Fullscreen programming on the Atari ST
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"Fullscreen"-Programming on the Atari ST
by Flix of Delta Force
Welcome to the first part of a series of articles in which the
Delta Force will explain how to code "demos", featuring fully
working source-codes on the Maggie-disk. This article will show
you how to display screens with a resolution of 416 by 276
pixels, using 16 colours per line.
The reader of this course should have basic knowledge of
assembler and the ST's hardware. The first topics are:
Annihilation of the screen borders (by Flix), Sync-Scrolling (by
New Mode) and Music programming using the YM-2149 (by Big Alec).
But now let's talk about the borders:
Allthough nobody expected it to be possible a long time ago, the
ST's borders can be removed by pure software. Demos, that
displayed graphics in the borders using nothing but software,
were the foundation for the development of the well-known
"Overscan"-hardware.
At first, I will write a bit about the history of the so-called
"border-programming".
The first human ever to display any graphics in a screen border
was Sven alias "Alyssa" from Mannheim/Germany. Back in 1987 he
wrote an intro, that showed graphics in the lower border. As
Sven vanished very fast from the ST-scene, it is completely
unsettled, how Sven had the idea to switch the screen frequency
to 60 Hz for a short period of time within the last line of the
screen. Without this fundamental idea, no fullscreens or sync-
scrolling would be possible on the ST. Unfortunately nobody in
the ST-scene knows what "Alyssa" is doing today. TEX (The
Exceptions) firstly used graphics in the lower border in the
famous "B.I.G. Demo". To open the lower border, the screen
frequency has to be switched to 60 Hz at the end of the 199th
line. The ST then reads nearly 50 more lines of the screen
memory! The Exceptions revealed this secret in the B.I.G. Demo
at the end of the longest scrolltext (42 Kybtes of text). After
The Exceptions took up Alyssa's trick, the upper border was the
next one to be removed. This is achieved by toggling the
frequency 13 or 29 lines above the "usual" beginning of the
screen. You may wonder, why the upper border can be 13 or 29
lines long. Unfortunately there are two different MMU-versions,
that have a difference of 16 lines. Due to the place to open the
upper border being above the screen memory and the Timer B
starting to count at the beginning of the screen memory, The
Exceptions used a quite complicated method. They set a Timer B
Interrupt in the last line and waited until the electron-ray
reached the right position in the upper border. Doing this, they
had to waste a lot of processor-time. In the "Musical Wonder -
1991" I used a routine that waited a few scanlines after the
VBL-Interrupt occured and then toggled the screen-frequency. But
there's still a better method. Timer B is counting just "real"
screen-lines (200), but there is another interrupt, the HBL,
which counts all scanlines (313). At the beginning of the VBL
the HBL starts counting until the right position to open the
upper border is reached. If you now open both, the upper and the
lower, border, you have 277 lines of graphics in the low
resolution.
Soon the next border was declared to vanish: The right border.
Unfortunately it is not possible to open this border with Timer B
or HBL interrupts. The ST displays graphics in the right border,
if you switch the frequency to 60 Hz at a certain position in
every line, in which the right border is supposed to be opened.
This "certain" position requires a completely new technique of
programming. If you switch colours in an ordinary Timer B, VBL or
HBL interrupt, you can see, that these interrupts do not occur at
exactly the same position. The colours shake from the left to the
right. In order to make some commands, like the colour or our
frequency switchings, occur at exactly the same position, you
have to become synchronized with the raster-electron-ray. An
ingenious method to achieve this effect is the following one:
WAIT: MOVE.B $FF8209.W,D0 ; Low-Byte
BEQ.S WAIT ; mustn't be 0
NOT.B D0 ; negate D0
LSL.B D0,D0 ; Synchronisation
If you execute this routine every VBL, all following commands
will be executed at the same position every VBL, that means that
the colour- or frequency-switches are stable. But what does this
little routine do? At first, the low-byte of the screen-address
is loaded into D0. This byte exactly determines the position
within the line. It is negated and the LSL-command is executed
(LSR, ASL or ASR work as well). As you can read in every
processor-book the LSL-command takes 8+2*n clock-cycles. That
means that the command needs more clock-cycles the bigger the
value in D0 is. That is exactly the shifting that we need! I hope
that you understood this part, because all fullscreens and sync-
scrolling-routines are based upon this effect. You should know
that one VBL (50 Hz) consists of 160000 clock-cycles (one
scanline consists of 512 clock-cycles). Now you have to switch
the frequency at a certain position and the border opens. Of
course this takes a lot more processor time than the opening of
the upper or lower border, because you've to open the border
every line. Your ST now displays 204 bytes per line! A line
consists of 25.5 instead of 20 words without the right border,
but should use only 23 of these 25.5 words, because the picture
is distorted on some STs (We made this mistake in the "Musical
Wonder - 1991") if you use too much words. Anyway, there are
hardly any monitors that have such a huge visible right border.
A demo-screen without the lower and the right border was
included in the "Amiga-Demo" by TEX.
