Introduction of an "Xtended Graphics Mode" for the Atari ST
Computers
from Stefan Hartmann (Hardware and article)
and Carsten Isakovic(Software:Overscan.PRG)
If You are a graphics freak and an owner of an Atari ST computer You
might have been looking with keen eyes into the direction of the
Commodore Amiga Computer and have been thinking about selling Your
32000 Bytes Sreen Memory Machine and gonna buy the Interlace-
HAM-Baby ?
Stop it man ! This article will tell You why !
With a simple switch (cost about a buck) and additionally only at the
STM520: 2 Rectifier diodes, 2 Resistances, a NPN Transistor and a
capacitor, we will show You how to get a visible 59640 Bytes big Screen
Memory (in 50 Hertz Mode). (the following description will apply for 50
Hertz mode , 60 Hertz Mode and Monochrome-Overscan-Mode has some other
sreen memory size)
What does this mean ?
This means in Low resolution mode there are now 420*284 Pixels
available to display, still 16 colors per pixel.
In midres resolution there we have now the gorgious resolution of
840*284 Pixel, still 4 colors per dot.
In monochrome mode we get now 688*480 pixel with a well adjusted SM124
monochrome monitor !
Well this means, in color mode there is no more screen window visible
but pixels all over the screen, indeed some pixels You'll never see,
because they hide behind the left and right monitor tube border till You
adapt Your horizontal screen width !
The same thing is available on the Amiga. There they call this mode :
OVERSCAN .
This overscan mode is now with this article also possible for all the
Atari ST computers, from the 512KB 260 machine to the Mega ST 4 !
That means the Atari ST can now like the Amiga be used for
semi-professionel Desktop-Video applications for examle together with
a genlock-interface to get a scrolled text message in the lower border
of a video movie.
How was this little hardware modification found ?
The story of the Overscan modification began, when some month ago I got
a demo disk on which a nice grahic screen appeared. But what the hack
was this ?
In the lower border of the ST screen was a scrolling text message ! How
to come ? Had my monitor been shifted away ?
Not at all !
I couldn`t trust my eyes ! This programmer called Alyssa (special
nickname of this hacker-guy) had programmed a very fine flowing
scrolling text message out of pixel graphics at the bottom of the screen
where normally only the color palette register 0 (that means the static
border color) is available !
Did this guy found an undocumented "poke" to switch off the lower border
and display further pixel graphics,which Mr. Shiraz Shivji (the father
of the ST) didn't told us ?
Not at all !
He had found the software-trick, that if the picture frequency is
shortly switched inside the 199th scan line from 50 to 60 Hertz the
result is that the Glue chip is disturbed, so that it holds its
Display-Enable-Signal longer at High(5 Volts)-Level and this tells the
MMU and the Video-Shifter to display Pixel-Graphics till the end of the
screen.(but still with a left and right border)
What a guy !
He must have been poked "till the shit hits the fan(Glue)" !
To understand all this screen manipulation I have shortly to explain
how the ST generates its picture in color mode: In 50 Hertz Mode the
screen is build up of 313 scan lines which are 50 times per second
written onto the tube.
(In 60 Hertz mode there are 263 scan lines displayed)
This means that the Atari doesn't work with Interlace, the so called
"half frames" (semi frames) are placed precisely one upon onother so
there is no placement shifting like in TV-Systems (e.g. NTSC).
Normally in 50 Hertz mode the first 39 scan lines are displayed without
pixel graphics but with background color palette 0.
This is the upper Border.
Then there are 200 scan lines in which the normal Screen window is
displayed but still with this left and right border, which is still
generated from palette 0.
The following 45 scan lines display the lower border, still palette 0.
(normally no pixel graphics available)
The last 29 scan lines of the total 313 lines are not visible, because
they are used for vertikal blanking. This is needed to synchronize the
picture frequency of a connected monitor.
The Blanking is done by the blank-signal that is also generated by the
Glue chip and this signal switches the RGB-D/A-Resistor-Converter to
Zero when the vertikal blank period is active. (In 60 Hertz Mode the
normal mode is : upper border 14 scan lines, 200 display lines, 24 lower
border scan lines and 25 scan lines blanked)
A few months ago there arrived the 2 new demo programs "Amiga-Demo" from
the TEX-Programmers ( some special assembler graphics wizzards ) and
the LT.PRG (the death of the left border) of the TNT-Crew.
This 2 programs blowed my mind because they already featured pixel
graphics inside the right (Amiga-Demo) and in either border(left and
right, LT.PRG).
