The 5x86 was always intended to scale to 200mhz, if AMD would not have gotten the K6 they may have officially started binning for them as the k5 just couldn’t scale fast enough. (Back in ye olde times I was told 1 out of 70 5x86 dies would hit 200mhz at stock voltage)

Outside the US a limited number of 5x86-150 and 5x86-160 escaped into the wild, AMD didn’t want to detract from k5/k6 sales so they left everything capped at 133mhz

I doubt the processor is running at 200 MHz if the bus is 40 MHz. If the board doesn't recognize the processor, it may display the frequency incorrectly. Another possible scenario is when the board doesn't recognize the processor, it may disable the L1 cache (for example, PC Chips M918), and with L1 disabled, many Am5x86 processors can boot at 200 MHz.

Benchmarks, or its a bios bug

brotherdg2@gmail.com wrote on 2026-07-03, 10:02:

I doubt the processor is running at 200 MHz if the bus is 40 MHz. If the board doesn't recognize the processor, it may display the frequency incorrectly. Another possible scenario is when the board doesn't recognize the processor, it may disable the L1 cache (for example, PC Chips M918), and with L1 disabled, many Am5x86 processors can boot at 200 MHz.

Yeah, it is strange. There is something with the BIOS or multiplier.
It runs at 133MHz when set to 25MHz bus.
It could (maybe?) be a multiplier issue. When set to 25MHz it runs at 33MHz and when set to 40MHz it runs at 50MHz.

Right now I tend to believe it's a BIOS bug, since the BIOS date precedes the CPU release date.
I dunno, will get a different BIOS and will check again.

myne wrote on 2026-07-03, 14:10:

Benchmarks, or its a bios bug

That is what I want to do by acquiring a different & newer BIOS.
With this one I can't boot the Am5x86-P75-S CPU even at stock settings.
AMD 486 DX2-66 boots fine.

Just run a benchmark.
Results are the truth. Numbers on a bios screen were calculated or hard coded by people who make typos and silly assumptions.

Performance will either scale or it won't.
Trust the results.

All I found is an unsupported BIOS for the Am5x86-P75-S CPU... oh well... no unicorn for me Today. 😀
Newer BIOS arrived dated 1997, everything works fine and boots fine. CPU jumpered to 25MHz bus boots at 100MHz and so on, up till 160MHz.
200MHz does not boot.

The first sign was that the CPU would not boot DOS boot disk or Windows boot disk at all at default speed.
The second sign was that the MB was jumpered to 25MHz bus and the CPU booted with 133MHz, which is impossible.

Yes!!!
486@200MHz and stable.

Had five AM5x86-133 CPUs.
This time it's a new! Soyo SY-4SAW. BIOS date: 1997. (killed the last one and had heavy acid damage)
At default 3,45V out of the 5 CPUs; 1 complained (BIOS beep) of memory missing(?). The other 4 where not doing anything at 50x4.
Took the one that gave BIOS beep messages and raised the voltage to 4V (or 3,6V not exactly sure: moved the JP39 from 3-4 to 5-6).
POST and boot!
Picky on memory modules, out of 10 it POST-ed with 3 and was stable with 1 or 2. The modules that I have are not the fastest.
Cooler: Socket A cooler with cooper core sitting right in the middle of the 5x86 CPU. (put some thermal grease in the middle and some super glue on the 4 corners, not too much. Left it with a 10Kg weight for 1 hour "to bake" 😁)
Temperature: To the touch it feels "normal", the cooler is not even warm. Tried to touch near the CPU: same.

I started naming the special PCs in my collection, this one is the: "The Monster!" 😀

Pictures:

Benchmarks:

On my board the JP15 is missing by default.
Because of this when IDE is connected to the mainboard, it fails to POST.
Using an ISA HDD and FDD controller.

Happy that the video card works with a PCI@50MHz... (the bus is set to 50MHz and since no JP15 I can't halve the PCI speed).

Update: I have artifacts in both Doom and Quake. It's runs stable; no freeze, crash or restart.

JAKra85 wrote on 2026-07-02, 20:35:

200MHz - POST! - The strangest thing is that it does this with the 40MHz bus setting.

I don't see how this can work, AFAIK x4 is the maximum multiplier and I assumed to get 200 MHz a 50MHz FSB would be needed, please can you enlighten me?

I have several CPUs that will when overclocked post, but crash when I try to start Windows NT V4.00 SP6a, which OS will boot/run @ 200 MHz? My AMD Am5x86-WT (133ADZ) happily runs @ 160MHz, I can't try 200 MHz as the maximum FSB my MB allows is 40 MHz.

I wonder what my SIV utility will report when the CPU is @ 200 MHz. Use SIV32L on W9x and SIV32N on NT4.

red-ray wrote on Yesterday, 22:07:

I don't see how this can work, AFAIK x4 is the maximum multiplier and I assumed to get 200 MHz a 50MHz FSB would be needed, plea […]
Show full quote

JAKra85 wrote on 2026-07-02, 20:35:

200MHz - POST! - The strangest thing is that it does this with the 40MHz bus setting.

I don't see how this can work, AFAIK x4 is the maximum multiplier and I assumed to get 200 MHz a 50MHz FSB would be needed, please can you enlighten me?

I have several CPUs that will when overclocked post, but crash when I try to start Windows NT V4.00 SP6a, which OS will boot/run @ 200 MHz? My AMD Am5x86-WT (133ADZ) happily runs @ 160MHz, I can't try 200 MHz as the maximum FSB my MB allows is 40 MHz.

