In the bios there should be an option to set hypertransport speed. On S939 Athlon64 it's 1000mhz by default, and is usually set by a combination of base clock 200 and multiplier 5. The cpu speed can not be lower than the hypertransport speed. So to run the cpu at 800mhz you need to find the bios option for the hypertransport multiplier and set it to 4.

pixel_workbench wrote on 2022-08-14, 14:52:

In the bios there should be an option to set hypertransport speed. On S939 Athlon64 it's 1000mhz by default, and is usually set by a combination of base clock 200 and multiplier 5. The cpu speed can not be lower than the hypertransport speed. So to run the cpu at 800mhz you need to find the bios option for the hypertransport multiplier and set it to 4.

Sorry, I somehow missed the reply notification for your post. I set the hypertransport to 800 MHz and it works, thank you very much! I can finally underclock the CPU to 800 MHz.

Since 4x is the lowest multiplier for Athlon 64, this is the lowest I could go with the default system clock of 200 MHz, which sadly cannot be underclocked on my board. If anyone had success with this, please post your board model and the tools you have used.

The Abit AV8 can underclock very well. I was using SysTool
https://www.techpowerup.com/systool/

Board will lock up in Windows when clocking below 200-MHz. You have to reboot but the FSB will stay below 200-MHz

https://valid.x86.fr/vy9hfi

Yes, I've had the same lockup when I tried to underclock the bus in Windows. Interesting that it persists with a reboot, I think I haven't tried that. I was wondering if there's a DOS tool that can underclock the CPU, maybe this way it wouldn't lock up. There's an open source library called WinRing0 that some of these tools use to write into the registers, I've had a look at it to see if it could be ported to DOS. I also have an Abit AV8, but I never liked it much because it does not allow to undervolt the CPU.

asdf53 wrote on 2022-09-23, 20:33:

...There's an open source library called WinRing0 that some of these tools use to write into the registers, I've had a look at it to see if it could be ported to DOS.

In DOS real mode you are already in Ring 0 (when no EMM386 like memory manager is loaded that puts CPU into protected/V86 mode). So you can do everything in DOS that is only possible in Windows with WinRing0 (such as reading/writing MSRs, reading/writing IO ports, reading/writing PCI address space etc.)
WinRing0 has no special functions that can set FSB or CPU multiplier directly, only generic functions that help you to access CPU MSRs, IO ports and PCI address space under Windows (protected mode).
So porting WinRing0 to DOS is absolutely unnecessary.

Thank you for the explanation. I shouldn't have said "porting" the library, I meant just taking the functions from it that the FSB tools use to write into the CPU and clock generator registers. It would have been useful as a starting point because I have never done that before. I remember looking at the official documentation from AMD for all the different CPU registers and such, but it was so overwhelming that I had no idea where to begin.

asdf53 wrote on 2022-09-24, 07:31:

Thank you for the explanation. I shouldn't have said "porting" the library, I meant just taking the functions from it that the FSB tools use to write into the CPU and clock generator registers. It would have been useful as a starting point because I have never done that before. I remember looking at the official documentation from AMD for all the different CPU registers and such, but it was so overwhelming that I had no idea where to begin.

Hi,
Under DOS implementing these functions actually is just writing a few assembly instructions.
Reading/writing CPU configuration registers is only 1 assembly instruction: RDMSR / WRMSR.
Reading/writing IO ports and PCI configuration space is also only 1 assembly instruction: IN register, DX / OUT DX, register.

reading MSR:

1mov ecx, MSRAddress
2rdmsr
3mov  Upper32bitValue, edx
4mov  Lower32bitValue, eax

writing MSR

1mov ecx, MSRAddress
2mov edx, Upper32bitValue
3mov eax, Lower32bitValue 
4wrmsr

reading 1 bye from PCI address space

1mov eax, PciBusDevFuncRegOffset
2mov dx, 0cf8h
3out  dx, eax
4mov dx, 0cfch
5in al, dx
6mov ByteValue, al

writing 1 byte to PCI address space

1mov eax, PciBusDevFuncRegOffset
2mov dx, 0cf8h
3out  dx, eax
4mov dx, 0cfch
5mov al, ByteValue
6out dx, al

The different MSR's and how to calculate PciBusDevFuncRegOffset can be found in AMD's documentation (BIOS and Kernel Developer's Guide for AMD AthlonTM 64 and AMD OpteronTM Processors)

Here is a sample program about how to change Athlon64 CPU multiplier from DOS (demonstrates the reading/writing of MSR's under DOS)
https://falcosoft.hu/a64lowp.zip

Also here is a sample program how to set Athlon 64 memory divider from 6 to 5 (demonstrates the reading/writing of PCI address space under DOS):

CPUSpd also works great
CpuSpd - A Hardware Based CPU Speed Control Utility for DOS/Win9X Retro Gaming

CPUSpd m12 = Multi 12
CPUSpd m10 = Multi 10
etc.

CPUSpd f1008 = 1.35V
CPUSpd f0c0A = 1.30V
CPUSpd f040C = 1.25V
CPUSpd f0C0e = 1.20V
CPUSpd f0410 = 1.15V
CPUSpd f0412 = 1.10V