How this is possible I don't know yet, because the assembler code is
very sophisticated crypted so no chance for a tracking monitor program
disassembler.
But it's possible that they use a 50 Hertz to 71 Hertz switching every
scanline synchronized with the video adress counter. Must be very
tricky code ! They wanna keep this still a secret !
(In the meantime February `89 the gorgious UNION-Demo was launched:
Level 16 has now implemented a full software controlled Overscan screen
with no borders at all!; unbelieveable, but very timeconsumming
interruptprogramming; not much time left for animation !)
I thought, wow !
If these guys are able to display some border graphics with some very
tricky but time consumming code how about trying to convince the little
Shifter to do this with a simple hardware modification and without any
time consumming calculations ?
No problem !
I fetched my scope, booted the Amiga-Demo and switched between the
additional right border pixel display and normal mode.
At this time I took a closer look with the scope next to the Shifter area
of the ST board.
That's it!
I recognized that if I switched the modes, the Display-Emable-Signal
changed its puls-stop relationship.
The Display Enable Signal determines if pixel graphics should be
displayed by the Shifter or if the border should be displayed (palette
register 0).
Every time if this signal goes high, pixel graphics is displayed, if it
is low, palette 0 is displayed.
The Display Enable signal is generated by the Glue, like the Blank-,
HSync- and VSync-Signal which are all used to generate the picture.
The DE-Signal also goes to the MMU and the Timer B Input of the MFP chip
(for counting the scan line through interrupt routines).
The screen is build up with the following sequence :
The vertical Sync signal which is not only going to the monitor
connector bush but which comes from the Glue and is also controling the
MMU is telling the internal counter of the MMU to take over the
adress of the next "semi frame" from the video.bas adress FF8201 and
FF8203.
If now the Display Enable signal goes high then the DCYC
(Display-Cycle-Clock) signal from the MMU is activated which is
strobing the videodisplay information (pixel graphics) from the RAM in
16 bits words into the Shifter for displaying.
That means the DCYC signal clocks the Load Input of the Shifter.
At this moment the internal counter of the MMU is counting upwards.
The actual adress is been written all 2 microseconds to the video adress
counter in FF8205/07/09.
Remains at this point the Display Enable Signal longer at high level,
accordingly more memory RAM is adressed as screen video ram by the MMU
and still transfered to the Shifter during every scan line!
So with the Puls Stop relationship of the DE signal we can determine the
size of the screen window.
At last we have the Blank signal which is also blanking the video signal
from the RGB-Converter during horizontal flyback time and vertikal
snchronisation time (as mentioned above).
To complete the description, there is still the CMPS Signal (Color Map
Chip Select), which is generated by the MMU and which is only changing
for telling the Shifter to load a new color palette. (what will Spectrum
512 look in Overscan mode: super gorgious !)
Conclusion: How do I find the right signal in exchange for the
DE signal ?
Looking around inside the ST I found the Composite Sync Signal which is
generated by an AND-Combining of the HSync and VSync Signals.
One can use the already installed Composite Sync Signal which feeds the
monitor bush connector.
Make sure to use the Composite Sync Signal directly from the Emitter of
the driver (buffer) transistor. (Take a look into Your circuit diagram
of Your ST!)
At the 520 STM (the STs with RF-modulators) there the internal
Composite Sync Signal is missing. Here You have to add the above
mentioned circuit of 2 rectifiers, 2 resistors a transistor and a
optional buffer capacitor( buffers the collector(5Volts) of the
transistor versus ground).
This Composite Sync Signal is ideal for controlling the new Overscan
mode. We have been trying other signal for example: only the HSync, the
VSync, the Blank signal or only permanent 5 Volts but this doesn't work
with either 50 and 60 Hertz and GEM couldn't be installed.
To find a better signal than the Composite Sync signal is not possible
with such a simple modification.
The ST has only 4 signal which come from the GLUE which could be used to
create a " new " Display Enable signal, because all other signals vary !
The only time invariant signals are :
1. The old DE-Signal
2. The HSync Signal
3. The VSync Signal
4. The BLANK Signal.
And still the divided clock frequencies 2 MHz and 500 kHz.
To create a better signal than the composite sync signal one would have
to use some counters and dividers to build up a better "DE"-signal.
This would mean a lot of additional hardware!
GEM needs a video scan line whose number has to be able to divide by 4 to
install it. So the Composite Sync Signal was the simplest solution to
find.
