5 ms·

It's the lowest end of the IBM PC lineup: 4.77Mhz 8088 CPU, max 256KB RAM, CGA graphics (4 color graphics and 16 color text modes), PC speaker sound (no sound c

by sedatk 4y ago

It's the lowest end of the IBM PC lineup: 4.77Mhz 8088 CPU, max 256KB RAM, CGA graphics (4 color graphics and 16 color text modes), PC speaker sound (no sound card). So, this is beyond incredible. IIRC, more than 4 color graphics modes are achieved by relying on pixel bleed with CGA's monitor signal output.

anyfoo 4y ago
"Pixel bleed" sounds like it isn't really doing it justice. More than 4 colors on CGA are usually achieved by using the composite output, which is actually plain NTSC. In NTSC, color is added to the monochrome signal by modulating a quadrature amplitude modulated signal onto a carrier whose frequency is actually within the monochrome image bandwidth itself. NTSC did this for compatibility with black and white TVs of the time. (If that sounds too complicated, it "just" means that changing brightness very fast creates color.)[1] To get more than 4 colors in CGA, you "just" switch pixels fast enough that you are effectively modulating the color signal yourself! But the problem with CGA is that it doesn't have a graphics mode with high enough resolution to get all those colors. But text mode does, so there's tricks to chop the text lines into pieces which allow puzzling back together a high res image that modulates the color signal. It's pretty cool. [1] PAL is the same with a twist, SECAM is very different but "changing brightness very fast" is still accurate.
- sedatk 4y ago
  Great explanation! I didn't know text mode's role in this.
- Scali 4y ago
  That was how the predecessor 8088 MPH worked. Area 5150 however is aimed at a TTL RGBI monitor, and as such, does not use NTSC composite artifacting. It can only display 16 colours at once. So in this demo, you get 640x200 in 16 colours, where additional colours are implied by using dither patterns. (Technically 8088 MPH did more or less the same, except these 'dither patterns' would result in NTSC artifact colours).
  - anyfoo 4y ago
    Thanks, I should have written "more than 16 colors", forgot that CGA has a 16 color mode without tricks. Dither patterns/"artifact colors" are exactly what I meant, they achieve color by modulating the luminance signal (which is of course the same as "normal" color, but the trick is to let the software influence only the luminance and getting "unintended" colors instead of directly telling the CGA adapter to modulate the color on, and over a separate circuit probably, like what normal 4/16 color mode does).
wbhart 4y ago
Actually the RAM was expanded in this demo out to 640kb. This was necessary for some of the effects in combination with the loader. Such expansion boards were available back in the day.
- jchw 4y ago
  That's fair; my understanding was that this was a pretty common thing to expand on a 5150, but I'd love to know if anyone has personal experience with having owned one back in the day.
  - Scali 4y ago
    Yes, especially somewhat later in the life of the 8088, most applications would demand quite a bit of memory, and the 640k configuration became standard for the later 5160s and most clones. So I wouldn't be surprised if 5150 owners also upgraded the memory on their machines. The AST Six Pak Plus card was quite a popular upgrade. It could add up to 384k, so if you had 256k on board (fully loaded later model 5150), there's your 640k. If you were unlucky enough to have an early model 5150, the board could only take 64k. That limit was however because 4116 memory chips of 16kbit capacity were used. The board could be modified to take the later 4164 chips of 64kbit capacity, in which case it would take 256k. Alternatively, you could use two memory expansion cards to bring the memory up from 64k onboard to 640k total. Although I doubt many people would use such a contraption in practice. Early model 5150s also required a BIOS upgrade, as a bug would prevent the BIOS from detecting more than 544k of installed memory. So in theory it is possible to have a 5150 with 640k, and no doubt such machines exist in the wild. But it's probably far more common to find a 5155 or 5160 with 640k. So just like with 8088 MPH I guess that's the 'practical' target of the demo.
  - wbhart 4y ago
    Technically it all runs in 500kb I think (not sure if this includes DOS). And this was intentional because the guys had in mind what people would typically have available. However I think everyone is going to have to temper expectations regarding what this will run on. Even my effects (the 3D Glenz objects) couldn’t be debugged on an emulator even though it’s a fairly simple tricked up video mode and a bunch of fairly ordinary assembly code doing VRAM writes that are not timing critical for the screaming fast 3D.

