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Comment by jezze | Hacker News Reader

One thing I remember was that DOOM did not run well on the 486SX. You needed the 486DX which had an FPU. Maybe there was a 486DX with 33mhz but most were 66mhz I think?

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Doom uses fixed point math so it doesn't matter if you have an FPU. But the fastest 486SX was only 33MHz, which isn't really fast enough for Doom, and they were typically used in cheaper systems with slower graphics cards/buses, which also made a big difference.

One of the biggest contributor to performance differences was the implementation of the L2 cache on the motherboard.

More expensive chipsets tended to have better cache implementations... Assuming the system even had cache installed, those cheaper systems often paired a 486SX with a cheap motherboard and zero L2 cache.

When installed on the same motherboard, same cache/memory config, with the same graphics card, same bus speed, a 486SX should run Doom identically to a 486DX.

I remember it being the case that Doom was aggressively tight-loop optimized, so the small amount of L1 cache was more significant especially on clock-multiplied 486s. Though you are right that the SX/DX distinction wasn't meaningful for Doom as it was all done with internet mathematics.

Yeah, the clock multiplier will also make a large difference.

Which might actually be a fair comparison, The DX2 and SX were launched at the same time, so there is a decent chance the "fast 486 DX" someone is talking about is actually a DX2.

The SX2 was launched later, with the same 2x clock multiplier and same size L1 cache, so would preform identically to the DX2 (in non-fpu tasks like doom). But the DX4 was launched at the same time, now with a 3x multiplier and L1 cache doubled to 16KB....

There were the 50 and 66MHz 486SX2's as well

IIRC the fastest base 486 (out of the box without trickery) was 50MHz. There was a DX2-66 (and DX2-50, which ran clock doubled with the rest of the motherboard at 25MHz). There was also the DX4 which despite the name was only click tripled, running as fast as 100MHz of a 33.3MHz. Other manufacturers produced faster clocked i486 compatible processors: I had an AMD 5x86 which was actually quad-clocked, running at 133MHz internally. For tight loops where the code and enough data for a fair few cycles fitted into its on-chip cache (something like 8K code & 8K data?) these were surprisingly speedy compared to Intel's 486s and even low-end (66MHz, and in some artificial tests 75MHz) Pentiums. Cyrix had some similar models that were quicker still for integer work, and IIRC cheaper so even better VFM for sure tasks, but had abysmal floating-point performance which was rapidly becoming pretty important for games around that time.

Doom used integer maths throughout, so would not benefit from and FPU, but did get a boost in complex maps from the faster internal clocks of doubled/tripled/quadded chips. The Quake era (maybe a little earlier for some more niche games like certain fight simulators) is when the FPU became a massively significant factor for home use (though it didn't exclusive use it: the original Pentiums' FPU wasn't trials fast enough so only something like 1/16th of the work was done there and integer approximations starting from those values were used between - improved further by the FPU being able to work on its next bits while the CPU ALUs could do there part, a hack somewhere between pipelining which 486+ units did naturally (& Pentiums more so due to extra ALU circuitry) and hyperthreading).

You are confusing Doom with Quake.

Doom run okish on 486SX.

"okish" by 1993 standards, but not a solid 35fps with full detail and full viewport size. Of course, that didn't stop people from enjoying it.

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If I recall, the 486-SX was a DX with the FPU disabled (or it was defective). There were 25,& 33MHZ versions.. I think the 66MHZ were "DX2"

IIRC AMD put out a chip that officially ran at 66MHz without clock doubling, though obviously you needed a motherboard (and RAM) capable of running at that rate.

I had a 486DX 33MHz (4MB IIRC).

Doom ran well on it.

I believe I had to start the machine with a boot floppy that configured the system before I run the game though.

486DX2/66 checking in

486DX2/66 with VLB S3 Vision864 checking in

I played Doom in my 486sx 25mhz..

I played it a little in a 386SX40. I had to reduce the rendered screen size, and even then it struggled in open or complex scenes and/or when there were a lot of enemies active.

I played Doom on my 486DX 33MHz.

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