Hi JAKra85,

Your Am5x86 issue can very well be caused by the ‘B8’ BIOS. If this is an Award BIOS from 10/1995 or earlier, it won’t have proper Am5x86 support.

What do you see as detected CPU on the BIOS boot screen? When the Am5x86 is supported, the Award BIOS should indicate “Am5x86-P75-S” when the multiplier is set to x4 mode, and “Enhanced Am486DX4-S” when the x3 mode is set.
If you don’t see Am5x86-P75, you need to upgrade the BIOS. All three 1996 BIOSes (C5, D2, and F2) at TRW support the Am5x86.

Cheers, Jan

Chkcpu wrote on 2026-04-28, 12:45:

Hi JAKra85, […]
Show full quote

Hi JAKra85,

Your Am5x86 issue can very well be caused by the ‘B8’ BIOS. If this is an Award BIOS from 10/1995 or earlier, it won’t have proper Am5x86 support.

What do you see as detected CPU on the BIOS boot screen? When the Am5x86 is supported, the Award BIOS should indicate “Am5x86-P75-S” when the multiplier is set to x4 mode, and “Enhanced Am486DX4-S” when the x3 mode is set.
If you don’t see Am5x86-P75, you need to upgrade the BIOS. All three 1996 BIOSes (C5, D2, and F2) at TRW support the Am5x86.

Cheers, Jan

Hello Jan,

Thanks for the confirmation! I was thinking the same thing!

“Am5x86-P75-S” when the multiplier is set to x4 mode, and “Enhanced Am486DX4-S” when the x3 mode is set.

- Correct, this is now it is reported.

Yesterday I requested a friend of mine to write me the F2 BIOS and send it to me.
In my opinion this old B8 BIOS does not handle the x4 multiplier well with this CPU, it's unstable.

I've tested with two CPUs and here is my conclusion:
CPU1
-Am5x86-P75-S (160MHz - 40x4) booting issue and unstable. (Reported as Am5x86-P75-S )
-Am5x86-P75-S (133MHz - 33x4) booting issue and unstable. (Reported as Am5x86-P75-S )
-Am5x86-P75-S (100MHz - 33x3) NO more booting issues, boots every time and stable. (Reported as Enhanced Am486DX4-S)
(I could not set this CPU to 120MHz to test that too, its 100 or 133.)
CPU2
-AMD 486DX4 (120MHz - 40x3) NO booting issues, boots every time and stable.
-AMD 486DX4 (100Mhz - 33x3) NO booting issues, boots every time and stable.

Will update this topic in the coming weeks with the results.

Kind Regards,
Attila

This board is very capricious when dealing with Am5x86-P75-S (133MHz) and (ATi) PCI based videocard!

I now have 3 boards:
ABit AB-PB4 - Rev 1.3 (1995 - B8 - BIOS)
ABit AB-PB4 - Rev 1.3 (1995 - 5E - BIOS)
ABit AB-PB4 - Rev 1.5 (1996 - D2 - BIOS)

The Rev 1.3 behave very similarly. Regardless of the BIOS version.
I do not recommend running any CPU above 486-DX4, or run the AMD X5-133 CPU at 100MHz (33x3). (no issues with F2 BIOS)

Rev 1.5 runs very stable with Am5x86-P75-S (133MHz) and PCI video.
There are still issues with some ATi PCI videocards!
Multiple ISA based card - no issues, boots all the time.
ATi Mach64 VT - sometimes (rarely) it boots, when it does it is stable.
ATi Mach64 GT (3D Rage) - no issues, boots all the time.
ATi Mach64 GT2 (3D Rage II) - no issues, boots all the time.

Also, the Rev 1.5 board supports 50Mhz base clock.
Runs stable at 150 MHz (3x50!) with ATi Mach64 GT2. 😉

The latest BIOS should arrive in a few days (1996 - F2). I will do more tests write the results here for future enthusiasts and "close the case".

Hi Attila,

Thanks for the update.
As both v1. 3 boards still have an older BIOS version, it will be interesting to see if the F2 BIOS will fix the stability issues on these boards.

Looking forward to your next test results.

Cheers, Jan

With the latest BIOS available there are no more PCI and booting issues when using Am5x86-P75-S CPU! 😀
BIOS:07/03/96-ALI-1487/89-2A4KDA1BC-F2
Double and triple tested on all motherboards Rev 1.3.
Rev 1.5 board died a few weeks ago when playing with OC... TIP127 transistor on it's way to revive it.

As a side note: 512K cache with (4 x IS61C1024-15N) is not supported even on the latest BIOS. Boots and hangs after memory count.

Hi Attila,

Thanks for reporting back and good to hear that updating to the F2 BIOS fixed your issues on the Rev 1.3 boards!
I hope you can revive that Rev 1.5 board as well.

All the best, Jan

Chkcpu wrote on 2026-06-14, 11:39:

Hi Attila, […]
Show full quote

Hi Attila,

Thanks for reporting back and good to hear that updating to the F2 BIOS fixed your issues on the Rev 1.3 boards!
I hope you can revive that Rev 1.5 board as well.

All the best, Jan

Hello Jan,

Rev 1.5 board is alive again, working nicely.
No more fooling around... stock 133 is perfect.
Will send this v1.5 MB to TheRetroWeb, only Rev 1.3 has a picture.

With Kind Regards,
Attila