Ham mode. So there's nothing stopping you from
producing some mind-blowing Ham games with this system!

  However you will encounter real problems when using 32 or 64 colour
screen in conjunction with three colour sprites. This is because the
colours used by these sprites are grouped together in the following
way:
           Hardware sprites  Colour registers
           ----------------  ----------------
                 0 / 1         17 / 18 / 19
                 2 / 3         21 / 22 / 23
                 4 / 5         25 / 26 / 27
                 6 / 7         29 / 30 / 31

   Colour registers 16,20,24 and 28 are treated as transparent.

The difficulty arises due to the way AMOS generates computed sprites.
The hardware sprites used to produce these objects vary during the
course of a game, so it's vital to ensure that the three colours used
by each individual sprite are set to exactly the same values, otherwise
the colours of your computed sprites will change unpredictably. Here's
a small AMOS procedure which will perform the entire process for you       151
automatically.

        Procedure INIT_SPRITES
          Get Sprite Palette
          For S=0 To 3
            For C=0 To 2
              Colour S*4+C+17,Colour(C)
            Next C
          Next S
        Endproc

The above restriction does not, of course, apply to fifteen-colour
sprites. If you want to make the most of the Extra Half Bright or
32-colour modes, you may find it easier to avoid using four-colour
sprites altogether.



                    GET SPRITE PALETTE (grab sprite
                         colours into screen)

GET SPRITE PALETTE [mask]

This loads the entire colour palette used for your sprite images into
the current screen. The optional "mask" allows you to load just a
selection of the colours from the sprite palette. Each of the 32
colours is represented by a single bit in the mask, numbered from right
to left. The rightmost bit represents the status of colour zero, the
next vit colour 1, and so on. To load a colour simply set the
appropriate bit to 1. If, for instance, you wanted to copy just the
first four colours, you would set the bit pattern to:

        Get Sprite Palette %0000000000001111

Identically, since bobs use the same sprite bank as sprites, this
command can also be used to load the colours of a bob.


Controlling sprites
===================


                 SET SPRITE BUFFER (set height of the
                           hardware sprites)

SET SPRITE BUFFER n

This sets the work area in which AMOS creates the images of the
hardware sprits. Acceptable values for n range from 16 to 256. TO set
the correct value for n, simply examine the sprites in the sprite
editor and work out which is the largest sprite length wise. ANy sprite
that is larger than "n" will simply be truncated at the appropriate cut
off point.

  SET SPRITE BUFFER is supplied for your use so that you can claim back
any redundant memory our game or application simply doesn't use.

  The amount of memory consumed by the sprite buffer can be calculated
using the formula:

        Memory = N*4*8*3 = N*96

  So the minimum buffer size is 1536 bytes and the maximum is 24k.
Note: This command erases all current sprite assignments and resets the
mouse cursor to its original state.



                    SPRITE OFF (remove one or more                         152
                       sprites from the screen)
SPRITE OFF [n]

The SPRITE OFF command removes one or more sprites from the screen. All
current sprite movements are aborted. In order to restart them, you'll
need to completely reinitialize your movement pattern.

SPRITE OFF    Removes all the sprites from display

SPRITE OFF n  Only deactivates sprite number n

Note that your sprites are automatically deactivated whenever you call
up the AMOS Basic editor. They will be automatically returned to their
original positions the next time you enter direct mode.



               SPRITE UPDATE (control sprite movements)

SPRITE UPDATE [ON/OFF]

The SPRITE UPDATE command provides you with total control of the
movements of your sprites. Normally, whenever you move a sprite, its
position is updated automatically during the next vertical blank period
(see WAIT VBL). But if you are moving a lot of sprites using the SPRITE
command, the updates will occur before all the sprites have been moved.
This may result in a noticeable jump in yur movement patterns. In these
circumstances, you can turn off the automatic updating system with the
SPRITE UPDATE OFF command.

  Once your sprites have been succesfully moved, you can then slide
them smoothly into place with a call to SPRITE UPDATE. This will
reposition any sprites which have moved since your last update.



               =X SPRITE (get x coordinate of a sprite)

x=X SPRITE(n)

Returns the current x coordinate of sprite n, measured the hardware
system. This command allows you to quickly check whether a sprite has
passed of the edge of the Amiga's screen.



               =Y SPRITE (get y coordinate of a sprite)                    153

y=Y SPRITE(n)

Y SPRITE returns a sprite's vertical position. As usual, n refers to
the number of the sprite and can range from 0 to 63. Remember, all
sprite positions are measured in hardware coordinates. See EXAMPLE 11.3



               GET SPRITE (load a section of the screen
                         into the sprite bank)

GET SPRITE [s,] i,x1,y1 TO x2,y2

This instruction enables you to grab images directly off the screen and
turn them into sprites. The coordinates x1,y1 and x2,y2 define a
rectangular area to be captured into the sprite bank. Normally all
images are taken from the current screen. However it's also possible to
grab the image from a specific screen using the optional screen number
"s".

  Note: There are no limitations to the region that may be grabbed in
this way. Providing your coordinates lie inside the existing screen
borders, everything will be fine.

  i denotes the number of the new image. If there is no existing sprite
with this number, a new image will be created automatically. AMOS wlil
also take the trouble of reserving the sprite bank if it hasn't been
previously defined. See EXAMPLE 11.4

  There's also an equivalent GET BOB instruction which is identical to
GET SPRITE in every respect. Since the sprite bank is shared by both
bobs and sprites, the images are in exactly the same format. So it's
perfectly acceptable to use both instructions in conjunction with
either bobs or sprites. Try changing the sprite instruction in the
previous example to something like:

        Bob 1,0,0,1


Conversion functions
====================


                 =X SCREEN (convert hardware coordinates
                 =Y SCREEN  into screen coordinates)

x=X SCREEN([n,] xcoord)
y=Y SCREEN([n,] ycoord)

Transforms a hardware coordinate into a screen cordinate relative to
the current screen. If the hardware coordinates lie outside the screen
then both functions will return relative offsets from the screens
boundaries. Type the following from direct mode:

        Print X Screen(130)

The result will be -2. This is because the x screen coordinate 0 is
equal to hardware coordinate 128 and thus the conversion of 130 to a
screen coordinate results in a position two pixels to the left of the
screen.

  If the optional screen number is included then the coordinates will
be returned relative to screen # n.



                  =X HARD (convert screen coordinates                      154
                  =Y HARD  into hardware coordinates)

X=X HARD ([n,] xcoord)

These functions convert a screen coordinate into a hardware coordinate.
There are four separate conversion functions, the above syntaz converts
xcoord from a coordinate relative to the current screen to a hardware
coordinate.

Y=Y HARD ([n,] ycoord)

Transforms a Y coordinate relative to the current screen into hardware
coordinate. As before, n specififes a screen number for use with the
functions. All coordinates will now be returned relative to this
screen.



             =I SPRITE (return current image of a sprite)

Image=I SPRITE(n)

This function returns the current image number being used by sprite n.
A value of zero will be reported if the sprite is not displayed.








                      12: BLITTER OBJECTS (BOBS)                           155
                     ----------------------------
While hardware sprites are certainly powerful, they do suffer from a
couple of annoying restrictions. The solution is to make use of the
Amiga's infamous Blitter chip. This is capable of copying images to
the screen at rates approaching a million pixels per second! With the
help of the blitter it's possible to create what are known as bobs.

  Bobs, like sprites, can be moved around completely independently of
the screen without destorying any existing graphics. But unlike
sprites, bobs are sroted as part of the current screen, so you can
create them in any graphics mode you wish. This allows you to generate
bobs with up to 64 colours. Furthermore the only limit to the number
of bobs you can display is dictated by the available memory.

  Bobs are slightly slower than sprites and they consume considerably
more memory. Therefore there's a trade-off between the speed of sprites,
and the flexibility of bobs. Fortunately there's nothing stopping you
from using both bobs and sprites in the same program.



                BOB (draw a bob on the current screen)

BOB n,x,y,i

The BOB command creates bob n at coordinates x,y using the image # i.

  n is the identification number of the bob. Permissible values
normally range from 0 to 63, but the number of bobs may be increased
using an option from the AMOS configuration program. Providing you've
enough memory, you can set this limit to any number you wish.

  x and y specify the position of the bob using standard screen
coordinates. These coordinates are not the same as the hardware
coordinates used by the equivalent SPRITE command. Like sprites, each
bob is controlled through a *hot spot*. This may be changed at any time
with the HOT SPOT command.

  i refers to an image which is to be assigned to the bob from the
sprite bank. The format of this image is identical to that used by the
sprites, so you can use the same images for either sprites or bobs.

  After you've created a bob, you can independently change either its
position or its appearance by omitting one or more parameters from this
instruction. Any of the numbers x,y or "image" may be left out, with
the missing parameters retaining their original values. This is
particularly useful if you are animating your bob with AMAL, as it
allows you to move your object anywhere you like, without disturbing
your existing animation sequence. However you must always include the
commas in their original order. Example:

        Load "AMOS_DATA:Sprites/Octopus.abk"
        Flash Off : Get Sprite Palette
        Channel 1 To Bob 1
        Bob 1,0,100,1
        Amal 1,"Anim 0,(1,4)(2,4)(3,4)(4,4)"                               156
        Amal On
        For X=1 To 320
          Bob 1,X,,
          Wait Vbl
        Next x

Whenever a bob is moved, the area underneath is replaced in its
original position, producing an identical effect to the equivalent
SPRITE command. Unlike STOS on the ST, each object is allocated its own
individual storage area. This reduces the amount of memory used by
bobs, and improves the overall performance dramatically. Due to the
Blitter, of course, therse's no real comparison between STOS sprites
and AMOS bobs.

  Although the BOB command works fine for small number of bobs, there's
an annoying flicker when you try to use more than three or four objects
on the screen at once. This happens because the bobs are updated at the
same time as the screen. You can therefore see the bobs while they are
being drawn which results in an unpleasant shimmering effect.

  One alternative for improving the quality of your animations is to
just limit your bobs to the bottom quarter of the screen. Since bobs
are redrawn extremely quickly, the updates can often be completed
before the lower part of the screen has been displayed. This provides
you with acceptably smooth movements while consuming very little
memory, so it's a useful trick if you're running short of space. See
EXAMPLE 12.1

  Obviously this cannot be seen as a serious solution to such a glaring
problem. So before you throw away your copy of AMOS Basic in disgust,
you'll be relieved to hear that there's a simple way of eliminating
this flicker completely, even when you're using dozens of bobs anywhere
on the screen:



             DOUBLE BUFFER (create a double screen buffer)

DOUBLE BUFFER

Creates a second invisible copy of the current screen. All graphics
operations, including bob movements, are now performed directly in this
*logical screen*, without disturbing your TV picture in the slightest.
Once the image has been redrawn, the logical screen is displayed, and
the original *physical* screen becomes the new logical screen. The
entire process now cycles continuously, producing a rock solid display
even when you're moving hundreds of bobs around the screen at once.

  The entire procedure is performed automatically by AMOS Basic, so
after you've executed this instruction you can forget about it
completely. Note that since the hardware sprites are always displayed
using the current physical screen, this system will have absolutely no
effect on any existing sprite animations.

  Double buffering works equally well in all of the AMIGA's graphics
modes. It can even be used in conjuction with dual playfields. But be
warned: Double buffering doubles the amount of memory used by your
screens. If you attempt to double buffer too many screens, you'll
quickly run out of memory. See EXAMPLE 12.2

  In practice, double buffering is an incredibly useful technique,
which can be readily exploited for most types of games. It has seen
service in the vast majority of commercial games, including Starglider
- that's why it's such an integral part of AMOS Basic. A detailed
explanation of this process can be found in the SCREENS chapter. ALso
see the SCREEN SWAP and AUTOBACK commands.



                   SET BOB (set drawing mode of bob)                       157

SET BOB n,back,planes,minterms

The SET BOB command changes the drawing mode used to display a bob on
the screen. n is the number of the bob you wish to affect.

  "back" chooses the way the background underneath your bob will be
redrawn. There are three possibilities:

 - A value of 0 indicates that the area underneath your bob should be
   saved in memory. The old image data is automatically replaced when
   the bob is moved, resulting a smooth movement effect.

 - if the "back" parameter is positive then the original background
   will be discarded altogether, and the area behind the bob will be
   filled with colour "back"-1. This is ideal for moving bobs over a
   solid block of colour such as a clear blue sky, as it's much faster
   than the standard drawing system.

