Re: TT Software overscan?
Posted: Sun Jun 17, 2018 7:10 pm
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Petari wrote: ↑Thu Jun 07, 2018 7:59 pmOn ST(E) overscan is achieved by making some writes to video registers (for freq. change) in very accurate moments of horizontal scanline. That's possible only with 100% CPU load, and knowing exact instruction execution times (cycle count) . Luckily, you can use there not only delays, but something like palette change - of course, with rigid execution time. I know only one SW, what does something else - Overscan Invaders. But that game is easy for ST, so was possible to make such code what moves objects and other, while cycle count is same. There is however one issue - it works only with keyboard. I tried to add joystick input, but that screwed timing (not because added cycles - I subtracted what it added, but because not same cycle count every time), and then graphic went corrupted. Could not solve it in simple way, so abandoned.
On TT must be same principle - and that means that CPU speed helps not - timing is what is problem. And you can not use some other way, because it will be not cycle accurate. Some will say: use Timer-B - but that's not good enough, because interrupt of it will not happen cycle accurate - it depends from what instruction executes in moment of interrupt. No wonder that it works only as slide show on TT too.
Just get some VME graphic card
Interesting work there Cyprian... One day when I get to my TT again I will have to pick your brains!Cyprian wrote: ↑Mon Jun 18, 2018 12:20 pm
sometimes ago I did some tests and a classic ST sync-code works fine on the TT:
Therefore there is a chance that software overscan could be reached.
Actually, IIRC this is a simple trick - it just shows very quickly two images alternately. Both are 640x480 16colors.mrbombermillzy wrote: ↑Mon Jun 18, 2018 10:01 pmIm still intrigued with what Ray did for the 1280x480i slideshow demo, as according to the HW TT overscan manual linked earlier by Frank Lucas, TT Med Res is only 832px wide. He has somehow traded vertical resolution for either a 1: LOWERING of H.Freq or 2: a higher dotclk?? I have managed such things on a more programmable Commodore video chip, but I didnt think the TT shifter was this flexible.