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OVERSCAN.TXT
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1989-02-22
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24KB
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535 lines
Stefan Hartmann
Electronic Research and Development
Keplerstrasse 11 B
1000 Berlin 10
West-Germany
Tel: West-Germany/30 344 23 66,
email on Compuserve ID: 72017,3216
"Hey You man, wanna have some Overscan ?"
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 writ