 - Turn of the redrawing process completely by loading "back" with a
   negative value such as -1. You can now deactivate the automatic
   updating process using BOB UPDATE, and manually move your bobs with
   a call to BOB DRAW. This allows you to regenerate the screen
   background using your own customised drawing routines.

  "planes" is a bit map which tells AMOS which screen planes your bob
will be drawn in. As you may know, the Amiga's screen is divided up
into a number of separate bit-planes. Each plane sets a single bit in
the final colour which is displayed on the screen.

  The first plane is represnted by bit one, the second by bit two and
so on. Normally the bob is drawn in all the bit-planes in the current
screen mode. This corresponds to a bitpattern of %111111.

  By changing some of these bits to zero, you can omit selected colours
from your bobs when they are drawn. This can be used to generate a
number of intriguing screen effects.

  "minterms" selects the blitter mode used to draw your bobs on the
screen. A full description of the available modes can be found in the
section on SCREEN COPY. "minterm" is usually set to one of two values:

        %11100010   If the bob is used with a mask
        %11001010   if NO MASK has been set

Feel free to experiment with the various combinations. There's no
danger of crashing your Amiga if you make a mistake. Advanced Amiga
users find the following information useful.

        Blitter source  Purpose                                            158
        --------------  ------------------
              A         Blitter mask
              B         Blitter object
              C         Destination screen

Note that you are recommended to use SET BOB *before* displaying your
bobs on the screen. If you don't, the Amiga won't crahsh, and you won't
get an error message, but your screen display may be corrputed.



                     NO MASK (remove blitter mask)

NO MASK [n]

As a default, a blitter mask is automatically created for every bob you
display on the screen. This mask is combined with the screen background
to make colour zero transparent. It's also used by the various
collision detection commands.

  The NO MASK command removes this mask, and forces the entire image to
be drawn on the screen. Any parts of the image in colour zero will now
be displayed directly over the existing background.

  n is the image number whose mask is to be removed. This mask should
never be erased if the image is active on the screen, otherwise the
sasociated bob will be corrupted. If you must remove the mask in this
way, it's important to deactivate the relevant bobs with BOB OFF first.
Here's an example:

        Centre "Click mouse button to remove mask"
        Double buffer : Load "AMOS_DATA:Sprites/Octopus.abk"
        Get Sprite Palette
        Do
          Bob 1,X Screen(X Mouse),Y Screen(Y Mouse),1
          If Mouse Click Then Bob Off : No Mask 1
        Loop

See MAKE MASK


                        AUTOBACK (set automatic
                         screen copying mode)

AUTOBACK n

When you are using a double bufferend screen, it's essential to
synchronize your drawing operations with the movements of your blitter
objects. Remember that each double buffered screen consists of two
separate displays. There's one screen for the current picture, and
another for the image whilst it's being constructed. If the background
underneath a bob changes while it's being redrawn, the contents of
these screens will be different, and you'll get an intense and annoying
flickering efect.

  The unique AMOS AUTOBACK system provides you with a perfect solution
to this problem. It allows you to generate your graphics in any one of
three graphics modes, depending on the precise requirements of your
program. Just for a change, we'll list tese options in reverse order.


AUTOBACK 2 (automatic mode - default)                                      159

  In this mode, all drawing operations are automatically combined with
  the bob updates. So anything you draw on the screen will be displayed
  directly underneath your bobs, as if by magic. The principles behing
  this system can be demonstrated by the following code:

        Bob Clear : Rem    Draw on first screen ... Remove Bobs
        Plot 150,100 : Rem This can be anything you wish
        Bob Draw : Rem     Redraw bobs
        Screen Swap : Rem  Next Screen
        Wait Vbl
        Bob Clear
        Plot 150,100 : Rem Perform your operation a second time
        Bob Draw
        Screen Swap : Rem  Get back to first screen
        Wait Vbl

  As you can see, all screen updates are performed exactly twice.
  There's one operation for both the logical and the physical screen.
  See EXAMPLE 12.3 for a demonstration.

    One obvious side effect, is that your graphics now take twice as
  long to be drawn. Furthermore, the program will be halted by at least
  2 vertical blanks, every time you output something to the screen.
  This may cause annoying delays in the execution of critical
  activities such as collision detection.


AUTOBACK 1 (half-automatic mode)

  Performs each graphical operation in both the physical and logical
  screens. Absolutely no account is taken of your bobs, so you should
  only use this system for drawing outside the current playing area.

    Unlike the standard mode, there's no need to halt your program
  until the next vertical blank. Mode 1 is therefore ideal for objects
  such as control panels or hi-score tables, which need to be updated
  continually during the game.


AUTOBACK 0 (manual mode)

  Stops the AUTOBACK system in it's tracks. All graphics are now output
  straight to the logical screen at the maximum possible speed. You
  should use this option if you need to repeatedly redraw large
  sections of your background screen during the course of a game.
  This will allow you to safely perform your collision detection
  routines at regural intervals, without destroying the overall quality
  of the animation effects. Here's a typical program loop for you to
  examine.

        Bob Update Off
        Repeat
        Screen Swap
        Wait Vbl
        Bob Clear
        Rem Now redraw any of your gfxs which have changed                 160
        Rem Perform your game routines (Collision detection etc...)
        Bob draw
        Until WIN

Note that this procedure will ONLY work if there's a smooth progression
from screen to screen. It's entirely up to you to keep the contents of
physical and logical screen in step with each other. An example of this
technique can be found in EXAMPLE 12.4

  Supposing for instance, you wanted to display a bob over a series of
random blocks. You might try to use a routine like:

        Load "AMOS_DATA:Sprites/Sprites.abk" : Flash Off
        Get Sprite Palette : Double Buffer : Cls 0 : Autoback 0
        Update Off : Bob 1,160,100,1
        Do
          Bob Clear
          X=Rnd(320)+1 : Y=Rnd(200)+1 : W=Rnd(80)+1
          H=Rnd(50)+1 : I=Rnd(15)
          Ink I : Bar X,Y To X+W,Y+H
          Rem <this would normally call your collision detection routine>
          Bob Draw
          Screen swap : Wait Vbl
        Loop

But since there's no relationship between the physical and logical
screens, the display will now flick continuously from screen to screen.
To overcome this problem, you'll need to mimic the original AUTOBACK
system. Replace the lines in the previous example between the lines
Do and Loop  as follows:

          Rem Update first screen
          Screen Swap : Wait Vbl
          Bob Clear
          X=Rnd(320)+1 : Y=Rnd(200)+1 : W=Rnd(80)+1
          H=Rnd(50)+1 : I=Rnd(15)
          Ink I : Bar X,Y To X+W,Y+H
          Bob Draw
          Rem Update second screen
          Screen Swap : Wait Vbl
          Bob Clear
          Ink I : Bar X,Y To X+W,Y+H
          Bob Draw

The two screens are now updated with exactly the same information, and     161
the display remains as steady as a rock, even though there's a great
deal of activity going on in the background.

  Autoback can be safely used at any point in your program. So it's
perfectly possible to use separate drawing methods for the different
parts of your screen. It's also totally compatible with all graphics
operations including Blocks, Icons, and Windowing.


Bob Control commands
====================


                   BOB UPDATE (control bob movements

BOB UPDATE [ON/OFF]

Normally all bobs are updated once every 50th of a second using a
built-int interrupt routine. Alhouth this is convenient for most
programs, there are some applications which require much finer control
over the redrawing process.

  BOB UPDATE OFF turns off the bob updates and deactivates all
automatic screen switching operations if they've been selected. You may
now redraw your bobs at the most appropriate point in your program
using the BOB UPDATE command. This is ideal when you are animating a
large number of objects as it enables you to move your bobs into
position before drawing them on the screen. Inevitably this results in
far smoother movements in your game.

  One word of warning: The bob updates will only occur at the NEXT
vertical blank. Also note that BOB UPDATE will always redraw the bobs
on the current logical screen, so if you forget to use the SCREEN SWAP
command, nothing will apparently happen.



            BOB CLEAR (remove all the bobs from the screen)

BOB CLEAR

Removes all active bobs from the screen, and redraws the background
regions underneath. It's inteded for use with BOB DRAW to provide an
alternative to the standard BOB UPDATE command



                        BOB DRAW (redraw bobs)

BOB DRAW

Whenever the bobs are redrawn on the screen, the following steps are
automatically performed:

 1. All active bobs are removed from the LOGICAL screen and the
    background regions are replaced. This step is performed by BOB
    CLEAR.
 2. A list is made of all bobs which have moved since the previous
    update.
 3. The background regions under the new screen coordinates are saved
    in memory.
 4. All active bobs are redrawn at their new positions on the logical      162
    screen
 5. If the DOUBLE BUFFER feature has been activated, the physical
    and logical screens are now swapped

The BOB DRAW command performs steps 2 to 4 of this process directly.
Supposing you wished to create a screen scrolling arcade game. In this
situation, it would be absolutely vital for your scrolling operations
to be perfectly synchronized with movement effects. If the aliens were
to move while the scrolling was taking place, their background areas
would be redrawn at the wrong place. This would totally corrupt your
display, and would result in a hopeless jumble on the screen. Load
EXAMPLE 12.5 for a demonstration of this process.



                   =X BOB (get X coordinate of bob)

x1=X BOB(n)

Returns the current X coordinate of bob number n. This coordinate is
measured relative to the current screen. See also Y SPRITE, X MOUSE and
Y MOUSE.



                   =Y BOB (get Y coordinate of bob)

y1=Y BOB(n)

Y BOB complements the X BOB command by returning the Y coordinate of
bob number n. This value will be returned using normal screen
coordinates.



                 =I BOB (return current image of bob)

Image=U BOB(n)

This function returns the current image number being used by bob n. A
value of zero will be reported if the bob isn't displayed.



                LIMIT BOB (limit a bob to a rectangular
                         region of the screen)

LIMIT BOB [n,] x1,y1 TO x2,y2

This command restricts the visibility of your bobs to a rectangular
screen area enclosed by the coordinates x1,y1 to x2,y2. The x
coordinates are rounded up to the nearest 16-pixel boundary. Note that
the width of this region must always be greater than the width of your
bobs, otherwise you'll get an "illegal function call" error.

  If it's included, n specifies the number of a single bob which is to
be affected by this instruction, otherwise *all* bobs will be
restricted. You can restore the visibility limit to the entire entire
screen by typing:

        LIMIT BOB



                 GET BOB (load a section of the screen                     163
                         into the sprite bank)

GET BOB [s,] i,x1,y1 TO x2,y2

This instruction is identical to the GET SPRITE command. It grabs an
image into the sprite bank from the current screen.

  x1,y1 to x2,y2 are the coordinates of the top and bottom corners of
the rectangular area to be grabbed.

  i specifies the image number which is to be loaded with this area. s
selects an optional screen number from which the image is to be taken.
See GET SPRITE for more details. See also EXAMPLE 12.6.



             PUT BOB (fix a xopy of a bob onto the screen)

PUT BOB n

This is the exact opposite of the previous GET BOB command. The action
of PUT BOB is to place a copy of bob number n at its present position
on the screen. It works by preventing the background underneath the bob
from being redrawn during the next vertical blank period. In order to
synchronise the bob updates with the screen display, you should always
follow this command with a WAIT VBL instruction.

  Note that after this instruction has been performed, the original bob
may be moved or animated with no ill efects.



               PASTE BOB (draw an image from the sprite
                          bank on the screen)

PASTE BOB x,y,i

The PASTE BOB command draws a copy of image number i at *screen*
coordinates x,y. Unlike PUT BOB this image is drawn on the screen
immediately, and all the normal clipping rules are obeyed. See PASTE
ICON.


                BOB OFF (remove a bob from the display)

BOB OFF [n]

Occasinoally, you may wish to remove certain bobs from the screen
altogether. The BOB OFF command erases bob number n from the screen and
terminates any associated animations. If n is omitted, all bobs will be    164
removed by this instruction.





                          13: OBJECT CONTROL                               165
                      ---------------------------
In this section you will find out how the various objects generated
using the sprite and bob commands can be controlled from within an AMOS
Basic program. The topics under discussion include collision detection,
using the mouse cursor and reading the joystick.


