CAUTION:
Here I explain a safe way to install a Low Vcore CPU on the Abit AX5/PX5 boards.
Read this file carefully to avoid damage to your K6-2(+)/III(+) CPU! 

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 2.9V Vcore would be supplied
to 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.
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.

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.

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 replacing 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 may already damage the CPU.

The solution
------------
When installing a Low Vcore CPU on the AX5/PX5, 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 by 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 loose your nice Low Vcore setting. ;)

Jan Steunebrink