by sedatk 4y ago

anyfoo 4y ago

"Pixel bleed" sounds like it isn't really doing it justice. More than 4 colors on CGA are usually achieved by using the composite output, which is actually plain NTSC. In NTSC, color is added to the monochrome signal by modulating a quadrature amplitude modulated signal onto a carrier whose frequency is actually within the monochrome image bandwidth itself. NTSC did this for compatibility with black and white TVs of the time. (If that sounds too complicated, it "just" means that changing brightness very fast creates color.)[1] To get more than 4 colors in CGA, you "just" switch pixels fast enough that you are effectively modulating the color signal yourself! But the problem with CGA is that it doesn't have a graphics mode with high enough resolution to get all those colors. But text mode does, so there's tricks to chop the text lines into pieces which allow puzzling back together a high res image that modulates the color signal. It's pretty cool. [1] PAL is the same with a twist, SECAM is very different but "changing brightness very fast" is still accurate.

sedatk 4y ago
Great explanation! I didn't know text mode's role in this.
Scali 4y ago
That was how the predecessor 8088 MPH worked. Area 5150 however is aimed at a TTL RGBI monitor, and as such, does not use NTSC composite artifacting. It can only display 16 colours at once. So in this demo, you get 640x200 in 16 colours, where additional colours are implied by using dither patterns. (Technically 8088 MPH did more or less the same, except these 'dither patterns' would result in NTSC artifact colours).
- anyfoo 4y ago
  Thanks, I should have written "more than 16 colors", forgot that CGA has a 16 color mode without tricks. Dither patterns/"artifact colors" are exactly what I meant, they achieve color by modulating the luminance signal (which is of course the same as "normal" color, but the trick is to let the software influence only the luminance and getting "unintended" colors instead of directly telling the CGA adapter to modulate the color on, and over a separate circuit probably, like what normal 4/16 color mode does).

wbhart 4y ago

Actually the RAM was expanded in this demo out to 640kb. This was necessary for some of the effects in combination with the loader. Such expansion boards were available back in the day.

jchw 4y ago
That's fair; my understanding was that this was a pretty common thing to expand on a 5150, but I'd love to know if anyone has personal experience with having owned one back in the day.
- Scali 4y ago
  Yes, especially somewhat later in the life of the 8088, most applications would demand quite a bit of memory, and the 640k configuration became standard for the later 5160s and most clones. So I wouldn't be surprised if 5150 owners also upgraded the memory on their machines. The AST Six Pak Plus card was quite a popular upgrade. It could add up to 384k, so if you had 256k on board (fully loaded later model 5150), there's your 640k. If you were unlucky enough to have an early model 5150, the board could only take 64k. That limit was however because 4116 memory chips of 16kbit capacity were used. The board could be modified to take the later 4164 chips of 64kbit capacity, in which case it would take 256k. Alternatively, you could use two memory expansion cards to bring the memory up from 64k onboard to 640k total. Although I doubt many people would use such a contraption in practice. Early model 5150s also required a BIOS upgrade, as a bug would prevent the BIOS from detecting more than 544k of installed memory. So in theory it is possible to have a 5150 with 640k, and no doubt such machines exist in the wild. But it's probably far more common to find a 5155 or 5160 with 640k. So just like with 8088 MPH I guess that's the 'practical' target of the demo.
- wbhart 4y ago
  Technically it all runs in 500kb I think (not sure if this includes DOS). And this was intentional because the guys had in mind what people would typically have available. However I think everyone is going to have to temper expectations regarding what this will run on. Even my effects (the 3D Glenz objects) couldn’t be debugged on an emulator even though it’s a fairly simple tricked up video mode and a bunch of fairly ordinary assembly code doing VRAM writes that are not timing critical for the screaming fast 3D.