Description of the Hardware Modification
The Display Enable Signal which is going to the Shifter Pin 37 and to
the MMU Pin 52 has to be cut and instead of this the Composite Sync
Signal has to be fed to this two Pins.
This could be done by a simple 3 Pins switch which switches between the
original and the new Overscan Mode.
The switch could be installed at the back of the ST for example.
It is also possible to switch it during operation of the ST without
hanging up.
If You switch back it might could happen, that the color palettes have
been cycled.
This is due to percussion during switching but could be removed by
switching 3 or 4 times again till the color palettes are in the right
order again.
Hint: If there will a shifted palette as the desktop appears, You have
to switch again a few times the change-over switch so due to percussion
this shifted palettes will disappear!
This shifted palettes only sometimes occur during booting but this is
the only time !
How to connect and wire the Overscan-change-over-switch:
Directly at the Emitter of the Composite Sync signal driver Transistor
You pick up the buffered Composite Sync Signal and feed it to the third
pin of the switch.
The second pin of the switch (the pin in the middle) goes to the pin 37
of the Shifter and pin 52 of the MMU.
The first pin of the switch is connected to pin 39 of the Glue (Display
Enable Signal)
Reminder: the MFP 68901 pin 20 still gets the Display Enable Signal in
both modes !!!
We have tried to use also the Composite Sync for the Timer B-Input in
Overscan mode, but then the ST doesn't boot himself after a Reset !
Softwareadaption to the Overscan Mode
If You switch directly from the normal desktop after booting without
any accessories or programs to the Overscan mode, You will see some nice
"pixel trash" in the upper half of the screen, but going from the very
left to the very right.(without a border)
But what has happened to the lower half screen ?
There is some pretty moiree jumping and changing in a strange pattern of
16 bits in various colors !
What is this, You will ask ?
Don't worry, be happy !
This is only the display of RAM databus signals !!!
This happens because the normal Video screen display Ram is installed
at the top of the available ST Memory.
If switched to Overscan the ST needs more than the 32kBytes but there at
the top is no more available, so behind the end of the RAM the ST
displays the undefinable Databus signals !
What a funny picture !
( I just wonder wether the first game will use this for displaying
"screen communication problems" like in the game: Carrier Command ?)
So the answer to get a correct full screen Overscan picture is :
Write a lower Adress into the Screen Starting Adress Pointer
FF8201/03 !
But then there is still the Pixel-Trash, but now all over the screen !
This is because the color bit planes have changed their place. This can
be corrected by adapting the GEM.
This was done by us by writing the Overscan.PRG which adapts the GEM.
It runs with the Blitter-TOS and the new ROM-TOS 1.4 and the BETA-RAMTOS
and the Developer-RAMTOS 1.4, but not with the old ROMTOS from 1985 !
You start it out of the AUTO-folder of a disk or from the harddisk and a
whole new OVERSCAN-world opens up.
If You push the CONTROL button during booting, it will come up with an
installation menu and You can change the Overscan-resolution, so it
will fit to Your monitor screen adjustment !
You can then save this Overscan-adjustment, so it will boot itself up
with the right size the next time !(The installation menu is only
available from Overscan.PRG version 1.5 up)
The GEM desktop with this resolution is just wonderfull !
With this solution one can use already a few programs, which get their
screen parameters directly from GEM and don't write the graphics
directly into the screen memory, for example the DOODLE.PRG, Wordplus,
GEM Paint, Easydraw, Kuma Graph and Spreadsheet etc.
These programs and some more already work with Overscan in color and
monochrome mode !
Unfortuneally the best two graphics programms, CAD3D from TOM HUDSON
and CYBERPAINT from JIM KENT don`t work at this moment with this higher
resolution.
TOM and JIM are YOU jet listening to this ???
Please try to adapt Your superb programs !!!
This Overscan.PRG patches the TOS with the negative LINE A-Variables,
so every application, for example CAD3D only would have to fetch the
actual SCREEN SIZE with the command OPEN WORKSTATION (VDI).
This will also apply for the the new 19" Matrix or Mat Screen Monitors
that are used espacially for DTP with Calamus and other applications.
Wouldn`t this be great to design Your objects on such a big screen with
CAD3D?
Imagine an OVERSCAN-Delta-Animation in color played back in realtime
from a 800 MBYTE Hardisk, synchronized by midi-clock with a
midi-sequenzer (a second Atari ST for example ) which is controlling a
complete orchestra of synthesizers !
