Chkcpu wrote on 2025-05-23, 10:28:

Hi 0xDEADBEEF, […]
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0xDEADBEEF wrote on 2025-04-12, 22:06:

Sweet! I have seen “CPU is unworkable or has been changed.” message too, happens when I switch to different CPU. I think there […]
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Sweet!
I have seen “CPU is unworkable or has been changed.” message too, happens when I switch to different CPU. I think there is some "check if user is doing something too stupid" logic in BIOS that prints it 😉
I am excited to hear you are working on it.
This board is pretty cool, 4 pci + 4 isa, built in usb and ATX form factor that makes it compatible with modern ATX cases.

Hi 0xDEADBEEF,

I made some progress with the patched Abit AX5/PX5 BIOS.
I’ve send you a PM with details about the changes I’ve made sofar, and a copy of a patched AB-AX5/PX5 interim BIOS for you to test.

When you are able to test this BIOS, please send a reply about your findings via PM or here in the thread.

Happy testing,
Jan

Hi 0xDEADBEEF,

I’m happy to report that the final patch J.3 BIOS for the jumperless Abit AX5/PX5 twin ATX/Baby AT boards is ready.
Thanks for testing the patch J2.4 BIOS on your AX5. After I sent you this interim BIOS, I performed a lot of additional tests on the stability of the new Low 2.4V/2.2V/2.1V Vcore voltage selections in the patched BIOS.
The outcome of this testing resulted in the below guidelines on how to safely install a K6-2(+)/III(+) CPU on the AX5/PX5. But first I will provide some background information.

The design
The jumperless Abit AX5/PX5 motherboard was not designed to support Low Vcore CPUs.
Even when using the latest Ver 'GP' 11/13/1998 BIOS, a default 2.9V Vcore would be set for any AMD K6 CPU, which could destroy 2.0V to 2.4V Vcore CPU models.
This limited the CPU support on the AX5/PX5 to:
- Single-Voltage: Intel Pentium, AMD K5, Cyrix 6x86, WinChip C6 and WinChip 2
- Dual-Voltage: Intel Pentium MMX, AMD K6 model 6, Cyrix 6x86L and 6x86MX.

Another limitation is that the BIOS only switches the CPU voltage automatically when changing from a Single to a Dual voltage CPU, and vice versa.
This switching supplies an initial voltage of 3.4V to a Single-voltage CPU and 3.1V to a Dual-voltage part.
In all other cases, like replacing a Dual-voltage CPU by another Dual-voltage model, the BIOS keeps the Vcore setting from the previous CPU, until you exit the BIOS via SAVE & EXIT SETUP. The Core Plane Volt setting as selected in the CPU SOFT MENU, will then become active.
To make the user aware of the need to check the BIOS settings after a CPU change, the BIOS will display a message to tell you to enter the CMOS Setup and check if the CPU SOFT MENU settings are correct.

Obviously this design worked fine when only 2.8V to 3.2V Dual-voltage CPUs were used, but it does create a problem now that I changed the BIOS to support 2.0V-2.4V K6-2(+)/III(+) CPUs.

The issue
In the AX5/PX5 patch J.3 BIOS, I kept the 2.8V and 2.9V settings but changed the 2.7V, 2.5V, and 3.2V settings to 2.4V, 2.2V, and 2.1V respectively.
These were the only CPU SOFT MENU Vcore changes I could make in the BIOS, given the boards hardware design.
In addition, I changed the CPU SOFT MENU default to set the correct 2.4V/2.2V/2.1V Vcore voltage for each AMD K6 model 7, K6-2, K6-III, and K6-2+/III+ CPU model.

But as described above, the BIOS keeps the Vcore setting from the previous CPU, until you select the CPU SOFT MENU, check or change the Core Plane Volt setting and exit the BIOS via SAVE & EXIT SETUP.
This last action will send the Vcore setting to the motherboard logic and applies the Vcore you set in the CPU SOFT MENU.

So when you replace a Pentium MMX (2.8V) by a K6-2+ (2.0V), the K6-2+ will receive a 2.8V Vcore untill the BIOS SETUP is entered, the CPU SOFT MENU options are set, and the BIOS is exited via SAVE & EXIT SETUP. This large over-voltage may already damage the CPU, even during this brief period.

The solution
When installing a Low Vcore CPU on the AX5/PX5, please follow these steps:
• With the old CPU still installed, flash the BIOS with the patch J.3 version (PX5_J3.BIN) and confirm the system still runs fine.
• If the old CPU was a Single-voltage model, replace it with a 2.8V/2.9V Dual-voltage CPU like a Pentium MMX, K6 model 6, or Cyrix 6x86L/6x86MX.
• Now enter the BIOS's CPU SOFT MENU and set the CPU Operating Speed to User Define, the External Clock to 50MHz, the Multiplication Factor to x2, and the Core Plane Volt. to 2.20v. Then exit the BIOS via SAVE & EXIT SETUP.
• The low 100MHz CPU speed setting will ensure that the 2.8V/2.9V CPU keeps running at the reduced 2.2V Vcore.
• After switching the system off, a Low voltage CPU model can now be installed. At the first power-up after the CPU change, the 2.2V Vcore from the previous step will be supplied to the CPU.
• Now enter the BIOS's CPU SOFT MENU again and check/set the speed and voltage settings as required. After a final SAVE & EXIT SETUP, the system will reboot with the required speed setting and correct voltage supply to the CPU.

At the end of this procedure I like to mention that the AX5/PX5 board uses a non-volatile memory in its Vcore control logic. This nice feature preserves the last CPU SOFT MENU voltage setting, even when the CMOS battery is dead or removed.
So, no worries that you suddenly lose your nice Low Vcore setting due to a CMOS clear or flat battery. 😉

Here is the Abit AX5/PX5 patch J.3 BIOS for the community:

In the zip I’ve included a CAUTION.TXT file with the above “safe install” procedure, and as usual you can find all changes I’ve made to the BIOS in the PATCH.TXT file.

Jan

Thank you so much for your work.

You, along with many other exceptional people, make it possible for someone like me—a simple enthusiast—to get the most out of these little treasures from the past.

Let’s be honest: while today’s PCs are incredible in terms of what they can do (especially regarding game graphics), back in the day, they were pure magic—with single-core CPUs pushed to the absolute limits of the technology of the time, 3D graphics cards, and sound cards with wavetable synthesis.

The magic—made possible in part by the efforts of special people like you—lies in the ability to push a motherboard, originally designed to support only one or two generations of CPUs, well beyond its intended limits, allowing it to run even the latest generations available for its socket.

Thank you so much!

Chkcpu wrote on Yesterday, 22:04:

At the end of this procedure I like to mention that the AX5/PX5 board uses a non-volatile memory in its Vcore control logic. This nice feature preserves the last CPU SOFT MENU voltage setting, even when the CMOS battery is dead or removed.
So, no worries that you suddenly lose your nice Low Vcore setting due to a CMOS clear or flat battery. 😉

Hi Jan,
thanks for your knowledge and support to the community.

Do you have more informations on how that NV-memory is accessed by that board(s)? Is it limited to a special VRM?
That's a nice feature to have.
I sometimes have worries on my QDI Titanium IB+ running my K6-3 400 (non + @66x6) on board's minimum of 2.0 Volts. That board is jumperless too.
It is because I don't want to fry my only K6-3 on imprinted 2.2 Volts since mobile parts are binned to run from 1.6 Volts and above.

Still looking forward to see you once in Keulen or Utrecht.
- dis