Video: Its Programmer, On SNES DOOM

I’ve been doing a lot of high-effort posts lately, even for things that should be fairly quick. Working to make this more sustainable, here’s a laid-back post that’s mostly just a Youtube video of a talk between the guys of Digital Foundry and Randy Linden, coder of the SNES port of DOOM, which uses the SuperFX chip to make the hardware push polygons at a rate that, while not stellar by PCs-of-the-time standards, at least not abysmal.

Let’s run down the differences of hardware:

PC running MS-DOS: targets VGA monitors, displays all its pixels in software, but makes up for it with a minimum requirement of a 386 running (if memory holds up over nearly 30 years) at 33 mHz.

SNES: Its processor is a much slower workalike of the 65C816, a 16-bit version of the 6502, running at 3.58 mHz. While it makes up for its slower clock speed with a simpler design, meaning instructions complete generally in fewer cycles, it’s hard to make up for that 10-fold difference in speed.

Their use of specialized graphics hardware was an important advantage, at the time, of consoles over personal computer hardware. Even many standard home microcomputers, like the Commodore 64 and Atari 800, had dedicated graphics hardware that helped games run better than what most PCs could do. Even when VGA came out, the standard had no hardware-level support for scrolling or sprites.

Consider what it takes to scroll a screen without hardware support: something in the system has to be able to move every pixel from one spot on-screen to another. The NES pulls this task off by having a bank of memory that its PPU can be pointed within, meaning the memory could stay in the same place, and the graphics chip would just work from a different region within it. Sadly, this technique is not amenable to 3D graphics, which usually do require every pixel on the screen to be recalculated every frame, either in software or hardware.

The SNES is known for having a rather slow chip for its time, but more demanding games tended to make up for it with co-processor chips included on the cartridges. The most well-known of these are the DSP-1, which functioned as a math co-processor; the SA-1, which was basically a second 65C816 running at around triple the speed and with a few added features; and the SuperFX, which ran at about the SA-1’s clock speed but functioned as a graphics accelerator. (The later SuperFX 2 ran at twice that speed.)

These were far from the only add-on chips included on Super Famicom and SNES carts. Since the SNES had a much larger address space than the NES’s 6502-clone, the need for mapper chips was much less, but these co-processors were used in a number of more notable games to help it make framerate goals.

Hm. Well, I tried making it a laid-back kind of post. Ah well, back to playing Live-A-Live.

DF Retro: The Making of Doom on Super NES – The Original ‘Impossible Port’

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