For a long time it was considered impossible to open the left
border, because the trick with the 60-hertz-switch did not work
in the left border (I was sure about that as well). But than came
the "Death Of The Left Border"-Demo by the TNT-Crew. Finally
there was a screen in the legendary Union-Demo that firstly in
ST-history displayed no borders at all: The first so-called
"Fullscreen"! The demo-coders made the impossible possible. But
how does this trick work? It's pretty simple: Instead of
switching to 60 Hz you switch to 71 Hz! If you disable the
interrupt that resets at 71 Hz, you can switch to 71 Hz in
coulour mode. You should do this only for short periods of time
(a few clock-cycles). Being in 71 Hz for a long time can damage
your monitor (allthough I've never seen such damages). It goes
without saying that neither Maggie nor the "Delta Force" take
responsibility for damages caused by this technique. If you now
open the right border additionally, the ST displays 230 instead
of 160 bytes per line! In order to make the opening of the
"side-borders" run on all STs, you've to install so-called
"Stabilisation"-switches at the end of each line (This is
sometimes called "Closing of the right border" which it isn't).
You can switch to 71 Hz (like in the source) or to medium
resolution (like ULM (Unlimited Matricks) does it). I prefer the
71 Hz method, because I heard that the other method causes
problems on few STs. The "Stabilisation" is needed because the
shifter waits for the last word at the end of the right border
(115 words are displayed in fullscreen) to fill the last plane.
This word never arrives and to avoid total confusion of the
shifter the 71-Hz-switch does something like a shifter-reset.
If you now open all borders, you've a screen memory that consists
of 160+230*276 (=63640) bytes. The first line is needed for
synchronisation, therefore it has 160 bytes. The screen memory is
almost twice as big as in the "usual" low resolution!
Two registers of the shifter are needed to switch the frequency:
$FFFF820A: 0: 60 Hertz 2: 50 Hertz
$FFFF8260: 0: Low resolution 2: High Res. (71 Hertz)
There are various methods to cause the frequency-switches. ULM
uses the "Immediate"-Adressation, that means a 60-Hz-switch would
look like this:
MOVE.B #0,$FF820A.W ; 60 Hz (16 clock-cycles)
MOVE.B #2,$FF820A.W ; 50 Hz (16 clock-cycles)
Besides that this method takes a lot of processor time, it is
said that it does not run on all STs. Another method is to use
registers. This is faster, but takes some Data- and Address-
registers. A 60-Hz-switch would look like that (D0=0, D1=2,
A0=$FF820A.W):
MOVE.B D0,(A0) ; 60 Hz (8 clock-cycles)
MOVE.B D1,(A0) ; 50 Hz (8 clock-cycles)
In the source-code on the this Maggie-disk only D0, A0 and A1 are
used for all frequency switches. How is this possible? Usually a
data-register is needed for synchronisation, but you can use the
fact that D0 should always contain zero:
WAIT: MOVE.B $FF8209.W,D0 ; Low-Byte
BEQ.S WAIT ; mustn't be 0
NOT.B D0 ; negate D0
LSR.B D0,D0 ; Synchronisation
After the LSR-command is executed, D0 contains zero for sure.
This zero can than be used for the border-routine. The other
data-register D1 must contain 2. This register is superfluous,
because you can get the 2 from somewhere else (D0=0,
A0=$FF820A.W):
MOVE.B D0,(A0) ; 60 Hz
MOVE.W A0,(A0) ; 50 Hz
This step is not very easy to understand. The second command
writes the word $820A to $FF820A. The second byte of the word is
meaningless, because the address $FF820B of the shifter is not
used. The shifter only reads the lower two bits of the first
byte ($82), that means that the byte is handled like $02!
The source-code on the disk includes a subroutine that tests
which MMU is present, that means how many lines the MMU displays
with an opened upper border (229 or 213). The program should run
on every ST, STe, Mega ST and Mega STe (not on TTs). It took me
more than a month to test the routine on all possible ST-chip-
configurations (Thanx to Alien/ST-CNX). There are more than two
dozens ST-versions. Atari changed the Glue, the MMU and the
Shifter quite often.
It is possible to open two more lines of the lower border by
switching to 60 Hz for a second time at the end of the "normal"
lower border. Unfortunately this does not work on the new Atari
colour monitors (Thanx to Aeon/Aura for telling me!). For my
"Punish Your Machine"-screen I had the idea to code a fullscreen
in medium resolution. It worked fine on my ST very soon, but on
the ICC #2 I discovered that the screen only worked on my ST! New
Mode made it work on all STs two months later, allthough all
"border-experts" on the ICC #2 said that it's impossible to code
a fullscreen in medium resolution.
The most difficult thing writing a fullscreen is that the entire
code has to fit synchronous in the waiting gaps of the
fullscreen-routine.
A few days after the ICC #2 I developed a new fullscreen-
routine. With this routine the main program does not have to be
synchronous with the electron-ray. The disadvantage of this
method is that the border annihilation takes up to 50 per cent
of the processor time. Therefore you can use keyboard-requests,
multiplications, divisions and interrupts. The routine is,
allthough it's awfully old, top secret and cannot be published
in Maggie (yet).
Ok, I hope you understood how you make your ST display a
resolution of 416*276 pixels. In the next part New Mode will
explain how you can scroll the whole screen in 4 planes using 7
scanlines of processor-time.
Bye...
Flix/DF (The Union)