Raster Interrupt Veterans: C64 and Atari 2600, etc.

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Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by Chief Blur Buster » 30 Dec 2013, 00:45

I've noticed several old-school computer programmers have signed up as members of Blur Busters Forums.

Check in this thread, if you have experience with raster programming of any kind. Scan line registers, etc. That used to be essential knowledge for programming on the primitive TIA chip of the Atari 2600, and was important for scroll zones, split screens, add extra colors beyond hardware limitations, and do sprite-multiplication effects on many 1980's platforms including the Commodore 64. Feel free to describe your raster programming exploits here.

Relevant Blur Busters Areas Of Study: People who have worked with rasters, and understands rasters, are also better-positioned to understand computer-code-to-photons-hitting-eyes -- including display scanning behavior. This is very important to easily understanding advanced display topics -- such as understanding and explaining the VSYNC OFF tearing artifact at a far more scientific level, ability to better understand why 250fps@60Hz (VSYNC OFF) looks/feels better than 125fps@60Hz, and know why there are input lag differences of different vertical regions of a display.
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spacediver
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Re: Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by spacediver » 30 Dec 2013, 01:32

I certainly don't fit the bill of a raster programmer, but for anyone who wants a good introduction to the basics of the raster scanning, see this awesome 13 page tutorial: http://easymamecab.mameworld.info/html/monitor1.htm

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Ahigh
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Re: Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by Ahigh » 30 Dec 2013, 17:41

We used to do this type of stuff. The good ol' days of the C-64. Yes, the kids who have been brought up on LCD screens often forget that the computer still scans out the pixels no matter how the display handles them.

Here is a link to some of these tricks from the past as documented by another fan.

http://www.antimon.org/dl/c64/code/raster.txt

twilen
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Re: Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by twilen » 28 Jan 2014, 05:44

I did raster stuff and still do.

I am developer of WinUAE (Amiga emulator), I have to know this stuff inside out (and I hope I actually do..), I also learned lots of new things (for example how interlace actually worked on PAL/NTSC CRT TVs, how it related to "progressive" 50Hz/60Hz modes that was abused by most 8/16-bit computers before TVs started to include digital circuits), unfortunately or fortunately most of this stuff is obsolete today :)

Amiga's Copper (and other chipset features) made raster stuff too easy to code but really complex to emulate. (Technically it looks simple but there is so many corner cases..). I also did some C64 raster stuff before I got my first Amiga (1987 I think).

Amiga also allows software to modify chipset's internal horizontal and vertical position register on the fly that some programs use to create "programmed" refresh rate timings. (From emulator coder point of view this feature is really annoying, there is few programs that abuse this feature quite horribly..). Did any other 8/16-bit computer allow it? Vertical counter modification isn't that bad but horizontal..
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Re: Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by Chief Blur Buster » 28 Jan 2014, 12:06

twilen wrote:Vertical counter modification isn't that bad but horizontal..
Raster register modification, wow. Varying the time of an individual scan line, and the number of scan lines per refresh. I imagine it would have been used to create "glitchy TV" special effects in some games. Must be a PITA to emulate.
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Re: Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by RealNC » 28 Jan 2014, 13:55

Heh, and I thought I was old-school. I still remember a "secrets of the demo-scene" book I bought when I was 14 and learning how to mess with VGA registers in Turbo Pascal and assembly to do coppers in text mode and stuff like that. Fun times. I have a lot of respect for anyone who could program the Atari 2600. That thing was a real b***h to work with. I never had the patience to learn how to do it, even when the first emulators came out that made it easier then ever to get started.
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twilen
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Re: Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by twilen » 29 Jan 2014, 11:17

Chief Blur Buster wrote:
twilen wrote:Vertical counter modification isn't that bad but horizontal..
Raster register modification, wow. Varying the time of an individual scan line, and the number of scan lines per refresh. I imagine it would have been used to create "glitchy TV" special effects in some games. Must be a PITA to emulate.
Fortunately there is only one known program (One scene demo. But even one is too much!) that modifies both vertical and horizontal counters every scanline, others "only" use it to create 60Hz mode (Newer chipsets have software PAL/NTSC switch support, older revision does not) which is easy to support.

Not worth the trouble to support but someday it has to be done :)
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Re: Raster Interrupt Veterans: C64 and Atari 2600, etc.

Post by Chief Blur Buster » 29 Jan 2014, 14:26

Related thread: Emulating GSync at fixed refresh rate [low-lag VSYNC ON]

(Emulating only one specific aspect of GSYNC: eliminating tearing in a low-latency manner. It doesn't fix stutters of variable frame rates).

Programmers reading this thread, might want to chime in over there in the other thread.
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       To support Blur Busters:
       • Official List of Best Gaming Monitors
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       • List of FreeSync Monitors
       • List of Ultrawide Monitors

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