The mouse pointer
=================
The mouse cursor provides the games programmer with a valuable
alternative to the standard joystick. With the CHANGE MOUSE command you
can replace the mouse with an image in the current sprite bank. There's
also a group of instructions which allow you to determine both the
position and status of this mouse at any time. These include the
X MOUSE, Y MOUSE and MOUSE KEY instructions.



              HIDE (remove mouse pointer from the screen)

HIDE [ON]

This command removes the mouse pointer from the screen completely. A
count of the number of occasions you have called this function is kept
internally by the system. This needs to be matched by an equal number
of SHOW instructions before the pointer will be returned on the screen.

  There's also another version of this instruction which can be
accessed with HIDE ON. This ignores the count and *always* hides the
mouse, no matter how many times you've called the SHOW command.

  Note that HIDE only makes the mouse pointer invisible. It has no
effect on any other AMOS commands, so you can still use X MOUSE and
Y MOUSE functions to read the coordinates of the mouse as normal.



                   SHOW (activate the mouse pointer)

SHOW [ON]

This returns the mouse pointer to the screen after a HIDE instruction.
Works the same way that HIDE does.



                   CHANGE MOUSE (change the shape of
                          the mouse pointer)

CHANGE MOUSE m

This allows you to change the shape of the mouse at any time. Three
mouse patterns are provided as standard. These can be assigned using
the numbers 1-3.

If you specify a value m greater than 3, this is assumed to refer to an    166
image stored in the sprite bank. The number of this image is determined
using the expression I=m-3. So image number 1 would be installed by a
value of 4.

  In order to use this option, your sprite image must be exactly 16
pixels wide and have no more than four colours. However there's no such
limit to the height of your image.



               =MOUSE KEY (read status of mouse buttons)

k=MOUSE KEY

Enables you to quickly check whether one or more of the mouse keys have
been pressed. It returns a bit-pattern which holds the current status
of the mouse buttons.

        Bit 0   Set to 1 if the LEFT   button pressed, otherwise zero.
        Bit 1   Set to 1 if the RIGHT  button pressed, otherwise zero.
        Bit 2   Set to 1 if the MIDDLE button pressed (if available).



                =MOUSE CLICK (check for a mouse click)

c=MOUSE CLICK

Checks wheter the user has "clicked" on a mouse button. Uses the same
bit pattern indication as =MOUSE KEY.

One shot tests are only set to 1 when the mouse key has just been
pressed. These bits are automatically reset to zero after they've been
tested once. So they will only check for a single key press at a time.



       =XMOUSE= (get/set the X coordinate of the mouse pointer)            167

x1=X MOUSE

X MOUSE returns the current X coordinate of the mouse pointer in
hardware notation. You can also use this function to move the mouse on
to a specific screen position. This can be achieved by assigning X
MOUSE with a value, just like a Basic variable, for example:

        X MOUSE=150



       =YMOUSE= (get/set the Y coordinate of the mouse pointer)

y1=Y MOUSE

Returns the Y coordinate of the mouse pointer. This can also be used to
set the Y position of the mouse pointer the same way as using X MOUSE.
See EXAMPLE 13.1 for an example of the X MOUSE and Y MOUSE.



                 LIMIT MOUSE (limit mouse to a section
                            of the screen)

LIMIT MOUSE x1,y1 TO x2,y2

Restricts mouse movements to the rectangular area defined by the
hardware coordinates (x1,y1) and (x2,y2). Note that unlike LIMIT BOB,
the mouse is completely trapped inside this zone and cannot be moved
beyond it. Simply use this instruction with no parameters to restore
the mouse to the full screen area.

        LIMIT MOUSE

See also EXAMPLE 13.2 from the manual folder for a demonstration.


Reading the joystick
====================
AMOS Basic includes six functions which allow you to immediately check
the movements of a joystick insterted in either of the available
sockets.



                         =JOY (read joystick)                              168

d=JOY(j)

This function returns a binary number which represnts the current
status of a joystick in port number j. Normally your joystick will be
placed in the left socket (number 1). However you can remove the mouse
from the right-hand socket and replace it with a joystick. This can be
accessed using port # 0.

  The state of the joystick can be read by inspecting the pattern of
binary bits in the result. Each bit indicates whether a specific action
has been performed by the user. If a bit is set to one then the test
has proved positive and the joystick has been moved in the appropriate
direction. Here's a list of the various bits and their meanings:

        Bit Number   Significance
        ----------   ------------
            0        Joy moved up
            1            "     down
            2            "     left
            3            "     right
            4        Fire button pressed

See EXAMPLE 13.3

You can also use the following commands, if you are not familiar with
this binary notation:



                =JLEFT(j) (test joystick movement left)
               =JRIGHT(j) (test joystick movement right)
                  =JUP(j) (test joystick movement up)
                =JDOWN(j) (test joystick movement down)                    169

x=JLEFT(j)
x=JRIGHT(j)    These functions return a value of -1(true) if the
x=JUP(j)       joystick in port j has been pulled to the associated
x=JDOWN(j)     direction. Value 0 is reported, if the condition is
               false (joystick hasn't been moved to the asked
               direction).


Detecting collisions
====================
If you're writing an arcade game it's vital to be able to accurately
check for collisions between the various objects on the screen. AMOS
Basic includes five powerful functions which allow you to perform these
tests quickly and easily.


Detecting collisions with a sprite
----------------------------------


                 SPRITE COL (detect collisions between
                         two hardware sprites)

c=SPRITE COL (n [,s TO e])

This provides you with a simple way of testing to see whether two or
more sprites have collided on the screen. The number n refers to an
active hardware sprite which is to be clicked for a collision. If a
collision has occurred a value of -1(true) will be returned, otherwise
the result will be set to 0 (false).

  The standard from of this function checks for all collisions. But you
can also test a whole group of sprites using an extended version of the
command:

        c=SPRITE COL n,s TO e

The above instruction checks for collisions between sprite n and
sprites s to e (inclusive). Once you've detected a collision, you can
then get the individual sprite numbers which have vollided using the
COL function.

  NOTE that in order to use this function, you'll need to create a
sprite mask with the MASK command first, otherwise your collisions will
not be detected. A detailed example of this command can be found in
EXAMPLE 13.4.


Detecting collisions with a bob                                            170
-------------------------------


                  BOB COL (detect collisions between
                         two blitter objects)

c=BOB(n, [,s TO e])

The BOB COL function checks bob number n for a collision with another
bob. If a collision has been detected, the value returned in c will be
set to -1 (true), otherwise it will be 0.

  Normally the command will check for all collisions, but you can
specify a collection of bobs to be tested using the optional range
parameters s to e. The status of these bobs can be individually
examined with the COL command. See EXAMPLE 13.5.


Collisions between bobs and sprites
-----------------------------------


              SPRITEBOB COL (test for a collision between
                           sprites and bobs)

c=SPRITEBOB COL(n [,s TO e])

This function checks for a collision between SPRITE n ane one or more
BOBS. The value of c will be either -1 if a collision has been
discovered, or 0 if there have been no collisions. The starting and
ending points specify that collisions will only be detected between the
bobs s to e. If they are not included then all active bobs will be
tested by this instruction.

  WARNING! Collision detection between a sprite and a bob is only
possible on a low resolution screen. In HiRes mode, the pixel sizes
used for bobs and sprites are totally different, and the results from
this function will be unreliable.



              BOBSPRITE COL (test for a collision between
                           bobs and sprites)

c=BOBSPRITE COL(n, [,s TO e])

The BOB SPRITE COL function checks for collisions between a single bob
and several sprites. The results and usage of this instruction are
same as in the SPRITEBOB COL. See EXAMPLE 13.6.



                 =COL (test the status of a sprite or
              bob after a collision detection intruction)

c=COL(n)

The COL array holds the status of all the objects which have been
previously tested by the collision detection functions.

  Each object you have checked is associated with one element in this
array. This element will be loaded with -1 if a collision has been
detected with object number n, or 0 if it has not. The numbering system
is simple: The first element in the array holds the status of object
number 1, the second represents object number 2, and so on. See EXAMPLE
13.7.

  If you are using the SPRITE COL or BOBSPRITE COL instructions then
the objects will be hardware sprites, otherwise they will be bobs. In
order to avoid confusion, it's sensible to call this instructoin
immediatly after the relevant detection command.



                HOT SPOT (set the hot spot for an image                    171
                          in the sprite bank)

HOT SPOT image,x,y
HOT SPOT image,p

This command sets the hot spot of an image stored in the current sprite
bank. The hot spot of the object is used as a reference point for all
coordinate calculations. There are two versions of this instruction.

        HOT SPOT image,x,y

x and y coordinates measured from the top left corner of the image.
These coordinates will be added to the sprite bank or bob coordinate to
position an object precisely on the screen.

              Sprite image
              +----------+        Note that it's perfectly
              :          :        lefal for the hot spot
              : x        :        to lie outside the
              :<-->*     :        actual image.
              :  hot spot:
              +----------+

        HOT SPOT image,p

This is a short form of the instruction which moves the hot spot to one
of nine predefined positions. The positions are shown in the diagram
below where the centre point of the image is represent by a value of       172
$11.

        $00     $10     $20
        $01     $11     $21        See EXAMPLE 13.8.
        $02     $12     $22



                        MAKE MASK (make a mask
               around an image for collision detection)

MAKE MASK [n]

Defines a mask around image number n in the sprite bank. This is used
by all the AMOS Basic collision detection commands. You should
therefore create a mask for every object you wish to check. If you omit
the image number n, then a mask will be generated for each image in the
sprite bank. This may take a little time.

  It's important to note that masks are generated automatically when a
bob is first drawn on the screen. This might cause a significant delay
in the running of your program, so it's worthwhile placing an explicit
call to MAKE MASK during your initialisation procedure.


Collisions with rectangular blocks
----------------------------------
AMOS Basic includes a number of functions which allow you to quickly
check whether a sprite or bob has entered a rectangular region of the
screen.

  These screen zones are especially useful for collision detection in
rebound games such as Arkanoid as each block can be assignet its own
individual screen zone. You can also use zones to construct the buttons
and switches needed for control panels and dialogue boxes.



           RESERVE ZONE (reserve space for a detection zone)

RESERVE ZONE [n]

RESERVE ZONE allocates enough memory for exactly n detection zones.
This command should always be used before defining a zone with SET
ZONE.

  The only limit to the number of zones is the amount of available
memory, so it's perfectly feasible to define hundreds or even thousands
of zones in one of your programs. To erase the current zone definitions
and restore the memory back to the main program, simply type

        RESERVE ZONE    with no parameters.



                   SET ZONE (set a zone for testing)

SET ZONE z,x1,y1 TO x2,y2

Defines a rectangular zone which can be subsequently tested using the
various ZONE commands. z specifies the number of the zone to be created
and x1,y1 and x2,y2 input the coordinates of the top left and bottom
right hand corners of the rectangle.

  Before using this instruction you'll need to reserve some space for
your zones with RESERVE ZONE.



                   =ZONE (return the zone under the                        173
                   the requested screen coordinates)

t=ZONE([s],x,y)

ZONE returns the number of the screen zone at the graphic coordinates
x,y. Normally the coordinates are relative to the current screen - you
can also include an optional screen number s in this function.

  After ZONE has been called, t will hold either the number of the zone
at the specified coordinates or a value of 0 (false).

  Note that ZONE only returns the first zone at these coordinates - it
won't detect any other zones which lie inside this region.

  It is possible to use this function in conjunction with X BOB and
Y BOB functions to detect whether a bob has entered a specific screen
zone. This can be accomplished using the following code:

        X=Zone(X bob(n),Y Bob(n))

See Examples 13.9 and 13.10.



                   =HZONE (return the zone under the
                    requested hardware coordinates)

t=HZONE([s],x,y)

HZONE is almost identical to ZONE except that the screen position is
now measured in hardware coordinates. You can therefore use this
function to detect when a hardware sprite enters one of your screen
zones. For example:

        X=Hzone(X Sprite(n),Y Sprite(n))

See also EXAMPLE 13.11, and ZONE, MOUSE ZONE, SET ZONE and ZONE$



              =MOUSE ZONE (check wheter the mouse pointer
                          has entered a zone)

x=MOUSE ZONE

The MOUSE ZONE function returns the number of the screen zone currently
occupied by the mouse pointer. It's equibalent to the line:

        X=Hzone(X mouse,Y mouse)



                       RESET ZONE (erase a zone)                           174

RESET ZONE [z]

This command permanently deactivats any of the zones created by SET
ZONE. If the optional zone number z is included then only this zone
will be reset, otherwise all the zones will be affected. Note that
RESET ZONE only erases the zone definitions, it does not return the
memory allocated by RESERVE ZONE.