What a realtime multi media show this would be !
This could also open up a whole new world of videomusic production in
the Music-Business Industry !
Still a few words to the screen-memory partitioning in OVERSCAN mode in
50 Hertz image frequency:
The visible screen memory is now 59640 Bytes (if Your monitor can be
regulated to this screen width and height).
But because of the Composite Sync Signal there are also pixels
displayed during the blanking time ( flybacktime) of one scan line.
This are 26 Bytes per scan line.
This adds up to 7384 Bytes more than the visible 59640 Bytes.
This means the actual size of the screen memory has to be 67024 Bytes
big. (calculated for 50 Hertz, in 60 Hertz some other size will fit)
This about 7 kByte can be used for storing palette information or other
stuff.
If it's to memory wasting the software could also store only the 59640
Bytes for oneOä ⇦ê✓נîÇ⇧נüâê r`⇧Ç éêOÇ⇩⇧üנüÄ✓OÄOä⇨Ç⇦üנÇ✓üנüÇê✓⇦IJ≡pëÇ✓OüÇOÇÇ◆נÇê⇦נüâêOגijנÇÇ⇦✓ נ£é⇧ê⇦Oéנêê⇦éOÇé êOä נüéê@≡p≡p£ê ⇦✓OÇ✓נüâêOçÇ⇧äOä⇧Ç◆נäê⇧Ä⇧éנüé✓OÇ✓נüâêO£◆Ç⇦⇩⇧êOä⇩îOä⇩✓Ä⇧≡pëÇ⇦éêOÇê⇦üé⇧נîüÄÇ⇩êOüé✓Oüâê⇦êOÄ⇦êOÄ⇧äOä✓Oçé⇨ê⇧äOüäÄä✓ê⇦äê ≡pÇ⇨é ⇨נÄ⇦êז⇧נÇé⇦⇩ üêOîÇ⇧נîüÄÇ✓ ij≡püâÄ⇧ז⇦OÇ⇨éOüâê⇦êOâÄ⇦OüÇOîêOÄנÇ✓êä✓⇧נä⇧Ä⇦üéêOÄ äê⇦⇦OÇ✓נÄ Ç⇧נijOÇé⇧ê⇦ppä üäÄ ⇧ê נêäÇOüâêOÇé ê@îÄ⇦OçÇ⇩üê⇦נäê✓⇩⇦⇧ê⇦äOÿÿבגוה@Gג@≡p≡p£Ç⇧נüâé⇦Oé⇦OÄ⇧äê éOëÇê⓪:7ÃÉ;œ´4⓪7´¨⓪'†2¨9õãÃ⑦()#É⓪åà$ø⓪;ØÉ;ôÀ2⓪40†2É:94ØØ⓪:7É0†7œØ⓪:42¨ØÉ6ittle two disadvantages we would
have been forced to generate a very special NEW DISPLAY ENABLE Signal
which would have been needed an anormous amount of counters and
flip-flops.
So it's easier to live with this a little more memory consuming but
simple to install Composite Sync signal.
The difference between 50 and 60 Hertz color mode :
In 50 Hz mode with using the Compositeä⇩îOä⇩✓Ä⇧נéOâÄêOגג@נ£é⇧ê⇦ppçê⇦נä נüéê@≡pגהGנ£é⇧ê⇦OÄ⇦êOÇé⇦⇩ üêOij≡püâêOÇ⇧âê⇦נגונ£é⇧ê⇦OÄ⇦êז⇧נÇé⇦⇩ üêOîê ⇦✓OÇ✓נer capacitor( buffers the collector(5Volts) of the
transistor versus ground).
This Composite Sync Signal is ideal for controlling the new Overscan
mode. We have been trying other signal for example: only the HSync, the
VSync, the Blank signal or only permanent 5 Volts but this doesn't work
with either 50 and 60 Hertz and GEM couldn't be installed.
To find a better signal than the Composite Sync signal is not possible
with such a simple modification.
The ST has only 4 signal which come from the GLUE which could be used to
create a " new " Display Enable signal, because all other signals vary !
The only time invariant signals are :
1. The old DE-Signal
2. The HSync Signal
3. The VSync Signal
4. The BLANK Signal.
And still the divided clock frequencies 2 MHz and 500 kHz.
To create a better signal than the composite sync signal one would have
to use some counters and dividers to build up a better "DE"-signal.
This would mean a lot of additional hardware!
GEM needs a video scan line whose number has to be able to divide by 4 to
install it. So the Composite Sync Signal was the simplest solution to
find.
