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Cachechk is slow only at 1MB on PCChips M912. What's that about?

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First post, by danieljm

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Hey all,

I've got a PCChips M912 and Cachechk seems to be slow only at 1MB. I'd love some help trying to figure out what's going on. Here's a pic.

The attachment cachechk.JPEG is no longer available

The rest of the system is an AMD 5x86-133, 1 stick of 16MB FPM RAM, and 256K of real cache chips. See pic below if you like.

I've tried fiddling with BIOS settings, but I haven't found anything that helps.

Could it maybe be a bad cache chip? Not sure if that makes any sense, but I haven't got any better ideas.

And here's the output of CTCM if that helps with anything.

The attachment ctcm.JPEG is no longer available
The attachment chips.JPEG is no longer available

Reply 1 of 4, by Deunan

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That is odd, even the on-die cache is not working and the access is slower than pure RAM cycle. What cards are installed? Also try with different RAM stick, smaller - 4 or 8 MiB.
Was this test done with HIMEM loaded? Does its presence or absence change anything? If so perhaps there is something weird going on with A20 gating.

Reply 2 of 4, by bakemono

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Deunan wrote on 2024-12-15, 13:05:

Was this test done with HIMEM loaded? Does its presence or absence change anything? If so perhaps there is something weird going on with A20 gating.

This. I have seen this exact weirdness before on this motherboard.

GBAJAM 2024 submission on itch: https://90soft90.itch.io/wreckage

Reply 3 of 4, by danieljm

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Thanks for the suggestions. I was running Cachechk without HIMEM loaded. When I ran it with HIMEM, it seemingly just skipped over 1MB. So 0, 2, 3, 4, etc... Very weird.

I tried different RAM in different slots, all different cards and storage. Nothing changed.

HOWEVER, then I tried changing write back to write thru in the BIOS and it fixed the problem. But oddly, both CHKCPU and CTCM report that write back is still enabled. Maybe I don't understand how that setting is supposed to work, but that doesn't make a lot of sense to me. Did some testing and the system seems stable. Doom benchmark shows similar or maybe very slightly higher fps.

Anyway, now I'm wondering if I have an old or maybe corrupted BIOS and need to flash a new one.

Reply 4 of 4, by rasz_pl

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1 megabyte with no L1 cache does look like BIOS misconfiguring chipset, setting that whole memory region as non-cacheable.

https://github.com/raszpl/FIC-486-GAC-2-Cache-Module for AT&T Globalyst
https://github.com/raszpl/386RC-16 memory board
https://github.com/raszpl/440BX Reference Design adapted to Kicad
https://github.com/raszpl/Zenith_ZBIOS MFM-300 Monitor

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