That's interesting. I figure it is also possible to use a ThrottleBlaster with this setup. I was looking up the Pentium 4 pinout and came across the STPCLK pin.. If this works, your build would be able to probably span the widest era of retro PCs I've ever seen (XT to XP 😅).

I find it interesting that the ISA card isn't much slower than the PCI/AGP ones. In my testing on 100Mhz Pentium MMX, inspired by PhilsComputerLab's 136 in 1 project, the ISA card was quite a bit slower, at all CPU speeds:

I think the above is using 3DBench 2

wbahnassi wrote on 2026-04-17, 08:54:

That's interesting. I figure it is also possible to use a ThrottleBlaster with this setup. I was looking up the Pentium 4 pinout and came across the STPCLK pin.. If this works, your build would be able to probably span the widest era of retro PCs I've ever seen (XT to XP 😅).

I hate to disappoint you, but throttling via STPCLK pin has the same drawback as ODCM: it reduces CPU performance but also restricts access to peripherals like graphics cards.
If you throttle to levels comparable to a 386 CPU, access to the VGA framebuffer becomes so slow that it causes stuttering and other undesirable effects in games.

Shponglefan wrote on 2026-04-16, 18:38:

Lately I’ve been experimenting with various Pentium 4 hardware combos and throttling capabilities. This is largely due to the re […]
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Lately I’ve been experimenting with various Pentium 4 hardware combos and throttling capabilities. This is largely due to the recently released P4Tool by OttoPS available here: Re: [b]p4tool - performance control utility for Pentium 4 / NetBurst[/b]

I’ve been wondering how slow a Pentium 4 can be throttled without relying on ACPI or ODCM throttling.

In testing with my i865 and Pentium 4 651 HT (Cedar Mill) setup, I could get down to speeds approximating a slow 486. I really want to see if it's possible to hit 386 speeds ideally the equivalent of a DX-25 or DX-33.

To that end, I’m testing a recently revived DFI ITOX G4E620-N motherboard with a 2.0GHz Pentium 4 (Northwood) processor. The nice thing about this board is it features both a working AGP x4 slot and three ISA slots with DMA support.

The attachment DFI ITOX G4E620-N motherboard testbench.jpg is no longer available

I was also inspired by Phil’s 136-in-1 Pentium MMX project, where he mentions using ISA cards to achieve slower performance levels. So I decided to try ISA VGA cards on this board to see how slow it can go, and how it compares to AGP or PCI cards.

One of the challenges with using an ISA video card is they tend to be the default video option, overriding either AGP or PCI. However, after reading about potential ways to disable that using jumpers, I discovered one of my cards, a Diamond Speedstar VGA (Tseng ET4000AX)) could be prevented from loading its video BIOS by removing the BIOS timing jumper.

On that note, I’ve been testing a triple GPU setup using a GeForce 4 MX440 SE (AGP), Voodoo3 3000 (PCI), and Diamond Speedstar VGA (ISA) concurrently.

The attachment DFI ITOX G4E620-N triple GPU setup.jpg is no longer available

In terms of performance, the ISA VGA card has a lower top end for performance compared to the AGP card. As the CPU is throttled to lower levels of performance, the gap narrows. At the lowest level of performance, there is only a 0.1 framerate (3%) difference in the PC Player benchmark.

The attachment Pentium 4 2.0GHz (Northwood) - PC Player Bench 320x200 AGP vs ISA.png is no longer available

I also tested a variety of ISA cards and a few PCI cards with the CPU highly throttled. For the most part everything was within a few percent of each other. The only exception was the Oak VGA 037c card.

The attachment Pentium 4 2.0 GHz (Northwood)_ PC Player Bench 320x200 AGP PCI ISA.png is no longer available

One issue I ran into was using sound cards at the same time as the ISA video card. I tested three different YMF-71x cards. In playing Decent, two of them stopped playing digital audio and would get stuck on GM playback. Only one of the three would playback properly. All three cards worked fine with the AGP video card.

I also tried an Orpheus I and AWE64 sound card. Both of these seemed to work fine alongside the ISA video card. But I will need to do a lot more testing to see if I run into any issues.

Overall, I don’t know how practical a setup this is. At the lowest levels of throttling, there isn’t much difference between AGP, PCI and most of the ISA video cards. I still need to test out a variety of games to see how well things performed throttled and in conjunction with ISA video and sound.