Bob priority
============


            PRIORITY ON/OFF (change between priority modes)

PRIORITY ON/OFF

Each bob is assigned a priority value ranging from 0-63. Amos basic
uses this number to decide which order the objects should be displayed
on the screen. As a rule, any bob with the highest priority will always
be displayed in front if any objects with a lower priority. The
priority value is taken directly from the number of a Bob.

  You should remember this fact when assigning numbers to your bobs.
The choise of number can have wide ranging effects on the appearance of
your objects on the screen.

  In addition to the standard system, it's also possible to arrange the
bobs according to their position on the screen. PRIORITY ON assigns the
greatest priority values to the bobs with the highest Y coordinates.
This allows you to create a useful illusion of perspective in your
games. Look at the example below:

        Load "AMOS_DATA/Sprites/Monkey_right.abk" : Cls : Flash Off
        Get Sprite Palette
        Priority Off : Rem Set normal mode
        Bob 1,160,100,2 : Bob 2,0,72,2 : Bob 3,320,128,2
        Channel 2 To Bob 2 : Channel 3 to Bob 3
        Amal 2," Loop: M 320,0,320 ; M -320,0,320 ; Jump Loop"
        Amal 3," Loop: M -320,0,320 ; M 320,0,320 ; Jump Loop"
        Amal On
        Wait Key
        Priority On : Rem Set Y mode
        Wait Key

Normally, both moving bobs pass below the object in the centre. When
you change the priority system with a call to PRIORITY ON, the bobs are
now ranked in order of their increasing Y coordinates. So bob three
moves aboce bob one while at the same time, bob two passes smoothly
behind it.

  HINT: It's usually best to position the Hot Spot of the sprite at its
base. This is because the Y coordinates used by this command relate to
the position of the Hot Spot on the screen. Also notice that the
PRIORITY OFF instruction can be utilised to reset the priority back to
normal.


Miscellaneous commands                                                     175
======================


             UPDATE (change automatic sprite/bob updates)

UPDATE [ON/OFF]

Normally any objects you draw on the screen will be automatically
redisplayed whenever they are animated or moved. This feature can be
temporarily halted using the UPDATE OFF command. When the updates are
not active the SPRITE, BOB and AMAL commands apparently have no effect.
Actually, all your animations are working correctly - it's just that
the results are not being displayed on the screen. You can force this
redrawing operation at any time using the UPDATE command. Here are the
three different forms of the UPDATE instruction.

        UPDATE OFF

Turns of the automatic updating.

        UPDATE

Redraws any sprites which have changed their original positions

        UPDATE ON

Returns the sprite updating to normal. See EXAMPLE 13.12.

14: AMAL                                    1
If you wish to generate the smooth movement required in an arcade game,
it's necessary to move each object on the screen dozens of times a
second. This is a real struggle even in machine code and it's way
beyond the abilities of the fastest version of Basic.

  AMOS sidesteps this problem by incorporating a powerful animation
language which is executed independently of your Basic programs. This
is capable of generating high speed animation effects which would be
impossible in standard Basic.

  The (AM)os (A)nimation (L)anguage (AMAL) is unique to AMOS Basic. In
can be used to animate anything from a sprite to an entire scren at
incredible speed. Up to 16 AMAL programs can be executed simultaneously
using interrupts.

  Each program controls the movements of a single object on the screen.
Objects may be moved in complex predefined attack patterns, created
from a separate editor accessory. You can also control your objects
directly from the mouse or joystick if required.

  The sheer versatility of the AMAL system has to be seen to be
believed.


AMAL principles
===============
AMAL is effectively just a simple version of Basic which has been
carefully optimised for the maximum possible speed. As with Basic,
there are instructions for program control (Jump), making decisions
(If) and repeating sections of code in loops (For...Next). The real
punch comes when AMAL program is run. Not only are the commands
lightning fast but all AMAL programs are *compiled* before run-time.

  AMAL commands are entered using short keywords consisting of one or
more capital letters. Anything in lowercase is ignored completely. This
allows you to pad out your AMAL instructions into something more
readable. So the M command might be entered as Move or the L
instruction as Let.

  AMAL instructions can be separated by parctically any unused
characters including spaces. You can't however, use the colon ":" for
this purpose, as it's needed to define a label. We advise you to use a
semi-colon ";" to separate commands to avoid possible AMAL headaches.

  There are two ways of creating your AMAL programs. The first is to
produce your animation sequences with the AMAL accessory program and
save them into a memory bank or you can define your animations inside
AMOS Basic using the AMAL command. The general format of this function
is:

        AMAL n,a$

"n" is the identification number of your new AMAL program. As a default
all programs are assigned to the relevant hardware sprite. So the first
AMAL program controls sprite number one, the second sprite number two,
and so on. You can change this selection at any time using a separate
CHANNEL command. a$ is a string containing a list of AMAL instructions
to be performed in your program. Here's a simple example:

        Load "AMOS_DATA:Sprites/Monkey_right.abk"
        Get Sprite Palette
        Sprite 8,130,50,1                                                  177
        Amal 8,"S: M 300,200,100 ; M -300,200,100 J S"
        Amal On 8 : Rem Activate AMAL program number eight
        Direct

The program returns you straight back to direct mode with the DIRECT
command. Try typing a few Basic commands at this point. You can see the
movement pattern continues regardless, without interfering with the
rest of the AMOS system. Also note we have used sprite 8 to force the
use of a computed sprite. All computed sprites from 8 to 15 are
automatically assigned to the equivalent channel number by the AMAL
system. So there's no need for any special initialisation procedures.
Unless you wish to restrict the amount of hardware sprites it's safest
to stick to just computed sprites in your programs. Notice how we've
activated the AMAL program using the AMAL ON command. This has the
format:

        AMAL ON [prog]

"prog" is the number of a single AMAL program. If it's omitted, then
*all* your AMAL programs will be executed at once!


AMAL tutorial
=============
We'll now provide you with a guided tour of the AMAL system. This
allows you to slowly familiarise yourself with the mechanics of AMAL
programs, without having to worry about too many technical details.

  For the time being we'll be concentrating on sprite movements, but
the same principles can also be applied to bob or screen animations.

  Start off by loading some examples into memory. These can be found in
in the SPRITES folder on the AMOS data disc. To get a directory of
Sprite files type the following from the direct windows:

        Dir "AMOS_DATA:"

To load a sprite file, type a line like:

        Load "AMOS_DATA:Sprites/Octopus.abk"


Moving an object
----------------
As you would expect from a dedicated animation language, AMAL allows
you to move your objects in a variety of different ways. The simplest
of these involves the use of the Move command.



                          Move (move object)
                          -
M w,h,n

The M command moves an object  w units to the right and h units down in
exactly n movement steps. If the coordinates of your subject were
(X,Y), then the object would progressively move to X+W,Y+H.

  Supposing you have a sprite at coordinates 100,100. The instruction
M 100,100,100  would move it to 200,200. The speed of this motion          178
depends on the number of movement steps. If n is large then each
individual sprite movement will be small and the sprite will move very
slowly. Conversely, a small value for n results in a large movement
steps which jerk the sprite across the screen at high speed. Here are
some examples of the Move command.

        Rem This moves an octopus down the screen using AMAL
        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Sprite 8,300,0,1
        Amal 8,"M 0,250,50" : Amal On 8 : Wait Key

        Rem Moves octopus down and across the screen
        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Sprite 10,150,150,1
        Amal 10,"M 300,-100,50" : Amal On 10 : Wait Key

        Rem Demonstrates multiple Move commands.
        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        M$="Move 300,0,50 ; Move -300,0,50"
        Sprite 11,150,150,1
        Amal 11,M$ : Amal On 11 : Wait Key

Notice how we've expanded M to Move in above program. Since the letters
"ove" are in lower case, they will be ignored by the AMAL system.

  At first glance, Move is a powerful but unexciting little
instruction. It's ideal for moving objects such as missiles, but
otherwise it's pretty uninspiring.

  Actually nothing could be further from the truth. That's because the
parameters in the move instruction are not limited to simple numbers.
You can also use complex arithmetical expressions incorporating one of
a variety of useful AMAL functions. Example:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Sprite 12,150,150,1 : Amal 12,"Move XM-X,YM-Y,32"
        Amal On 12 : Wait Key

This smoothly moves computed sprite 12 to the current mouse position. X
and Y hold the coordinates of your sprite, and XM and YM are functions
returning the current coordinates of the mouse.

        It's possible to exploit this effect in games like Pac-Man to
make your objects chase the player's character. Example:

        Load Iff "AMOS_DATA:IFF/Frog_Screen.IFF",1
        Channel 1 To Screen Display 1                                      179
        Amal 1,"Move 0,-200,50 ; Move 0,200,50"
        Amal On 1 : Direct

Channel assigns an AMOS program to a particular object. We'll be
discussing this command in detail slightly later, but the basic format
is:

        CHANNEL p TO object n

"p" is the number of your AMAL program. Allowable values range from 0
to 63, although only the first 16 of these programs can be performed
using interrupts.

  "object" specifies the type of object you with to control with your
AMAL program. This is indicated using one of the following statements:

        Sprite          (values >7 refer to computed sprites)
        Bob             (blitter object)
        Screen Display  (used to move the screen display)
        Screen Offset   (Hardware scrolling)
        Screen Size     (Changes the screen size using interrupts)
        Rainbow         (Animates a rainbow effect)

"n" is the number of the object to be animated. This object needs to be
subsequently defined using the SPRITE, BOB or SCREEN open instructions.


Animation
---------


                       Anim (animate an object)
                       -
A n,(image,delay)(image,delay)...

The Anim instruction cycles an object through a sequence of images,
producing a smotth animation effect. "n" is the number of times the
animation cycle is to be repeated. A value of zero for this parameter
will perform the animation continuously.

  "image" sprcifies the number of an image to be used for each frame of
your animation. "delay" determines the length of time this image is to
be displayed on the screen, measured in units of a 50th of a second.
Example:

        Load "AMOS_DATA:Sprites/Monkey_right.abk" : Get Sprite Palette
        Sprite 9,150,50,11
        M$="Anim 12, (1,4)(2,4)(3,4)(4,4)(5,4)(6,4) ;"                     180
        M$=M$+"Move 300,150,150 ; Move -300,-150,75"
        Amal 9,M$
        Amal On 9
        Direct

This program combines a sprite movement with an animation. Notice how
we've separated the commands with a semi-colon. This ensures that the
two operations are totally independent of each other. Once the
animation sequence has been defined, AMAL will immediatly jump to the
next instruction, and the animation will begin.

  It's important to realize that Anim only works in conjunction with
sprites and bobs. So it's not possible to animate entire screen with
this command.


Simple Loops
------------


                   Jump (redirects an AMAL program)
                   -
J label

Jump provides a simple way of moving from one part of an AMAL program
to another. "label" is the target of your jump, and must have been
defined elsewhere in your current program. All AMAL labels are defined
using a single uppercase followed by a colon. like instructions, you
can pad them out with lower case to improve readability.

  Remember that each label is deinfed using just a *single* letter. So
"S:" and "Swoop:" refer to the same label! If you attempt to define two
labels starting with an identical letter, you'll be presented with a
"label already defined in animation string" error.

        Each AMAL program can have its own unique set of labels. It's
perfectly acceptable to use the identical labels in several different
programs. Example:

       Load "AMOS_DATA:Sprites/Octopus.abk"
       Get Sprite Palette
       For S=8 to 20 Step 2 : Rem Set up 7 computed sprites
        Sprite S,200,(S-7)*13+40,1
       Next S
       Rem : Now let's create seven AMAL programs
       For S=1 to 7
        Channel S To Sprite 6+(S*2)
        M$="Anim 0,(1,2)(2,2)(3,2)(4,2) ; Label: Move "+Str$(S*2)"+,0,7 ;"
        Amal S,M$
       Next S
       Rem Okay, now animate it all!
       Amal On : Direct

Since AMAL commands are performed using interrupts, infinite lopos         181
could be disasterous. So a special counter is automatically kept of the
number of jumps in your program. When the counter exceeds ten, any
further jumps will be totally ignored by the AMAL system.

