Chkcpu wrote on 2025-02-27, 19:55:

Hi evanboonie, […]
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Hi evanboonie,

That is quite a challenge you have there, getting a K6-2 to run on the Apple 1997 PC Compatibility Card!

Looking into the AMI BIOS9.ROM, I see this card uses the OPTi 557M Viper-M chipset. This is a 1995 chipset for socket 5 boards. But the BIOS9.ROM looks like it is from 1997 and it has the expected CPU support from that era: Intel Pentium and Pentium MMX, Cyrix 6x86 and 6x86MX, and AMD K5 and K6. So no support for any IDT WinChip.

The K6 support in this BIOS is only for the original K6 model 6, the 2.9V/3.2V Vcore models. The 2.2V K6 model 7 and all later K6(-2/III) models are not supported.

If your K6-233 is a 3.2V model 6, it is supported by the BIOS. But the BIOS also contains an Apple Compatibility BIOS v1.6.4 module that may prevent other than the original Pentium from running.
Although I’m just guessing here, it would be interesting to see if a Pentium MMX, Cx686MX, or AMD K5 work on this card. If you have one of these CPUs, please try.

Another issue may be the power supply. The K6-233 draws a lot more current than a Pentium 166/200. If you can lower the multiplier, you could test the K6-233 at 200 or 166MHz. At these speeds, this CPU will run fine at a lower 2.9V Vcore.

Now to answer your question, with basic K6 support it should be possible to add K6-2 support to this BIOS. But this is a lot of work, so please try the above CPU alternates first, so I know that the time spent on patching this BIOS is not waisted. 😉

Cheers, Jan

Jan, I have finally gotten around to testing the K6-233 again with an externally powered voltage adapter and I'm pleased to report that it worked! So it does appear that the K6 was just a little too power hungry for the built-in regulator. I also realized that I gave you the modded BIOS file (BIOS9.ROM) which I had attempted to replace the VBIOS module using amibcp. It is likely best to start any CPU additions on the base BIOS and I should be able to easily add my VBIOS and second cache bank mods on top of that if it works. The original BIOS I have attached here (BIOS 8.bios) is the file exactly as I extracted it from the Mac application.

Sphere478 wrote on 2025-10-28, 02:52:

If there was PCI I would say to try to make an adapter to a radeon 9250. But with ISA, the onboard would be better.

You may be able to boge wire attach a breakout for pci still though if you tap into enough of the legs, pins, and traces.

The data book for the chipset is archived at the retro web: https://theretroweb.com/chipset/documentation … b7863333286.pdf. According to it, the PCI signals come from the 82C557M and 82C558M chips and are mostly isolated to one corner of each. I'm thinking something like one of those "quick solder" FPC boards I've seen in console modding kits could be an option to quickly tap off those signals and send them somewhere else. A PCI to PCI adapter card could then be designed to get power from the Mac's PCI bus and the rest of the lines from the FPC. It would only allow for low-profile cards, but it should work... Excessive? Maybe. But possible, almost certainly.

This card has kept me busier than any piece of retro gear I've had before!

Yes, you and I are on the same page with that idea. I had the exact same concept, churning around in my brain

I think some of the PCI configuration though might be in the bios so some bios mods may also be required

Sphere478 wrote on Yesterday, 02:02:

I think some of the PCI configuration though might be in the bios so some bios mods may also be required

Thankfully, amibcp works on this BIOS and allows access to most of the registers and embedded device configurations. I have a third mod going where I'm attempting to stack cache chips to give a dedicated dirty bit and two banks for a total of 512k of L2 cache. The default configuration has combined dirty and tag bits with a single bank for 256k of cache. It is hard coded to write-back and the chipset only supports 32MB of cache-able memory in that configuration. The cards support up to 80MB of RAM though, which I should be able to have all cache-able if the mod succeeds. I hope to try even faster chips and see if timings can be improved too. I was able to easily compare the registers in the databook with their configurations using amibcp; which is especially nice because getting into a BIOS menu doesn't seem possible on these cards, much less saving the configuration.