I further want to test 1.6GHz and 1.8GHz P4 processors to see how they compare to the 2.0GHz Northwood, though I'll need to obtain those processors first.

Very nice retro battle station! I think the DFI ITOX G4E620-N motherboard is the best option to cover everything from early DOS to early Windows XP.

I'm experimenting with a new dsbts option in the p4tool utility that might help you. The idea is to use an invalid, non-aligned address as a pointer to ds_area. Somehow, the microcode doesn't like this, or it generates stalls that further reduce performance compared to dsbts alone.

You can try it with this command line: "p4tool.com cd dsbts idsa"

This is the new version of the tool:

wbahnassi wrote on 2026-04-17, 08:54:

That's interesting. I figure it is also possible to use a ThrottleBlaster with this setup. I was looking up the Pentium 4 pinout and came across the STPCLK pin.. If this works, your build would be able to probably span the widest era of retro PCs I've ever seen (XT to XP 😅).

ThrottleBlaster would be an option. I was also considering building one for my other LGA775 Pentium 4 system, though I'm not sure how I would rig it up given the LGA775 socket configuration.

That said, as OttoPS pointed out, ODCM and ACPI (Southbridge) throttling basically do the same thing. They all introduce CPU halt states via the STPCLK pin. Just ODCM and ACPI do it at predefined frequencies, versus ThrottleBlaster which is tunable.

So it is possible to drive the P4 a lot slower with ODCM and ACPI throttling, I just wanted to see what is possible without using those options.

RetroPCCupboard wrote on 2026-04-17, 09:18:

I find it interesting that the ISA card isn't much slower than the PCI/AGP ones. In my testing on 100Mhz Pentium MMX, inspired by PhilsComputerLab's 136 in 1 project, the ISA card was quite a bit slower, at all CPU speeds:

The attachment 20260417_100356.jpg is no longer available

I think the above is using 3DBench 2

I did testing with 3D Bench 2 (1.0c) as well. There is more of a spread between the scores compared to PC Player bench.

At the slowest performance, I get 9.1 FPS with the ISA card versus 10.1 with AGP.

However I find 3D Bench seems to generate scores that are lower than indicative of relative performance, at least when throttled. So I've been using PC Player Bench instead.

OttoPS wrote on 2026-04-18, 16:09:

Very nice retro battle station! I think the DFI ITOX G4E620-N motherboard is the best option to cover everything from early DOS […]
Show full quote

Very nice retro battle station! I think the DFI ITOX G4E620-N motherboard is the best option to cover everything from early DOS to early Windows XP.

I'm experimenting with a new dsbts option in the p4tool utility that might help you. The idea is to use an invalid, non-aligned address as a pointer to ds_area. Somehow, the microcode doesn't like this, or it generates stalls that further reduce performance compared to dsbts alone.

You can try it with this command line: "p4tool.com cd dsbts idsa"

This is the new version of the tool:

The attachment p4tool.zip is no longer available

Awesome, I'll test this out! More throttling is good. 😁

OttoPS wrote on 2026-04-18, 15:56:

I hate to disappoint you, but throttling via STPCLK pin has the same drawback as ODCM: it reduces CPU performance but also restricts access to peripherals like graphics cards.
If you throttle to levels comparable to a 386 CPU, access to the VGA framebuffer becomes so slow that it causes stuttering and other undesirable effects in games.

Interesting. Is this a P4-only thing? Because I am using a ThrottleBlaster on a P2 and it doesn't cause that behavior. At least I haven't seen any stuttering on games like Test Drive 3 or the Sierra games when throttling down to 25MHz.

wbahnassi wrote on 2026-04-18, 16:35:

Interesting. Is this a P4-only thing? Because I am using a ThrottleBlaster on a P2 and it doesn't cause that behavior. At least I haven't seen any stuttering on games like Test Drive 3 or the Sierra games when throttling down to 25MHz.

It might be game specific.

I have a copy of Police Quest 2 that has a speed sensitive intro. With ODCM or ACPI throttling, the intro can run at speeds similar to my 12MHz 286 with animation and music playing back correctly.

I've also used ODCM throttling to slow down Decent to Pentium levels of performance and it seems okay from what I've played.

On the other hand, with Ultima VII I ran into an issue with the screen shake effect used in the earthquake at the beginning of the game. With ODCM throttling it causes screen redraw and music to slow down during that specific sequence.