  NOTE: if you rely on this system, and allow your programs to loop
continually, uou'll waste a great deal of the Amiga's computer power.
In practice, it's much more effecient to limit yourself to just a
single jump per VBL. This can be achieved by adding a simple PAUSE
comand before each Jump in your program. See PAUSE for more details.


Variables and expressions
-------------------------


                  Let (assigns a value to a register)
                  -
L register=expression

The L instruction assigns a value to an AMAL register. The action is
very similar to normal Basic, except that all expressions are evaluated
strictly from left to right.

  Registers are integer variables used to hold the intermediate values
in your AMAL programs. Allowable numbers range between -32768 to +32768.
There are three basic types of register:

  Internal registers
  - - - - - - - - -
  Every AMAL program has its own set of 10 internal registers. The
  names of these registers start with the letter R, followed by one of
  the digits from 0 to 9 (R0-R9). Internal registers are like the local
  variables inside an AMOS Basic procedure.

  External registers
  - - - - - - - - -
  Ecternal registers are rather different because they retain their
  values between separate AMAL programs. This allows you to use these
  registers to pass information between several AMAL routines. AMAL
  provides you with up to 26 external registers, with names ranging
  from RA to RZ. The contents of any internal or external register can
  be accessed directly from your Basic program using the AMREG function.

  Special registers                                                        182
  - - - - - - - - -
  Special registers are a set of three values which determine the
  status of your object. X,Y contain the coordinates of your object. By
  changing these registers you can move your object around on the
  screen. Example:

        Load "AMOS_DATA:Sprites/Frog_Sprites.abk" : Channel 1 To Bob 1
        Flash Off : Get Sprite Palette : Bob 1,0,0,1
        Amal 1,"Loop: Let X=X+1 ; Let Y=Y+1; Pause; Jump Loop"
        Amal On 1 : Direct

"A" stores the number of the image which is displayed by a sprite or
bob. You can alter this value to generate your own animation sequences
like so:

        Load "AMOS_DATA:Sprites/Frog_Sprites.abk" : Get Sprite Palette
        Flash Off : Channel 2 To Bob 1 : Bob 1,300,100,1
        M$="Loop: Let A=A+1 ; "
        M$=M$+"For R0=1 To 5 ; Next R0 ; Jump Loop"
        Amal 2,M$
        Amal On 2 : Direct

The For To Next lop will be explained in more detail below. It is used
here to slow down each change to Bob 1's image. When the "Next" of the
loop is executed, AMAL won't continue until a vertical blank has
occurred. Also note the use of ";" to separate the AMAL instructions -
although a space " " will serve just as well.


Operators
---------
AMAL expressions can include all the normal arithmetic operations,
except MOD. You can also use the following logical operatoins in your
calculations:

        &       Logical AND
        |       Logical OR

Note that it's not possible to change the order of evaluation using
brackets "()" as this would slow down your calculations considerably
and thus reduce the allowable time in the interrupt. Type the following
example:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Hide
        Get Sprite Palette
        Sprite 8,X Mouse,Y Mouse,1
        Amal 8,"Loop: Let X=XM ; Let Y=YM ; Pause ; Jump Loop"
        Amal On 8

        Load "AMOS_DATA:Sprites/Octopus.abk" : Hide
        Get Sprite Palette
        Sprite 8,X Mouse,Y Mouse,1
        Amal 8,"Anim 0,(1,4)(2,4)(3,4)(4,4) ; Loop: Let X=XM ; Let
                                          Y=YM ; Pause ; Jump Loop"
        Amal On

The above examples effectively mimic the CHANGE MOUSE command. However
this system is much more powerful as you can easily move bobs, computed
sprites, or even screens using exactly the same technique.


Making decisions                                                           183
----------------


                   If (branch within an AMAL string)

If test Jump L

This instruction allows you to perform simple tests in your AMAL
programs. If the expression test is -1 (true) the program will jump to
label L, otherwise AMAL will immediately progress to the next
instruction. Note that unlike it's equivalent, you're limited to a
single jump operation after the test.

  It's common practice to pad out this instruction with lowercase
commands like "then" or "else". This makes the action of the command
rather more obvious. Here's an example:

        If X>100 then Jump Label else Let X=X+1
        -             -    -          -
"test" can be any logical expression you like, and may include:

        <>  Not equals
        <   Less than
        >   Greater than
        =   Equals

Example:

        Load "AMOS_DATA:Sprits/Octopus.abk"
        Get Sprite Palette
        Sprite 8,130,50,1
        C$="Main: If XM>100 Jump Test: "
        C$=C$+"Let X=XM "
        C$=C$+"Test: If YM>100 Jump Main "
        C$=C$+"Let Y=YM Jump Main"
        Amal 8,C$ : Amal On : Direct

WARNING! Don't try to combine several tests into a single AMAL
expression using "&" or "|". Since expressions are evaluated from left
to right, this will generate an error. Take the expression:
X>100|Y>100. This is intended to check whether X>100 OR Y>100. In
practice, the expression will be evaluated in the following order:

        X>100   May be TRUE or FALSE                                       184
        |Y      OR result with Y
        >100    Check if (Y>100|Y)>100)

The result from the above expression will obviously be no relation to
the expected value. Technically-minded users can avoid this problem by
using boolean algebra. First assign each test to an single AMAL
register like so:

        Let R0=X>100; Let R1=Y>100

Now combine these tests into a single expression using | and & and use
it directly in your If statement.

        If R0 | R1 Jump L ...

This may look a little crazy, but it works beautifully in practice.


                    For To Next (loop within AMAL)
                    -   -  -
For reg=start To end
  :   :
Next reg                      This implements a standard FOR...NEXT
                              loop which is almost identical to its
Basic equivalent. These loops can be exploited in your programs to move
objects in complex visual patterns. "reg" may be any normal AMAL
register (R0-R9 or RA-RZ). However you can't use special registers for
this purpose.

  As with Basic, the register after the Next must match with the
counter you specified in the For, otherwise you'll get an AMAL syntax
error. Also note that the step size is always set to one. Additionally,
it's possible to "nest" any number of loops inside each other.

  Note that each animation channel will only perform a single loop per
VBL. This synchronizes the effects of your loops with the screen
display, and avoids the need to add an explicit Pause command before
each Next.


Generating an attack wave for a game
------------------------------------
These lopos can be used to create some quite complex movement patterns.
The easiest type of motion is in a straight line. This can be generated
using a single For...Next loop like so:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Sprite 8,130,60,1
        C$=For R0=1 To 320 ; Let X=X+1 ; Next R0" : Rem Move sprite
        Amal 8,C$ : Amal On 8 : Direct

You can now expand this program to sweep the object back and forth
across the screen.

        Load "AMOS_SATA:Sprites/Octopus.abk" : Get Sprite Palette
        Sprite 8,130,60,1
        C$="Loop: For R0=1 To 320 ; Let X=X+1 ; Next R0 ;"                 185
        C$=C$+" For R0=1 To 320 ; Let X=X-1 ; Next R0 ; Jump Loop"
        Amal 8,C$ : Amal On 8 : Direct

The first loop moves the object from left to right, and the second from
right to left. So far the pattern has been restricted to just
horizontal movements. In order to create a realistic attack wave, it's
necessary to incorporate a vertical component to this motion as well.
This can be achieved by enclosing your program with yet another loop.

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Sprite 8,130,60,1 : C$=For R1=0 To 10 ;"
        C$=C$+"For R0=1 To 320 ; Let X=X+1 ; Next R0 ; "
        C$=C$+"Let Y=Y+8 ; "
        C$=C$+"For R0=1 To 320 ; Let X=X-1 ; Next R0 ; "
        C$=C$+"Let Y=Y+8 ; Next R1"
        Amal 8,C$ : Amal On 8

The above programs generates a smooth but quite basic attack pattern. A
further demonstration can be found in EXAMPLE 14.1 in the MANUAL
folder.


Recording a complex movement sequence
-------------------------------------


                                 PLay
                                 --
PLay path

If you've looked at the smooth attack waves in a modern arcade game,
and thought them forever beyond your reach, think again. The AMAL Play
command allows you freely animate your objects through practically any
sequence of movements you can imagine. It works by playing a previously
defined movement pattern stored in the AMAL memory bank.

  These patterns are created from the AMAL accessory on the AMOS
program disc. This simply records a sequence of mouse movements and
enters them directly into the amal memory bank. Once you've created
your patterns in this way, you can effortlessly assign them to any
object on the screen, reproducing your original patterns perfectly.
Both the speed and direction of your movement can be changed at any
time from your AMOS Basic program.

  The first time AMAL encounters a Play command, it checks the AMAL
bank to find the recorded movement you specified using the "path"
parameter. "path" is simply a number ranging from one to the maximum
number of patterns in the bank. If a problem crops up during this
phase, AMAL will abort the play instruction completely, and will skip
to the next instruction in your animation string.

  After the pattern has been initialised, register R0 will be loaded
with the tempo of the movement. This determines the time interval
between each individual movement step. All timings are measured in
units of a 50th of a second. By changing this register within your AMAL
program, you can speed up or slow down your object movements
accordingly.

  Note that each movement step is *added* to the current coordinates of
your object. So if an object is subsequently moved using the Sprite or
Bob instructions, it will continue its manoeuvres unaffected, starting
from the new screen position. It's therefore possible to animate dozens
of different objects on the screen using a single sequence of
movements.

  Register R1 now contains the flag which sets the direction of your       186
movements. There are three possible situations:

    *  R1>0   Forward

A value of one for R1 specifies that the movement pattern will be
replayed from start to finish, in exactly the order it was created
(this is the default).

    *  R1=0   Backward

Many animation sequences require your objects to move back and forth
across the screen in a complex pattern. To change direction, simply
load R1 with a zero. Your object will now turn around and execute your
original movement steps in reverse.

    *  R1=-1  Exit

If a collision has been detected from your AMOS program, you'll need to
stop your object completely, and generate an explosion effect. This can
be accomplished by setting R1 to a value of minus one. AMAL will now
abort the play instruction, and immediately jump to the next
instruction in your animation sequence.

  The clever thing about these registers is that they can be changed
directly from AMOS Basic. This lets you control your movement patterns
directly from within your main program. There's even a special AMPLAY
instruction to make things easier for you.

  The PLay comand is perfect for controlling the aliens in an arcade
game. In fact, it's the single most powerful instruction in AMAL.



                      AMAL (call an AMAL program)

AMAL n,a$
AMAL n,p
AMAL n,a$ to address          The AMAL command assigns an AMAL program
                              to an animation channel. This program can
    be taken either from a string in a$ or directly from the AMAL bank.

  The first version of the instruction loads your program from the
string a$ and assigns it to channel n. a$ can contain any list of AMAL
instructions. Alternatively you can load your program from a memory
bank stored in bank number 4.

  n is the number of an animation channel ranging from 0 to 63. Each
AMOS channel can be independently assigned to either a bob, a sprite or
a screen.

  Only the first 16 AMAL programs can be performed using interrupts. In
order to exceed this limit you need execute your programs directly from
Basic using the SYNCHRO command.

  The final version of the AMAL insturction is provided for advanced
users. Instead of moving an actual object, this simply copies the
contents of registers X,Y and A into a specific area of memory. You can
now use this information directly in your own Basic routines. It's         187
therefore possible to exploit the AMAL system to animate anything from
a Block to a character. The format is:

        AMAL n,a$ To address

"address" must be EVEN and must point to safe region of memory,
preferably in an AMOS string or a memory bank. Every time your AMAL
program is executed (50 times per second), the following values will be
written into this memory area:

     Location     Effect
     --------     ------
     Address      Bit 0 is set to 1 if the X has changed
                  Bit 1 indicates that Y has been altered
                  Bit 2 will be set if the image (A) has changed since
                        the last interrupt.
     Address+2    Is a *word* containing the latest value of X
     Address+4    Holds the current value of Y
     Address+6    Stores the value of A

These values can be accessed from your program using a simple DEEK.
NOTE: This option totally overrides any previous CHANNEL assignments.