I wonder what my SIV utility will report when the CPU is @ 200 MHz. Use SIV32L on W9x and SIV32N on NT4.

I have CHKCPU image added.
Yes, bus is set to 50. The CPU runs @ 4x50.

EDIT:
"200MHz - POST! - The strangest thing is that it does this with the 40MHz bus setting."
That is the old board with an old BIOS! That BIOS was before Am5x86-133 was released. Please disregard. 😀
The post I made Today is the relevant one:
Re: I think I found a unicorn! Am5x86-P75-S @ 200MHz @ 3,45V (Edit:Now I really did!)

Doom and Quake
(8MB RAM)

As stated before the PCI is running at 50MHz and getting artifacts in games.

Doom:
On min. details: 337FPS
On max. details: 128FPS

Quake timedemo: 17,8FPS

red-ray wrote on Yesterday, 22:07:

I don't see how this can work, AFAIK x4 is the maximum multiplier and I assumed to get 200 MHz a 50MHz FSB would be needed, plea […]
Show full quote

JAKra85 wrote on 2026-07-02, 20:35:

200MHz - POST! - The strangest thing is that it does this with the 40MHz bus setting.

I don't see how this can work, AFAIK x4 is the maximum multiplier and I assumed to get 200 MHz a 50MHz FSB would be needed, please can you enlighten me?

I have several CPUs that will when overclocked post, but crash when I try to start Windows NT V4.00 SP6a, which OS will boot/run @ 200 MHz? My AMD Am5x86-WT (133ADZ) happily runs @ 160MHz, I can't try 200 MHz as the maximum FSB my MB allows is 40 MHz.

I wonder what my SIV utility will report when the CPU is @ 200 MHz. Use SIV32L on W9x and SIV32N on NT4.

I just built it Today. Only booting to MS-DOS 6.22 from floppy and running games and benchmarks from DVD drive.
I will check installing Win98 once I get rid of the artifacts.
Will check a couple of PCI videocards that I have.
Also, need more and better RAM. Currently only 8MB.

I have these CPUs:
2-133ADW
1-133ADY
2-133ADZ
The one that is running @200MHz is an ADW CPU, the one in the picture.
I thought it will be one of the ADZ parts that better overclock. I was wrong. 😀

I also choose this mainboard because of the 50MHz BUS.

JAKra85 wrote on 2026-07-02, 20:35:

200MHz - POST! - The strangest thing is that it does this with the 40MHz bus setting.

I would edit the original post.

JAKra85 wrote on Yesterday, 22:54:

I will check installing Win98 once I get rid of the artefacts.

I also choose this mainboard because of the 50MHz BUS.

OK, I just checked and was surprised your MB supports 256MB. Does it have a 256KB or 512KB cache and what is the maximum cacheable memory size?

After a Google it seems to be:

256 KB L2 Cache: Can cache up to 64 MB of main memory.
512 KB L2 Cache: Can cache up to 128 MB of main memory.

I keep looking for such as https://theretroweb.com/motherboards/s/fic-486-vip-io2 which supports PCI with 25+33+40+50MHz FSB and PS/2 KB + PS/2 Mouse

red-ray wrote on Today, 06:52:

I would edit the original post.

Done, thanks!

red-ray wrote on Today, 06:52:

Does it have a 256KB or 512KB cache and what is the maximum cacheable memory size?

Currently using Winbond cache chips - 256K. The TAG cache (Winbond) is 12ns the rest are 15ns.
I have and tested ISSI cache chips - 512K. The TAG cache (ISSI) is 12ns the rest are 15ns. Would POST, but froze before boot. I double checked all ISSI chips one-by-one. Whatever I've done had no effect. It's either multiple chips are bad (e-bay chips, so very likely) or this board does not like ISSI cache chips.

red-ray wrote on Today, 06:52:

I keep looking for such as https://theretroweb.com/motherboards/s/fic-486-vip-io2 which supports PCI with 25+33+40+50MHz FSB and PS/2 KB + PS/2 Mouse

I don't really like the layout of this board. Also, did you consider USB? 😜
I am using USB driver stacks and USB mouse driver to natively use USB optical mouse in DOS.
Have multiple PCI (VIA chip based) USB cards and all work nicely. (not in this PC! I don't want to mess with 50MHz PCI...)

JAKra85 wrote on Today, 07:18:

red-ray wrote on Today, 06:52:

I keep looking for such as https://theretroweb.com/motherboards/s/fic-486-vip-io2 which supports PCI with 25+33+40+50MHz FSB and PS/2 KB + PS/2 Mouse

I don't really like the layout of this board. Also, did you consider USB? 😜

Yes and no, I would install Windows NT V4.00 SP6a on it and as it doesn't by default support USB I have no real need for USB.

In order to run the MSVC 6 remote debugger I need to have all my systems networked so I don't need to transfer files via USB.

Aren't the Matrox Milleniums in the better end of high PCI bus compatibility?

H3nrik V! wrote on 44 minutes ago:

Aren't the Matrox Milleniums in the better end of high PCI bus compatibility?

I don't have that, however I am not done tweaking it.
Managed to get the ISSI 512K cache working, by setting the VGA to a different PCI slot. Stange, but it works.
Then I changed the VGA to ATi 3D Rage IIC - PCI.
No more corruption! - Yay!

Quake demo is currently at: 18.6FPS.

I think this is the max. I can do with these RAMs.