Gla




This one won't be especially deep or hevy but I think it might be 
useful,  yeas all the information in this article can be found in 
"Internals" and "Concise" but,  their chapters about Line-A don't 
feature  the  experience and trouble shooting that  only  can  be 
experienced  by "trial and error".  Therefor read careful  caurse 
this article will feature several extreamly well (modesy,  what's 
that?  is  it  a new pizza?) documented sources as well  as  some 
plain raw facts.  But why waste time let's get right on with  the 
stuff,  by the way it's good if you have some previous experience 
about  the bit mapped grapics (see the previous issue of the  Inc 
for that).  This article is splitted up into two parts, why? Well 
thatre is some utterly extreame calls that requires a deeper look, 
they will be described in the Part two while the more normal ones 
is  found  in Part one  article in this  issue.  One  more  thing 
before  I'll  get on with it,  there is something  utterly  badly 
documented  thing called "Line-F" I'll comeback in a later  issue 
(hopefully in the next one) with a deep description of what  that 
is. The Line-F are called with DC.W $F00x instead of the line-A's 
DC.W $A00x.

                    Line-A grapics, part one.

Before I'll describe an DC.W $A000
ything in the deep I'll start with a brief 
overview  of  the  commands and  what  Line-A  actually  is.  The 
commands are:
     
$A000 The "initzialiation" of Line-A it returnes some values.
$A001 The dot plotting routine
$A002 This one get's the color of a dot on screen.
$A003 Draw line, the most intresting.
$A004 Draw horizontal line (like above but only horizontal)
$A005 Draw a filled Rectangle/box.
$A006 Draw a filled polygon (slow!)
$A007 Bit block transfer (see the other Line-A article)
$A008 Text block transfer (see above)
$A009 Mouse on 
$A00A Mouse off, it don't removes the interrupt just the pointer.
$A00B Change mouse style "look".
$A00C Remove sprite (software, slow)
$A00D Draw sprite (above!)
$A00E Copy raster form, haves nothing to bo with raster bars.
$A00F Fill

The  line-A  isn't very user friendly it's rather a  bit  hostile 
against  the  user  since  if you  pass  a  completly  messed  up 
coordinate  to  $A006 it won't return a error  message  like  the 
GEMDOS  and BIOS/XBIOS traps does it  will crash of mess up  your 
poor  ST/STE  compleately.  I guess it's not included as  a  trap 
caurse it dosen't have anything to do with GEMDOS nor  BIOS/XBIOS 
in case you didn't know it,  the BIOS and Xbios are both  calling 
the  GEMDOS and the otherway around,  but Line-A is Atari's  idea 
and  have  nothing to do with the IBM-PC,  from where  Gemdos  is 
originaly).  Though the Xbios is ST specific.  Let's now continue 
with  explaining the most important Line-A  variables.  The  most 
anoying ones must be the X1,Y1 and X2,Y2 pairs.  They are used in 
most calls though they arent used in $A001 (plot) where one could 
expect to find a such command. Do keep in mind that the registers 
D0-D2 and A0-A2 will be thrashed. Save them away with this line.

   MOVEM.L D0-D2/A0-A2,-(SP)

And recover them with:

   MOVEM.L (SP)+,D0-D2/A0-A2

The  usage of line A is very easy as soon as one gets a  hang  of 
the build up of it.  To draw a horizontal line for  example,  one 
will have to do like this:

   * Call line A to get the start of the line-A calls.
   * Pass the required values to line A.
   * Call the function with DC.W $A004

In  "The Consice Atari St" is the parameters required to be  sent 
organized like this.

   $26 X1 coordinate
   $28 Y1 coordinate
   $2A X2 coordinate

What  is  the  hex value written before the  description  of  the 
parameter?  Well,  it's the offset from the start of Line-A where 
the values should be passed into.  With this example I hope  that 
you'll understand.

   DC.W  $A000
   MOVE.W 10,$26(A0)
   MOVE.W 10,$28(A0)
   MOVE.W 300,$2A(A0)
   MOVE.W 120,$2C(A0)

Since A0 contains the base address of the line-A parameter  table 
is  it that simple to do it.  Another way to do that and  perhaps 
easyer to understand is like this:
   
   DC.W  $A000
   ADD.W #$26,A0
   MOVE.W #10,(A0)

I hope you'll understand what the "offset" is now.  Now wont this 
article feature any more,  now load the Part one article and read 
about the first 7 calls in that one.  In Part two (allso in  this 
issue)  is the last 9 calls featuerd.  The reason why  I  devided 
them  up  was becurse the first 7 are user  friendly  and  "easy" 
while the last 9 is lousy documented (with a few exceptions  such 
as  $A00A and $A009) and don't have any source examples.  But  if 
you  reads the blitter article then will ya understand  that  you 
don't need Line-A for most of those 9 calls since all of them can 
be  done  with some rather easy blitter calls and they  are  done 
much faster in that way.  Any questions?  If so send me a  letter 
(or two) to:

Kent Johansson
Mariehmsv„gen 37a
S-902 36 Ume†
Sweden.

Or give me a call:
090-139994 
+46(0)90139994