AMAL commands
=============
Here is a full list of the available amal commands:

M (Move)        Move deltaX, deltaY, steps
A (Anim)        Anim cycles,(image,delay)(image,delay)...
L (Let)         Let reg=exp                                                188
J (Jump)        Jump L
I (If)          If exp Jump L
For To Next     For Reg=start To end ...Next Reg
PL (PLay)       PLay path                                                  189
P (Pause)       Pause

AU (AUtotest)   AU (list of tests)             See the Autotest System     190

X (eXit)        eXit          Exits from an AUtotest and re-enters the
                              current AMAL program.

W (Wait)        Wait          Freezes your AMAL program and only
                              executes the AUtotest.

O (On)          On            Activates the main program after a Wait.

D (Direct)      Direct        Sets the section of the main program
                              to be executed after an autotest.

AMAL functions                                                             191
==============

=XM          Returns the X coordinate of the mouse
=YM          Returns the Y coordinate of the mouse
=K1          Status of left mouse key (-1, if pressed, otherwise 0)
=K2          Status of right mouse key
=J0          Test right joystick. Result in bit-map.
=J1          Test left joystick. See the JOY command.

=Z(n)        Random number. Returns a random number between -32767
             to 32768. This number can be limited to a specific
             range using the bit-mask n. A logical AND operation
             is performed between the bit mask n and the random
             number to generate the final result. So setting n to
             a value of 255 will ensure that the numbers will be
             returned in the range 0 to 255. Since this function has
             been optimized for speed, the number returned isn't
             totally random. If you need really random numbers, you
             would be better to generate your values using Basic's
             RND and then load them into an external AMAL register
             with the AMREG function.

=XH(s,x)     Converts a screen x coordinate into a hardware coordinate.    192
=YH(s,y)     Converts a screen y coordinate into hardware format.
=XS(s,x)     Hardware to screen conversion
=YS(s,y)     Hardware to screen conversion

=BC(n,s,e)   Check for collisions between bobs. BC is identical to the
             equivalent AMOS Basic BOB COL instruction. It checks bob
             number n for collisions between bobs s to e. If a
             collision has been detected, then BC will return a value
             of -1, otherwise 0. This instruction may NOT be performed
             within an iterrupt. So it's only available when you are
             executing your AMAL routines directly from Basic with the
             SYNCHRO instruction.

=SC(n,s,e)   This is equivalent to the SPRITE COL function. Like BC
             function, it's only allows in conjuction with the SYNCHRO
             instruction.

=V(v)        VU-meter. The VU function samples one of the sound
             channels and returns the intensity of the current voice.
             This is a number in the range 0-255. You can use this
             information to animate your objects in time to the music.
             An example of this can be found in EXAMPLE 14.3. Also see
             the VUMETER function from AMOS Basic


Controlling AMAL from Basic                                                193
===========================


               AMAL ON/OFF (start/stop an AMAL program)

AMAL ON [n]

Once you've defined your AMAL program you need to execute it using the
AMAL ON command. This activates the AMAL system and starts your
programs from the first instruction.

  AMAL ON activates all your programs. The optional parameter n allows
you start just one routine at a time.

        AMAL OFF [n]

Stops one or all AMAL programs from executing. These programs are
erased from meomry. They can only be restarted by redefining them again
using the AMAL instruction.



                    AMAL FREEZE (temporarily freeze
                           an amal program)
AMAL FREEZE [n]

Stops one or more AMAL programs for running. Your programs can be
restarted at any time using a simple call to AMAL ON. Note that this
instruction should always be used to stop AMAL before a command such as
DIR is executed, otherwise problems with timing can cause visual
mishaps.



                     =AMREG= (get the value of an
                        external AMAL register)

r=AMGER(n, [channel])
AMREG(n, [channel])=expression

The AMREG function allows you to access the contents of internal and
external AMAL register directly from within your Basic program.

  "n" is the number of the register. Possible values range from 0 to 25
with zero representing register RA and twenty-five denoting RZ.

  By using the optional "channel" parameter you can reference any AMAL
internal register. In this mode "n" ranges between 0 and 9 representing
R0 to R9.

  The following examples shows how it is possible to retrieve a
sprite's current X-position from Basic:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Channel 1 To Sprite 8 : Sprite 8,100,100,1
        A$="Loop: Let RX=X+1; Let X=RX; Pause; Jump Loop"
        Amal 1,A$ : Amal On : Curs Off
        Do
          Locate 0,0
          Z=Asc("X")-65 : Rem Note the use of ASC to get the register #
          Print Amreg(Asc("X")-65)
        Loop



                     AMPLAY (control an animation                          194
                          produced with PLay)

AMPLAY tempo,direction [start TO end])

Any movement sequences you've produced using the AMAL PL command are
controlled through the internal registers R0 and R1. Each object will
be assigned it's own unique set of AMAL registers. So if you're
animating several objects, you'll often need to load a number of these
registers with exactly the same values.

  Although this can be achieved using the standard AMREG function, it
would obviously be much easier if there was a single instruction which
allowed you to change R0 and R1 for a whole batch of objects at a time.
That's the purpose of the AMPLAY command.

  AMPLAY takes the "tempo" and "direction" of your movements, and loads
them into the registers R0 and R1 in the selected channels.

  "tempo" controls the speed of your object on the screen. It sets a
delay (in 50ths of a second) between each successive movement step.

  "direction" changes the direction of the motion. Here's a list of the
various different options.:

     Value Direction
     ----- ---------
      >0   Move the selected object in the original movement direction.
       0   Reverses the motion and moves the object backwards
      -1   Aborts movement pattern and jumps to the following
           instruction in your AMAL animation sequence.

As a default, this instruction will affect all current animation
channels. This can be changed by adding some explicit "start" and "end"
points to the command. "start" is the channel number of the first
object to be adjusted. "end" holds the channel number assigned to the
last object in your list. Note that either the "tempo" or the
"direction" can be omitted as required. Examples:

        Amplay ,0         : Rem reverse your objects
        Amplay 2,         : Rem Slow down your movement patterns
        Amplay ,-1 3 To 6 : Rem stop movements on channels 3,4,5 and 6.



                     =CHANAN (test AMAL animation)                         195

s=CHANAN(channel)

This is a simple function which checks the status of an AMAL animation
sequence and returns -1 (true) if it's currently active or 0 if the
animation is complete. "channel" holds the number of the channel to be
tested.



                   =CHANMV (checks whether an object
                           is still moving)
s=CHANMV(channel)

Returns a value of -1 if the object assigned to "channel" is currently
moving, otherwise 0 (false).

  This command can be used in conjunction with the AMAL Move
instruction to check whether a movement sequence has "run out" of
steps. You can now restart the sequence at the new position with an
appropriate movement string if required. Example:

        Load "AMOS_DATA:Sprites/Monkey_right.abk" : Get Sprite Palette
        Sprite 9,150,50,11
        M$=Move 300,150,150; Move -300,-150,75"
        Amal 9,M$ : Amal On
        While Chanmv(9)
        Wend
        Print "Movement complete"



AMAL errors
===========


              =AMALERR (return the position of an error)

p=AMALERR

Returns the position in the current animation string where an error has
occurred. Careful inspection of this string will allow you to quickly
correct your mistakes. Example:

        Load "AMOS_DATA:Sprites/Octopus.abk"
        Sprite 8,100,100,1
        A$="L: IF X=300 then Jump L else X=X+1; Jump L"
        Amal 8,A$

This program will generate a syntax error because IF will be
interpreted as the two instructions I and F. To find the position in
the animation string of this error, type the following instruction from
the direct window.

        Print Mid$(A$,Amalerr,Amaller+5)


Error messages                                                             196
--------------
If you make a mistake in one of your AMAL programs, AMOS will exit back
to Basic with an appropriate error message. Here's a full list of the
errors which can be generated by this system, along with an explanation
of their most likely causes.

 Bank not reserved: This error is caused if you attempt to call the
                    PLay instruction without first loading a bank
         containing the movement data into memory. This should be
         created with the AMAL accessory program. If you're not using
         PLay at all then check that you've correctly separated any
         Pause and Let instructions.

 Insturction only valid in Autotest: You've inadvertently called either
                                     the Direct or the eXit
         instructions from your main AMAL program.

 Illegal instruction in Autotest: Autotest may only be used in
                                  conjunction with a limited range of
         AMAL commands. It's not possible to move or animate our
         objects in any way inside an autotest. So check for erroneous
         commands like Move, Anim or For...Next.

 Jump To/Within Autotest in animation string: The commands inside an
                                              autotest function are
         completely separate from your main AMAL program. So AMAL does
         not allow you to jump directly inside an AUtotest procedure.
         To leave an autotest, and return to your main AMAL program you
         must use either eXit or Direct.

 Label already defined in animation string: You've attempted to define
                                            the same label twice in
         your AMAL program. All AMAL labels consist of just a single
         CAPITAL letter. So "Test" and "Total" are just different
         versions of the same label (T). This error is also generated
         if you have accidentally separated two instructions by a ":"
         (colon). Use a semi-colon instead.

 Label not defined in animation string: This error is generated when
                                        you try to jump to a label
         which doesn't currently exist in your animation string.

 Next without For in animation string: Like it's Basic equivalent each
                                       For command should be matched
         by a corresponding Next statement. Check any nested loops for
         an spurious Next command.

 Syntax error in animation string: You've made a typing mistake in one
                                   of your animation strings. It's easy
         to cause this error by accidentally entering an AMAL
         instruction in full, just like its Basic equivalent.


Animation channels                                                         197
==================
Amos allows you to execute up to 64 different AMAL programs
simultaneously. Each program is assigned to a specific animation
channel.

  Only the first 16 channels can be performed using interrupts. If you
need to animate more objects you'll have to turn off the interrupts
using SYNCHRO OFF. You can now execute the AMAL programs step by step
using an explicit call to the SYNCHRO command in yur main program loop.
As a default, all interrupt channels are assigned to the relevant
hardware sprite.



             CHANNEL (assign an object to an AMAL channel)

CHANNEL n TO object s

The CHANNEL command assigns an animation channel to a particular screen
related "object". In AMAL, you're not restricted to a single channel
per object. Any single screen object can be safely animated with
several channels if required. There are various different forms of this
instruction.


Animating a computed sprite
---------------------------

CHANNEL n TO SPRITE s

This assigns sprite number s to channel n. As a default, channels 0-7
are automatically allocated to the equivalent hardware sprite, and 8-15
are reserved for the appropriate computed sprites.

  In order to animate the computed sprites from 16 onwards, you'll need
to allocate them directly to an animation channel with the CHANNEL
command. As normal , sprite numbers from 8 to 63 specify a computed
sprite rather than a single hardware sprite. For example;

        Channel 5 To Sprite 8 : Rem Animates Computed sprite 8 using
                                    Channel 5.

The X,Y  registers in your AMAL program now refer to the hardware
coordinates of the selected sprite. Similarly the current sprite image
is held in register A.


Animating a blitter object
--------------------------

CHANNEL n TO BOB b

Allocates blitter object b to animation channel n. This object will be
treated in an identical way to the equivalent hardware sprite. The only
difference is that registers X and Y now contain the position of your
bob in *screen* coordinates.

  Note that if you've activated screen switching with the DOUBLE BUFFER
command, this will be automatically used for all bob animations.


Moving a screen                                                            198
---------------
AMOS Basic allows you to freely position the current screen anywhere on
your TV display. Normally this is controlled with the SCREEN DISPLAY
instruction. However, sometimes it's useful to be able to move the
screen using interrupts.

CHANNEL n TO SCREEN DISPLAY d

This sets the channel n to screen number d. Screen d can be defined
anywhere in your program. You'll only get an error if the screen hasn't
been opened when you start your animation.

  The X and Y variables in AMAL now hold the position of your screen in
hardware coordinates. Register A is *not* used by this option and you
can't animate screens using Anim. Otherwise all standard AMAL
instructions can be performed as normal. So you can easily use this
system to "bounce" the picture aroud the display. Examples:

        Load Iff "AMOS_DATA:IFF/Frog_screen.IFF",1
        Channel 0 To Screen Display 1
        Amal 0,"Loop: Move 0,200,100 ; Move 0,-200,100 ; Jump Loop"
        Amal On 0 : Direct

        Load Iff "AMOS_DATA:IFF/Frog_screen.IFF",1
        Channel 0 To Screen Display 1
        Rem Screen can only be displayed at certain positions in the X
        Amal 0,"Loop: Let X=XM; Let Y=YM; Pause; Jump Loop"
        Amal On : Direct

For a further example of this technique, load EXAMPLE 14.4. This
demonstrates how the SCREEN DISPLAY can be used in conjunction with the
menu commands to slide the menu screen up and down your display. It's
similar to the display system found in Magnetic Scrolls' excellent
series of adventures.