Description of the Hardware Modification
The Display Enable Signal which is going to the Shifter Pin 37 and to
the MMU Pin 52 has to be cut and instead of this the Composite Sync
Signal has to be fed to this two Pins.
This could be done by a simple 3 Pins switch which switches between the
original and the new Overscan Mode.
The switch could be installed at the back of the ST for example.
It is also possible to switch it during operation of the ST without
hanging up.
If You switch back it might could happen, that the color palettes have
been cycled.
This is due to percussion during switching but could be removed by
switching 3 or 4 times again till the color palettes are in the right
order again.
Hint: If there will a shifted palette as the desktop appears, You have
to switch again a few times the change-over switch so due to percussion
this shifted palettes will disappear!
This shifted palettes only sometimes occur during booting but this is
the only time !
How to connect and wire the Overscan-change-over-switch:
Directly at the Emitter of the Composite Sync signal driver Transistor
You pick up the buffered Composite Sync Signal and feed it to the third
pin of the switch.
The second pin of the switch (the pin in the middle) goes to the pin 37
of the Shifter and pin 52 of the MMU.
The first pin of the switch is connected to pin 39 of the Glue (Display
Enable Signal)
Reminder: the MFP 68901 pin 20 still gets the Display Enable Signal in
both modes !!!
We have tried to use also the Composite Sync for the Timer B-Input in
Overscan mode, but then the ST doesn't boot himself after a Reset !
Softwareadaption to the Overscan Mode
If You switch directly from the normal desktop after booting without
any accessories or programs to the Overscan mode, You will see some nice
"pixel trash" in the upper half of the screen, but going from the very
left to the very right.(without a border)
But what has happened to the lower half screen ?
There is some pretty moiree jumping and changing in a strange pattern of
16 bits in various colors !
What is this, You will ask ?
Don't worry, be happy !
This is only the display of RAM databus signals !!!
This happens because the normal Video screen display Ram is installed
at the top of the available ST Memory.
If switched to Overscan the ST needs more than the 32kBytes but there at
the top is no more available, so behind the end of the RAM the ST
displays the undefinable Databus signals !
What a funny picture !
( I just wonder wether the first game will use this for displaying
"screen communication problems" like in the game: Carrier Command ?)
So the answer to get a correct full screen Overscan picture is :
Write a lower Adress into the Screen Starting Adress Pointer
FF8201/03 !
But then there is still the Pixel-Trash, but now all over the screen !
This is because the color bit planes have changed their place. This can
be corrected by adapting the GEM.
This was done by us by writing the Overscan.PRG which adapts the GEM.
It runs with the Blitter-TOS and the new ROM-TOS 1.4 and the BETA-RAMTOS
and the Developer-RAMTOS 1.4, but not with the old ROMTOS from 1985 !
You start it out of the AUTO-folder of a disk or from the harddisk and a
whole new OVERSCAN-world opens up.
If You push the CONTROL button during booting, it will come up with an
installation menu and You can change the Overscan-resolution, so it
will fit to Your monitor screen adjustment !
You can then save this Overscan-adjustment, so it will boot itself up
with the right size the next time !(The installation menu is only
available from Overscan.PRG version 1.5 up)
The GEM desktop with this resolution is just wonderfull !
With this solution one can use already a few programs, which get their
screen parameters directly from GEM and don't write the graphics
directly into the screen memory, for example the DOODLE.PRG, Wordplus,
GEM Paint, Easydraw, Kuma Graph and Spreadsheet etc.
These programs and some more already work with Overscan in color and
monochrome mode !
Unfortuneally the best two graphics programms, CAD3D from TOM HUDSON
and CYBERPAINT from JIM KENT don`t work at this moment with this higher
resolution.
TOM and JIM are YOU jet listening to this ???
Please try to adapt Your superb programs !!!
This Overscan.PRG patches the TOS with the negative LINE A-Variables,
so every application, for example CAD3D only would have to fetch the
actual SCREEN SIZE with the command OPEN WORKSTATION (VDI).
This will also apply for the the new 19" Matrix or Mat Screen Monitors
that are used espacially for DTP with Calamus and other applications.
Wouldn`t this be great to design Your objects on such a big screen with
CAD3D?
Imagine an OVERSCAN-Delta-Animation in color played back in realtime
from a 800 MBYTE Hardisk, synchronized by midi-clock with a
midi-sequenzer (a second Atari ST for example ) which is controlling a
