Intel and AMD 486 subtle differences \ VOGONS

Intel and AMD 486 subtle differences

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First post, by MastErAldo

Posted on 2021-02-07, 13:19

MastErAldo Offline

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This post is intended to provide an answer to my older post regarding hangs on my 486 setup with mainly EGA games.
You can find the original topic here: Cirrus Logic GD5424 (maybe)timing problems on some games
But also to ask for opinions about what I discovered.

Just for recap, I have a 486 system and the problem was that every single EGA game randomly hangs within seconds after started (games like doom, descent, monkey island 2 and in general VGA games run fine). The system has the following specs:
Motherboard: Micronics Gemini VLB, like this one: https://i.ebayimg.com/images/g/YBkAAOSwdJtevmle/s-l400.jpg part number 09-00144
CPU: am486dx4 NV8T
Video: Cirrus Logic GD5428 VLB equipped with 1MB of RAM
RAM: 16mb FPM ECC ram 60 ns

I recently managed to get a "new" system and what is important about it is that is equipped with an Intel 486 DX2 66mhz (sSpec sx750).

I placed the intel DX2 on my original system and every single game worked flawlessly.
Just to be sure that it wasn't my old CPU fault I also tried to use an SV8B dx4 (the amd writeback cache model) and the problem was still there.

Since the old motherboard is a 5v only, the amd CPUs used a voltage adaptor which also lets you set te multiplier to x3 or x2, I though that maybe dx4 was just too fast.
So I switched to x2, every benchmark confirmed that the actual speed was 66mhz, but the games still hang.
As a counter test I used my amd chips on the new board (also equipped with the same VLB card i used in the original one) and every EGA game run flawlessly.

So I'm pretty sure now that my original system has some incompatibilities with AMD chips regardless of their speed/cache/multiplier.

I'm happy to have found a solution to a problem that, searching on the internet, may be not that common, but now, just out of curiosity I wonder if anyone has an idea about what can cause this problem.
I mean, I know that even though they are compabile chips they are made by 2 different companies and the later am486 models had to come with their own microcode, but are you aware of some subtle differences between these 2 chip models that can in some circustances, cause timing issues?
I though about a particular instruction that on amd chips completes a cycle before/after intel ones, but so far I have not found something to support my theory.

Reply 1 of 5, by bakemono

Posted on 2021-02-08, 06:35

bakemono Offline

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My guess is an incompatibility with the motherboard chipset. I have a socket 1 VLB board (SARC RC4018 chipset) which works fine with an Intel 486DX-33, but won't run stable with AMD DX4, 5x86, or Cyrix DX4 no matter what the clock speed or memory timings are.

again another retro game on itch: https://90soft90.itch.io/shmup-salad

Reply 2 of 5, by Cyberdyne

Posted on 2021-02-08, 08:25

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Yes, I also think that your motherboard just somehow does not like your AMD CPU, or it is flawed somehow. Because I have many AMD 486 and Intel 486 computers, and never noticed any incompatibility issues. Even with EGA and even with old CGA games.

Intel: 486SX25(many of them) usually run it at 33. 486DX33. 486DX2 66(many of them).
Amd: 486SX2 50 usually run it at 66. 486DX2 66(many of them, many versions 3V, WT, WB and so on.). 486 DX4 120. And somehow I usually run it at 100.... just my OCD. In reality it will happily run at 150MHz.

I am aroused about any X86 motherboard that has full functional ISA slot. I think i have problem. Not really into that original (Turbo) XT,286,386 and CGA/EGA stuff. So just a DOS nut.
PS. If I upload RAR, it is a 16-bit DOS RAR Version 2.50.

Reply 3 of 5, by douglar

Posted on 2021-02-08, 16:25

douglar Offline

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What is the date on your motherboard BIOS?

My experience is that it is difficult to find a motherboard with a BIOS earlier than October 1994 that correctly support the P24D processor family, whether it is AMD, Cyrix or Intel.

Sometimes the older BIOS can identify a P24D CPU, but the performance is usually lower than expected and or the board isn't completely stable. And in my limited experience, I've seen 3 boards that make me pump the reset button a couple times after applying power to get the board posts when using a 5x86 or DX4 CPU, but boot fine with a DX or DX2 CPU. I recently upgraded the bios on one of those boards and the issue seems to have gone away and CPU performance jumped by 15% before enabling the WB cache.

So I suspect that you have an old BIOS.

Reply 4 of 5, by Am386DX-40

Posted on 2021-02-08, 19:34

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I would point my fingers to the BIOS. The 486 cpus from intel and AMD are _identical_ in every respect, except as you correctly said, the microcode in the "N" AMD chips. What happens if you "cheat" your motherboard and setup your AMD processor as if it was an Intel one? Do the jumpers differ or the bios auto-detects which is which?

Reply 5 of 5, by MastErAldo

Posted on 2021-02-20, 23:31

MastErAldo Offline

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Posts: 5
Joined: 2020-08-03, 20:20
Location: Italy

Thanks for the responses.
As suggested, it would appear that in the end the culprit was a compatibility issue between the mobo, the cpu and/or the graphic card.

The motherboard and the bios are indeed way older than the first dx4.
I updated the bios to the latest available version and it reports "Phoenix 80486 bios plus 0.10 GLB05" 1985-1990.
The seller was kind enough to send me, the time I bought the mobo, a list of reports from users that had the same board and tried newer hardware.
One guy reported to make it work even with an amd 5x86 @133mhz.

But here's another new thing I discovered. I managed to get my hands on a Tseng ET4000/W32i VLB 1MB.
Now the setup with this old motherboard, a NV8T @ 100mhz and this graphic card let me play every single game I tried, even the EGA ones that previously always hanged.

Interestingly enough the NV8T inside a newer motherboard I have hangs if HIMEM is loaded while the dx2 doesn't give any problem.
So in all these trials and errors on chip/card swapping I figured out 2 things:
- it really seems that newer amd chips (at least the ones with amd's own microcode) behave differently than their intel counterparts on some very specific cases.
- My old motherboard is a little choosy on both CPUs and graphic cards, but in the end I managed to get a dx4 working with a nice vlb card for every game I like so I'm happy with that 😁

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