Hardware scrolling
------------------
Although hardware scrolling can be performed using AMOS Basic's SCREEN
OFFSET command, it's often easiest to animate your screens using AMAL
instead as this generates a much smoother effect.

CHANNEL n TO SCREEN OFFSET d

This assigns AMAL program number n to a screen d, for the purpose of
hardware scrolling. The X and Y registers now refer to the section of
the screen which is to be displayed through your TV. Changing these
registers will scroll the visible screen area around the display.
Here's an example:

        Screen Open 0,320,500,32,lowres : Rem Open an extra tall screen
        Screen Display 0,,45,320,250
        Load Iff "AMOS_DATA:IFF/Magic_screen.IFF"
        Screen copy 0,0,0,320,250 To 0,0,251
        Screen 0 : Flash Off : Get Palette (0)
        Channel 0 to Screen Offset 0
        Amal 0,"Loop: Let X=XM-128; Let Y=YM-45; Pause; Jump Loop"
        Amal On : Wait Key

This program allows you to scroll through the screen using the mouse.
Try moving the mouse in direct mode. For a further example of hardware
scrolling, see EXAMPLE 14.5


Changing the screen size                                                   199
------------------------

CHANNEL n TO SCREEN SIZE s

This allows you to change the size of a screen using AMAL. s is the
number of the screen to be manipulated. Registers X and Y now control
the width and height of your screen respectively. They're similar to
the W and H parameters used by the SCREEN DISPLAY command. Example:

        Load Iff "AMOS_DATA:IFF/Magic_screen.IFF",0
        Channel 0 to Screen Size 0
        Screen display 0,,,320,1 : Rem set the screen size to 1
        A$=Loop: For R0=0 To 255 ; Let Y=R0 ; Next R0; "
        A$=A$+"For R0=0 To 254; Let Y=255-R0; Next R0; J Loop"
        Amal 0,A$ : Amal On : Direct


Rainbows
--------

CHANNEL n TO RAINBOW r

This option generates a rainbow effect within an AMAL program. As usual
n is the number of an animation channel from 0 to 63. r is an
identification number of your rainbow (0-3).

  X holds the current BASE of your rainbow. This is the first colour of
your rainbow palette to be displayed. Changing it will make the rainbow
appear to turn. Y contains the line on the screen at which the rainbow
effect will start. If you alter this value, the rainbow effect will
move up or down. All coordinates are measured in *hardware* format.

  Register A stores the height of your rainbow on the screen. See the
AMOS Basic RAINBOW command fore more details.


Advanced tehcniques
===================

The AUTOTEST system
-------------------
Normally all AMAL programs are performed in strict order from start to
finish. Inevitably some commands such as Move and For...Next will take
several seconds to complete. Although this will be fine in the vast
majority of cases it may lead to significant delays in the running of
certain programs. Take the following simple program:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Sprite 8,130,50,1
        Amal 8,"Loop: Let R0=XM-X; Let R1=YM-Y; Move R0,R1,50; Jump Loop"
        Amal On : Direct

As you move the mouse, the sprite is supposed to follow it around on
the screen. However in practice the response time is quite sluggish,
because the new values of XM and YM are only entered after the sprite
movement has totally finished. Try moving the mouse in a circle. The
octopus is completely fooled!

  Autotest solves this problem by performing your tests at the start of
every VBL, before continuing with the current program. You tests now
occur at regular 1/50 intervals, leading to a practically instantanous
response!


Autotest commands                                                          200
-----------------
The syntax of Autotest is:

AUtotest (tests)
--
    "tests" can consist of any of the following AMAL commands.

Let reg=exp
-
    This is the standard AMAL Let instruction. It assigns the result
    of an expression to register "reg".

Jump label
-
    The Jump command jumps to another part of the current autotest.
    "label" is defined using the colon ":" and *MUST* lie inside the
    autotest brackets.

eXit
 -
    Leaves the autotest and re-enters the main program from the point
    it left off.

Wait
-
    Wait turns off the main AMAL program completely, and only executes
    the Autotest.

If
-
    In order to simplify the testing process inside an autotest routine
    there's a specially extended version of the AMAL If statement. This
    allows you to perform one of three actions depending on the result
    of the logical expression "exp".

    If exp Jump L   (Jumps to another part of the autotest)
    If exp Direct L (Chooses part of the prog to be executed after AU)     201
    If exp eXit     (Leaves autotest)

On
-
    Restarts the main program after a previous Wait instruction. This
    lets you wait for a specific event such as a mouse click without
    wasting processor time.

Direct label
-
    Direct changes the point at which the main program will be resumed
    after your test. AMAL will now jump to this point automatically at
    the next vertical blank period. Note that label *must* be defined
    outside the Autotest brackets.


Inside Autotest
---------------
Here's the previous example rewritten using the Autotest feature

        Load "AMOS_DATA:Sprites/octopus.abk"
        Sprite 8,130,50,1 : Get Sprite Palette
        A$="AUtotest (If R0<>XM Jump Update"
        A$=A$+"If R1<>YM Jump Update else eXit"
        A$=A$+"Update: Let R0=XM; Let R1=YM; Direct M)" : Rem End of AU
        A$=A$+"M: Move R0-X,R1-Y,20 Wait;" : Rem Try changing 20 to
                                                 different values!
        Amal 8,A$ : Amal On

The sprite now smoothly follows your mouse, no matter how fast you move
it. The action of this program is as follows:

  Every 50th of a sec the mouse coordinates are tested using the XM and
YM functions. If they are unchanged since the last test, the Autotest
is aborted using the eXit command. The main program now resumes
precisely where it left off.

  However if the mouse has been moved, the autotest routine will
restart the main program again from the beginning (label M) using the
new coordinates in XM and YM respectively.


Timing considerations
---------------------


                   UPDATE EVERY (save some time for
                         your Basic programs)
UPDATE EVERY n

Although most AMAL programs are performed practically instantaneously,
any objects they manipulate need to be explicity drawn on the Amiga's
screen.

  The amount of time required for this updating procedure is
unpredictable and can vary during the course or your program. This can
lead to an annoying jitter in the movement patterns of certain objects.

  The UPDATE EVERY command slows down the updating process so that even
the largest object can be redrawn during a single screen update. This
regulates the animation system and generates delightfully smooth
movement effects.

  n is the number of vertical blank periods between each screen update.
In practice you should start off with a value of two, and gradually
increase it until movement is smooth.

  One useful side effect of UPDATE EVERY, is to reserve more time for
Basic to execute your programs. With a judicious use of this
instruction, it's sometimes possible to speed up your programs by as
much as 30%, without destroying the smoothness of your animation
sequences.


Beating the 16 object limit                                                202
---------------------------


              SYNCHRO (execute an AMAL program directly)

SYNCHRO [ON/OFF]

Normally AMOS Basic will allow you to execute up to 16 different AMAL
programs at a time. This limit is determined by the overall speed of
the Amiga's hardware. Each AMAL program takes its own slice of the
available processor time. So if you're using the standard interrupt
system, there's only enough time to execute around 16 separate
programs.

  The SYNCHRO command allows you to exceed this restriction by
executing your AMAL programs directly from Basic. Instead of using
interrupts, all AMAL programs are now run using a single call to the
SYNCHRO command. Since AMAL programs execute far faster than the
equivalent Basic routines, your animations will still be delightfully
smooth. But you will now able to decide when and where yur AMAL
routines will be performed in your program.

  One additional bonus is that you can now include collision detection
commands such as Bob Col or Sprite Col directly in your AMAL routines.
These are not available from the interrupt system as they make use of
the Amiga's blitter chip. This would be impossible using iterrupts.

  Before calling SYNCHRO you first need to turn off the interrupts with
SYNCHRO OFF. It's imporatnt to do this *before* defining your AMAL
programs, otherwise you won't be allowed to use channel numbers greater
than 15 without an error.

  Due of the sheer power of the animation system, it's nearly possible
to write entire arcade games completely in AMAL. This leaves your Basic
program with simple jobs such as managing the hi-score table and
loading your attack waves from the disc. The results will be
indistinguishable from pure machine code. A good example is Cartoon
Capers, the first commercial games release that's written entirely in
AMOS.

  A demonstration of SYNCHRO can be found in EXAMPLE 14.6.


STOS compatible animation commands
----------------------------------
The original STOS Basic included a powerful animation system which
allowed you to move your sprites in quite complex patterns using
interrupts. At the time, these commands were hailed as a breakthrough.

  Although they've now been overshadowed by the AMAL system, they do       203
provide a simple introduction to animation on the Amiga. So AMOS
provides you with the entire STOS animation system as an extra bonus!

  If you're indenting to convert STOS programs to AMOS, you'll need to
note the following points:

  * Unlike STOS, the movement patterns in AMOS Basic can be assigned to
    any animation channel you like. The Move commands can therefore be
    used to move bobs, sprites or screens, using exactly the same
    techniques.

      As a default, all animation channels are assigned to the
    equivalent hardware sprites. In practice you may find it easier to
    substitute blitter objects as these are much close to the standard
    STOS Basic sprites. Add a sequence of CHANNEL commands to start of
    your program like so:

        Channel 1 to bob 1
        Channel 2 to bob 2
          :        :

    Don't forget to call DOUBLE BUFFER during your initialisation
    procedure, otherwise your bobs will flicker annoyingly when they're
    moved.

  * The same channel can be used for both STOS animations and AMAL
    programs. So it's easy to extend your programs once they've been
    succesfully converted into AMOS Basic. The order of execution is:

    AMAL
    MOVE X
    MOVE Y
    ANIM


                  MOVE X (move a sprite horizontally)

MOVE X n,m$

Defines a list of horizontal movements which will be subsequently
performed on animation channel number n.

  n can range from 0 to 15 and refers to an object you have previously
assigned using the CHANNEL command. m$ contains a sequence of
instructions which together determine both the speed and direction of
your object. These commands are enclosed between brackets and are
entered using the following format:

        (speed,step,count)

There's no limit to the number of commands you can include in a single
movement string, other than the amount of available memory.

  "speed" sets a delay in 50ths of a second between each successive
movement step. The speed can vary from 1 (very fast) to 32767
(incredibly slow).

  "step" specifies the number of pixels the object will be moved during
each operation. If the step is positive the sprite will move to the        204
right, and if it is negative it will move left.

  The apparent speed of the object depends on a combination of the
speed and step size. Large displacements coupled with a moderate speed
will move the object quickly but jerkily across the screen. Similarly a
small step size combined with a high speed will also move the object
rapidly, but the motion will be much smoother. The fastest speeds can
be obtained with a displacements of about 10 (or -10).

  "count" determines the number of times the movement will be repeated.
Possible values range from 0 to 32767. A count of 0 performs the
movement pattern indefinitely.

  In addition to the above commands, you can also add one of the
following directives at the end of your movement string.

  The most important of these extensios is the L instruction (for
loop), which jumps back to the start of the string and returns the
entire sequence again from the beginning. Example:

        Load "AMOS_DATA:Sprits/Octopus.abk" : Get Sprite Palette
        Sprite 1,130,100,1 : Rem Define Sprite 5
        Move X 1,"(1,5,60)(1,-5,60)L"
        Move On

The E option allows you to stop your object when it reaches a specific
point on the screen. Change the second to last line in the above
example to:

        Move X 1,"(1,5,30)E100"

Note that these end-points will only work if the x coordinate of the
object exactly reaches the value you originally designated in the
instruction. If this increment is badly chosen the object will leap
past the end-point in a single bound, and the test will fail. Example:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Channel 1 To Sprite 8 : Channel 2 To Sprite 10
        Print At(0,5)+"Looping OK"
        Sprite 8,130,100,1
        Move X 1,"(1,10,30)(1,-10,30)L"
        Move On
        Print At(0,10)+"Now press a key" : Wait Key
        Sprite 10,140,150,2
        Move X 2,"(1,15,20)L" : Move On 2
        Print At(0,15)+"Oh dear!" : Wait Key



                   MOVE Y (Move an vertical object)

MOVE Y n,m$

This instruction complements the MOVE X command by enabling you to move
an object vertically along the screen. As before, n refers to the
number of an animation sequence you've allocated using the CHANNEL
command, and ranges between 0 and 15.

  m$ holds a movement string in an identical format to MOVE X. Positive
displacements now correspond to a downward motion, and negative values
result in an upward movement. Examples:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette          205
        Channel 1 to Sprite 8 : Sprite 8,130,10,1
        Move Y 1,"10(1,1,180)L"
        Channel 2 To Screen Display 0
        Move Y 2,"(1,4,25)(1,-4,25)
        Move On : Wait Key



                  MOVE ON/OFF (start/stop movements)

MOVE ON/OFF [n]

Before your movement patterns will be executed they need to be
activated using the MOVE ON command.

