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Tiny correction: it's 320x240 also called mode X [0], an undocumented VGA graphical mode discovered by Michael Abrash which made him famous at the time. One of

Tiny correction: it's 320x240 also called mode X [0], an undocumented VGA graphical mode discovered by Michael Abrash which made him famous at the time. One of the big advantage of the mode X is that the pixels are square, unlike mode 13h (320x200). It was previously used in the commander keen series and was in great part to credit to their success because for the first time on PC a game was matching console graphical quality. It has square pixels. IIRC Carmack improved on the mode X and him and Michael started a correspondance. Who knew such a great story was behind such a tiny mistake: nothing less than the advent of games on PC. [0] http://archive.gamedev.net/archive/reference/articles/article373.html http://archive.gamedev.net/archive/reference/articles/articl...

Beldin 5y ago
> One of the big advantage of the mode X is that the pixels are square On the then-ubiquitous 4:3 resolution monitors. (Made this addition because on first reading, I wondered why square pixels would occur on anything but 320x320 resolution)
rasz 5y ago
1 Abrash wasnt the first person to discover modeX. 2 All Commander Keen game were EGA, with some having additional CGA mode. 3 Carmack didnt improve modeX, he hired Abrash to help with optimizing Doom. As a result Doom ended up with the stupid modeY (variation of modeX) which was a wash performance wise on slow 486, while tanking performance on faster processors. https://github.com/viti95/FastDoom https://github.com/viti95/FastDoom switching original Doom sourcecode back to mode 13h (or even better VESA 320x200) results in substantial to huge performance improvements.
- hyperman1 5y ago
  Can you explain point 3 a bit more? Specifically where the performance impact came from: A) Mode 13h has not enough ram to have 2 pages, so either you get screen flicker as you see Doom draw, or you loose 64k ram for an offscreen buffer, and the extra data transfer also takes time. B) The core of ModeX/ModeY is disabling the chain4 mechanism and manually dispatching bytes to the 4 planes. The cost of this is some 'out' instructions, I assume 1 group per vertical slice. I don't know how doom did floors here, maybe draw them 4 times?. Even so, the cost seems not that expensive.
  - rasz 5y ago
    Doom, like Wof3d, renders screen in 1pixel wide columns. https://www.youtube.com/watch?v=ujXrQVyl610 https://www.youtube.com/watch?v=ujXrQVyl610 8-bit accesses only, this alone tanks performance for faster VGA cards. The second problem is slow plane switching in some implementations, especially later cards didnt bother optimizing this. When you write to VGA (ISA in general) CPU is paused for the duration of the transfer. Switching back to 13h and rendering to RAM buffer benefits from motherboard cache and lets you actually use whole bandwidth when copying to the card (rep movsd). Typical 8-Bit VGA write bw is between 700KB/s for very bad Trident up to almost 2000KB/s for ET4000. At the same time fast 386 will do >20MB/s cache writes and extremely fast VGA on overclocked ISA bus can do 7MB/s. Heretic performance is higher compared to Doom because Raven dropped modeY. Fastdoom: Cyrix MediaGX https://www.youtube.com/watch?v=3PAfvDsmzVY https://www.youtube.com/watch?v=3PAfvDsmzVY 24 vs 64 fps and extreme case of very bad modeY handling by Rendition Verite https://www.youtube.com/watch?v=6unNuPympE4 https://www.youtube.com/watch?v=6unNuPympE4 modeY 14, 13h 60, VBE2 300 fps
    - rasz 5y ago
      I forgot one more thing: reading! Doom reads back VGA memory when painting invisible sprites (Spectres, player sprite with partial invisibility blur sphere), this is extremely slow because absolutely nobody optimized VGA cards for reading. Ram buffer removes this disadvantage.
      - hyperman1 5y ago
        I think we played on different machines. You talk about high end VGA and cache. But most 386's had no cache, mine surely hadn't. My VGA was slow enough not to care about transferring 16 bits instead of 8, it just got half as fast or worse. OTOH, the low res mode, 160x200, was created by enabling 2 of the planes, and made a big difference. And 64K on a 4MB meant less of hard disk activity, which was my main bottleneck. The semi translucent spectres causing VGA reads were very bad on that machine, causing <5FPS if there were multiple on screen. I learned to shoot them on sight and from a distance.

