Chkcpu wrote on 2025-10-03, 20:39:

Ciao Elio, […]
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Ciao Elio,

I’m sorry to hear the 66MHz FSB trial didn’t work and the Camaro board still refuses to POST.

I made some progress analyzing this Compaq BIOS but POST step 2F has a lot of code and I haven’t figured out yet how it works.
Luckily, someone made a copy of this Compaq BIOS with Uniflash and posted it on TRW, so I have something to work with.

The top 16KB of this 256KB BIOS is the bootblock and contains readable code and data. The rest of the BIOS is not recognizable as code and is probably compressed or encrypted. I don’t recognize any known structures in this BIOS, so it is probably not AMI but Compaq’s own product.

All POST code up to step 2F is in the bootblock so I was able to disassemble that. When I follow the code from the start address to POST_2F, I see the follow POST steps in this order:
00, 08, 09, 01, 02, 03, 04, 05, 0D, 14, 15, 10, 11, 12, 13, 0E, 42, 43, 0F, 28, 2A, 2D, 42, 43, 2E, 2F.
This coincides nicely with what you found, only steps 28 and 2D are missing from your list.
In POST steps 42 and 43 the RAM is tested and all POST code up till step 0F runs from ROM and POST_0F copies the bootblock to RAM. Step 28 jumps to this code in RAM and from step 2D onwarts the POST runs from RAM, including a second call to steps 42 and 43 to test another part of memory. So also steps 2E and 2F run from RAM.
If one of the two steps 43 reports a memory error, the code jumps straight to POST 47 and halts, and never reaches steps 2E and 2F.

Because of this initial analysis, I agree that the RAM is fine and that the problem is either the BIOS or some hardware failure.
I haven’t found any checksum code yet and I don’t know if this BIOS has a recovery mode, so shorting 2 address lines on the BIOS chip to trigger a BIOS checksum error may not work on this BIOS.

I will study the POST_2F code further and hope to find a clue there.
Cheers, Jan

Ciao Jan
Thank you very much for your analysis on the BIOS, and in fact as I thought, the first part of this Compaq BIOS is working, at least to the point where it detects the RAM, here maybe I can try to see the post codes, starting without RAM, in this case it stops at 47, this should confirm the sequence obtained from both, even if in my list, there are not those two codes that you reported, or it does not show them (I don't know why), or it is possible that when pressing the button, it inadvertently pressed a second time (?).
On the origin of this BIOS, that is, if it is a modified AMI, this seemed very likely to me, but looking at what 😨 they correspond to, they seem more likely to those of the link (put in my last comment), so maybe as you rightly suggest, it could be a BIOS made entirely by Compaq, maybe using an AMI inspiration scheme, so maybe the 2F is in fact an error, corresponding to "Write to diagnostic byte", unfortunately I still don't understand what it means.
So the problem of the Recovery BIOS would remain, if it is as you say, that is, that there may not be a Recovery BIOS, if it is forced by joining the two corresponding PINs of the BIOS chip, it would be a big problem, because there would be nothing else to do, in fact perhaps the only solution would be to remove the chip (with the welder), after adding a socket on the MB, reprogram and try again.
Last chance, it would be perhaps 🤔 the easiest, if there was an extension cable, for 8 PIN chips there are tweezers, which attach to the welded chip, for PLCC32 chips I don't think there is anything like that, it seems to me that the only solution would be to build it yourself, and then connect the programmer, I think the battery 🔋 CR2032 of the motherboard should be removed, I don't know the reason why you can't find anything like that, maybe for those who deal with reprogramming the BIOS chips, it's not complicated to remove and replace the chip (welding it), but I don't think that in a service center, they do it on purpose to make the restoration uneconomical, maybe the reprogramming could be done using something else (?), I don't think there is a special port like SPI ASUS, or who knows if Compaq used USB, or even the Serials or the Parallel.

Doing some research 🔍, I found the PC it should come from, it would be a Compaq PC, model Presario 7350, the 7350 had AMD K6-2 475 MHz CPU, 40 GB disk 🇬🇧, and 64 MB of RAM, for the rest having both audio and video integrated, no PCI cards were needed, in fact the slots were free, although I believe that at least one was occupied by an internal modem (PCI), but I'm not sure 🤔, because there were external modems, and that PC was used to connect to the internet, yes today it seems a bit strange, but it was a PC to allow entry to the web, maybe it was a cheap PC (?), but there were other faster models, the 7360 for example was a 500 MHz, while the 7370 was a 533 MHz, the latter would be the upgrade I had in mind.
Unfortunately 😣 as seen, until now, even changing CPU, RAM or jumper position, the PC does not go beyond the Post Code 2F 2E, so I would rule out a problem due to a defective CPU, also because usually only — —, so no post code flows.
Even if I find the pdf manual, I don't think I would find useful indications for the resolution of this problem, although there are usually some solutions to simple startup problems, this case I think it would be a problem to solve by taking the PC to service.
Looking at the BIOS screens, it looks similar to that of my AST Bravo Pro (Pentium Pro 200), and I think it is based on the Phoenix BIOS, which was used in Intel production motherboards, but I don't know if it actually is, or if it is just similar to the view, while the post codes could refer to something else.

As Socket 7, in addition to the Compaq Camaro, I have on the bench a similar PCChips M560 Rev:4.1A, the motherboard is working, except for the L2 cache on board, which should be 512 KB, there are two UT6164C32Q-6 chips, but there are no jumpers to select the L2 cache, it means that there is a bridge, and the selection is fixed.
There is an RTC chip VIA VT82885N, which must be fixed, the battery is missing 🪫, for this reason, turning it off it does not store the BIOS settings and the changes, fixing it is relatively simple, but I don't think the L2 cache is not working, it depends on the lack of the battery in the RTC chip, it is a problem that I imagine is due to something else, unfortunately the cache chips are welded, and it is not possible to exclude one or the other, to see if maybe 🤔 one of the two works, in that case you would have 256 KB, which anyway are always better than 0 KB.
The BIOS is an AMI, Relase 11/26/1997S, and the chipset is rebranded as TX PRO, in practice it should be an ALI M1531, maybe 🤔 not the best for this kind of MB, but anyway at most the FSB reaches 83 MHz, but if you settle for 400 MHz with FSB 66, and if you have a CPU with integrated L2 cache, the on-board one of 512 KB becomes L3, in this case the performance with L3 cache of 512 KB or with 0 KB, should be quite similar, if not even the same.
Currently with the cache enabled I read on the post card PCI, the 71 AC codes, just out of curiosity, I could write down the previous codes, of course I already know that the problem is due to the on-board L2 cache (defective?), I have the same problem on another PCChips M549, initially I would suspect L2 cache chips, one or both could be not working, but sometimes there could be a different problem, such as a non-powered pin, for an interrupted track or a faulty SMD component, maybe first it would be the case to do some checks, because to manage to make the L2 cache work, it will work best, I have also seen some that replace the chips with others of double capacity, in this case getting 1 MB of L2 cache, honestly I don't know if this requires the movement of the bridge, in the missing jumper area to select the amount of cache.