  "n" refers to the animation sequence you wish to start, and can range
from 0 to 15. If it's omitted then all your movements will be activated
simultaneously.

  MOVE OFF has exactly the opposite effect: It stops the relecant
movement sequences in their tracks.



          MOVE FREEZE (temporatily suspend sprite movements)

MOVE FREEZE [n]

The MOVE FREEZE command temporarily halts the movements of one or more
objects on the screen. These objects can be restarted again using
MOVE ON.

  "n" is completely optional and specifiew the number of a single
object to be suspended by this instruction.



                    =MOVON (return movement status)

x=MOVON(n)

MOVON checks whether a particular object is being moved by the MOVE X
and MOVE Y instructions. It returns -1 if object n is in motion, and 0
if it's stationary. Do not confuse this with the MOVE ON command. Also
note that MOVON searches for movement patterns generated using the MOVE
commands, so it will not detect any animations generated by AMAL.



                       ANIM (animate an object)

ANIM n,a$

Automatically flicks an object through a sequence of images creating a
smooth animation effect on the screen. These animations are performed
50 times a second using interrupts, so they can be executed
simultaneously with your Basic programs.

  "n" is the number of the channel which specifies a sprite or bob to
be animated by this instruction.

  "a$" contains a series of instructions which define your animation
sequence. Each operation is split into two separate components enclosed
between round brackets.

  "image" is number of the image to be displayed during each frame of      206
the animation. "delay" specifies the length of time this image will be
hled on the screen (in 50ths of a sec.). Example:

        Load "AMOS_DATA:Sprites/Octopus.abk" : Get Sprite Palette
        Channel 1 to Sprite 8 : Sprite 8,200,100,1
        Anim 1,"(1,10)(2,10)(3,10)(4,10)"
        Anim On : Wait Key

Just as with the MOVE instruction, there's also an L directive which
enables you to repeat your animations continuously. So just change the
ANIM command in the previous example to the following:

        Anim 1,"(1,10)(2,10)(3,10)(4,10)L"



                   ANIM ON/OFF (start an animation)

ANIM ON/OFF [n]

ANIM ON activates a series of animations which have been previously
created using the ANIM command. n specifies the number of an individual
animation sequence to be initialised. If it's omitted, then all current
animation sequences will be started immediately.

ANIM OFF [n]

Halts one or more animation sequences started by ANIM ON.



                   ANIM FREEZE (freeze an animation)

ANIM FREEZE [n]

Temporarily freezes the current animation sequence on the screen. n
chooses a single animation sequence to be suspended. If it's not
included, all current animations will be affected. They can be
restarted at any time with a simple call to the ANIM ON instruction.








                        15: BACKGROUND GRAPHICS                            207
                    ------------------------------

Nowadays, it's not uncommon for an arcade game to contain hunderds of
different screens. With compaction, it's possible to crap a single 32
colour screen into about 30k of memory. So 100 screens would be the
equivalent of about 3 Megabytes of data. Imagine how difficult this
would be to fit into a standard A500!

  The classic way of avoiding this restriction, is to construct your
backgrounds out of a set of simple building blocks. Once these "tiles"
have been created, they can be placed on the screen in any order you
like. So the same set of tiles can be reused to generate a vast number
of potential screens. Each screen is now stored as a simple list of its
components, and requires a tiny fraction of the original memory.

  In order to exploit this system, you'll obviously need some way of
defining your various screen maps. As you might have guessed, we've
helpfully provided you with a powerful map definer accessory on the
AMOS program disc. Full details can be found in the accompanying
documentation file.

  AMOS Basic also includes a number of special instructions for drawing
your tiles on the screen. These make it easy to generate the fast
scrolling backgrounds that are the hallmark of a modern arcade game.


Icons
=====
Icons are separate images which have been especially designed for
producing your background screens. Once you've drawn an icon, it's
fixed permanently into place. So you can't move it to a new position
using the AMAL animation system.

  All icons are stored in their own AMOS memory bank (#2). This bank is
created using the Sprite definer accessory (on the AMOS Program disk),
and will be automatically saved along with your Basic programs.

  Like Bobs, Icons are displayed using the Amiga's amazing Blitter
chip. But since Icons are essentally static objects, they are usually
drawn in REPLACE mode. Your icons will therefore totally erase any
existing graphics at the current screen position.



                       PASTE ICON (draw an icon)

PASTE ICON x,y,n

Draws icon number n on the screen at GRAPHIC coordinates x,y. n is the
number of the icon which is to be displayed. This must have been
previously stored in the ICON bank.

  Icons can be freely positioned anywhere on the screen, subject to the
normal clipping rules. Example:

        Load "AMOS_DATA:Icons/Map_icons.abk"
        Screem Open 0,320,256,32,Lowres : Cls 0 : Get Icon Palette
        For X=1 To 11 : Paste Icon X*32,0,1 : Next X
        For Y=1 To 6 : Paste Icon 0,Y*32+11 : Paste Icon 288,Y*32,1
        Next Y
        For X=1 To 11 : Paste Icon X*32,223,1 : Next X

Note that if you're using double buffering, a copy of your icons will
be drawn into both the physical and logical screens. Since this is
rather slow, it's common practive to add a call to AUTOBACK 0 before
drawing your icons on the screen. This restricts straight to the
physical screen using SCREEN COPY, saving a considerable amount of
time.

  For a further example, see the MAPVIEW program on the AMOS DATA disc.
This displays a background screen you've created using the AMOS Map
Editor.



                       GET ICON (create an icon)                           208

GET ICON [s,] i,tx,ty TO bx,by

Captures an image from the screen and loads it into icon "i". If this
icon does not presently exist, it will be created for you in bank 2.
This bank will be automatically reserved by the system if required.

  i is the number of your icon, starting from 1. tx,ty to bx,by define
the rectangular zone which encloses the selected region.

  s determines the number of the screen which will be used as the
source of your image. If it's omitted, the image will be taken from the
current screen instead. Example:

        Erase 2
        F$=Fsel$("*.*","","Load a screen") : If F$="" Then Direct
        If Exist(f$) Then Load Iff f$,0 Else Direct
        SH=Screen Height : H=SH/32-1 : SW=Screen Width : W=SW/32-1
        For Y=0 to H
          For X=0 to W
            Get Icon X+Y*W+1,X*32,Y*32 To X*32+31,Y*32+31
          Next X
        Next Y
        Cls 0
        Do
          Paste Icon Rnd(Sw-1),Rnd(SH-1),Rnd/(H*W)+1
        Loop



                  GET ICON PALETTE (get icon colours)

GET ICON PALETTE

Grabs the colours of the icon images in bank 2, and loads them into the
current screen palette. This command is normally used to initialize the
screen after you'be loaded some icons from the disc. Example:

        Load "AMOS_DATA:Icons/Map_icons.abk"
        Get Icon Palette
        Paste Icon 100,100,1



                       DEL ICON (deletes icons)                            209

DEL ICON n[ TO m]

Deletes one or more icons from the icon bank. n is the number of the
first icon to be removed.

  m is the optional number of the last icon to be deleted in the list.
If it's included all the icons from first to last will be erased one
after another.

  When the final icon in a bank has been deleted, the entire bank will
be removed from memory.



            MAKE ICON MASK (set colour zero to transparent)

MAKE ICON MASK [n]

Normally, any icons you draw on the screen will completely replace the
existing background. The icon will seem to be displayed in a
rectangular box filled with colour zero.

  If you want to avoid this effect and overlay your icons directly over
the current graphics, you'll need to create a *mask* for your icons.
This informs AMOS that colour zero should be treated as transparent.

  n is the number of the icon to be affected. If it's omitted, a mask
will be defined for all icons in the bank. See EXAMPLE 15.1


Screen blocks
=============
AMOS Basic supplies you with a set of powerful BLOCK commands which
allow you to grab part of an image into memory and paste it anywhere on
the screen.

        These instructions are mainly used for holding temporary data,
since your blocks cannot be saved along with your Basic programs.

  Blocks are especially effective in the construction of dialogue
boxes, as they can be used to save the background areas before
displaying your new graphics.

  They can also be exploited in puzzle games like Split Personalities.
Each block can be loaded with a single section of your image. You can
then jumble your pictures by rearranging the blocks on the screen with
PUT BLOCK.



              GET BLOCK (grab a screen block into memory)

GET BLOCK n,tx,ty,w,h[,mask]

GET BLOCK grabs a rectangular area in block number n, starting at
coordinates tx,ty.

  n is the number of the block ranging from 1-65535. tx, ty set the
coordinates of the top left hand corner of your block. w,y hold the
width and height of the block respectively.

  "mask" is a flag which chooses whether a mask will be created for
your new block.

  mask=0        Replace mode. When the block is drawn on the screen,
                it will totally destroy any graphics at that current
                position.
  mask=1        Calculates a mask for the block. Colour zero will now
                be treated as if it were transparent.



                PUT BLOCK (copies a previously created                     210
                        block onto the screen)

PUT BLOCK n[,x,y]
PUT BLOCK n,x,y,planes[,minterms]

PUT BLOCK copies block number n to the current screen. x,y specify the
position of your new block on the screen. If they are omitted the block
will be redrawn at its original screen coordinates.

  Note that all drawing operations will be clipped to fit into the
current screen, starting from the nearest 16 pixel boundary.

  For a demostration of the BLOCK commands see the routine in EXAMPLE
15.2. We've also provided experienced programmers with a couple of
optional extras. These are not needed for the vast majority of
applications, they're only required when you want to achieve weird
special effects on the screen!

  "planes" holds a bit-map which sets the range of colours which will
be drawn in your block. The Amiga's screen is divided up into segments
known as bit-planes. Each plane contains a single bit for every point
on the Amiga's screen. When the Amiga's hardware displays this point,
it combines the bits from each plane to calculate the required colour
number. Each bit in "planes" represents the status of a single
bit-plane. If it's set to one, then the selected plane will be drawn by
the instruction, otherwise it will be completely ignored. The first
plane is represented by bit zero, the second by bit one, etc.

  Usually, the block will be displayed in all the available bit-planes.
The corresponds to a bit-pattern of %111111

  "minterm" selects the blitter mode used to copy your block on the
screen. A full description of the possible drawing modes can be found
in the section on SCREEN COPY. The best way to loearn about these
options is to experiment!



                   DEL BLOCK (delete a screen block)

DEL BLOCK n

Deletes one or more blocks and restores the memory used to AMOS Basic.

DEL BLOCK       Erases *all* current blocks
DEL BLOCK n     Deletes block number n.



             GET CBLOCK (save and compact a screen image)                  211

GET BLOCK n,x,y,sx,sy

The GET BLOCK command saves and compacts a rectangular area of the
screen. The compaction system used by this command has been especially
optimized for speed. So it's nowhere near as efficient as the dedicated
AMOS compression routines provided by the PACK or SPACK instructions.

  CBLOCKS are often used to grab the area underneath your dialogue
boxes. After the dialogue has been completed, the screen can quickly
restored back to its original state. See EXAMPLE 15.3.

  n specifies the number of your block and can range between 1-65535.

  x,y are the top left coordinates. The x coordinate is rouded to the
nearest multiple of 8.

  w,h hold the dimensios of the area to be saved. The width is always
rounded to an exact multiple of 8.

                     PUT CBLOCK (displays a block
                         created using CBLOCK)

PUT CBLOCK n [,x,y]

Places block n on the current screen at coordinates x,y. If the target
coordinates are omitted, the block will be redrawn at its original
screen position. Also note that x is automatically rounded to the
nearest eight pixel boundary.

                  DEL CBLOCK (deletes a screen block
                       defined with GET CBLOCK)

DEL CBLOCK [n]

Erases all blocks from memory. If n is present only block n will be
deleted.