Beldin 5y ago

> One of the big advantage of the mode X is that the pixels are square On the then-ubiquitous 4:3 resolution monitors. (Made this addition because on first reading, I wondered why square pixels would occur on anything but 320x320 resolution)

rasz 5y ago

1 Abrash wasnt the first person to discover modeX. 2 All Commander Keen game were EGA, with some having additional CGA mode. 3 Carmack didnt improve modeX, he hired Abrash to help with optimizing Doom. As a result Doom ended up with the stupid modeY (variation of modeX) which was a wash performance wise on slow 486, while tanking performance on faster processors. https://github.com/viti95/FastDoom https://github.com/viti95/FastDoom switching original Doom sourcecode back to mode 13h (or even better VESA 320x200) results in substantial to huge performance improvements.

hyperman1 5y ago
Can you explain point 3 a bit more? Specifically where the performance impact came from: A) Mode 13h has not enough ram to have 2 pages, so either you get screen flicker as you see Doom draw, or you loose 64k ram for an offscreen buffer, and the extra data transfer also takes time. B) The core of ModeX/ModeY is disabling the chain4 mechanism and manually dispatching bytes to the 4 planes. The cost of this is some 'out' instructions, I assume 1 group per vertical slice. I don't know how doom did floors here, maybe draw them 4 times?. Even so, the cost seems not that expensive.
- rasz 5y ago
  Doom, like Wof3d, renders screen in 1pixel wide columns. https://www.youtube.com/watch?v=ujXrQVyl610 https://www.youtube.com/watch?v=ujXrQVyl610 8-bit accesses only, this alone tanks performance for faster VGA cards. The second problem is slow plane switching in some implementations, especially later cards didnt bother optimizing this. When you write to VGA (ISA in general) CPU is paused for the duration of the transfer. Switching back to 13h and rendering to RAM buffer benefits from motherboard cache and lets you actually use whole bandwidth when copying to the card (rep movsd). Typical 8-Bit VGA write bw is between 700KB/s for very bad Trident up to almost 2000KB/s for ET4000. At the same time fast 386 will do >20MB/s cache writes and extremely fast VGA on overclocked ISA bus can do 7MB/s. Heretic performance is higher compared to Doom because Raven dropped modeY. Fastdoom: Cyrix MediaGX https://www.youtube.com/watch?v=3PAfvDsmzVY https://www.youtube.com/watch?v=3PAfvDsmzVY 24 vs 64 fps and extreme case of very bad modeY handling by Rendition Verite https://www.youtube.com/watch?v=6unNuPympE4 https://www.youtube.com/watch?v=6unNuPympE4 modeY 14, 13h 60, VBE2 300 fps
  - rasz 5y ago
    I forgot one more thing: reading! Doom reads back VGA memory when painting invisible sprites (Spectres, player sprite with partial invisibility blur sphere), this is extremely slow because absolutely nobody optimized VGA cards for reading. Ram buffer removes this disadvantage.
    - hyperman1 5y ago
      I think we played on different machines. You talk about high end VGA and cache. But most 386's had no cache, mine surely hadn't. My VGA was slow enough not to care about transferring 16 bits instead of 8, it just got half as fast or worse. OTOH, the low res mode, 160x200, was created by enabling 2 of the planes, and made a big difference. And 64K on a 4MB meant less of hard disk activity, which was my main bottleneck. The semi translucent spectres causing VGA reads were very bad on that machine, causing <5FPS if there were multiple on screen. I learned to shoot them on sight and from a distance.