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AMD K6-2+/K6-III+ freezing in Quake 2018-04-22 02:54:07

Recently was helping my brother with a K6-III+ build for DOS gaming and had to debug a rather odd issue. When enabling write combining Quake would freeze or sometimes crash quickly when running in high resolutions (the few other games I tested seemed to be fine). Didn't matter if the CPU was running at 200MHz, underclocked FSB, you name it. The board I was using is a Biostar M5ALA Rev 1.1 with the ALi Aladdin V chipset, which is supposedly the more stable of the Super Socket 7 chipsets. I did eventually figure out that disabling the L2 and L3 caches or using a K6-2 (non-plus) would make the game stable, but also using a SiS530 based PCChips board (probably the cheapest board I've ever used) would be perfectly stable as well ruling out issues with the CPU.

However it turns out the real issue was much simpler than a hardware issue. Running speedsys produced a rather anomalous result for memory writes.

I initially ignored the result, but after spending a few hours with a friend trying to figure out what could be wrong with k6wcx, we tried installing k6dos.sys. Not only did this boost frame rates well beyond what k6wcx was doing (90+fps in 320x200, 30+fps in 640x480 vs about 70 and 20 respectively), but Quake was suddenly stable. Furthermore setting the memory regions with k6wcx gave an additional boost in speed without issues! But more importantly the write test were correct in speedsys:

So it appears that the M5ALA sets the cache to write-through instead of the generally preferable write back! It also seems that write through doesn't agree with write combining.

 

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