Is there a way to manually change BIOS POST codes on motherboard LCD?

Question

I was wondering if anyone knew if it was possible to change the BIOS POST Code that is displayed on the motherboard LCD. I want to develop a program that can manipulate the LCD screen on the motherboard to display any set of desired characters. I haven't been able to find anyone who has done something similar. Does anyone have any ideas on if this is possible? Thank You!

Answer 1

POST codes are usually displayed on LED devices on the motherboard, not LCD. Historically, POST codes can be output via IO port 0x80 on IBM/Intel compatible systems. Been a while since I have done x86 assembly, but would be something like this:

mov al, 41h   ;41h, the value to output
out 80h, al   ;send the value to IO port 80h

This will make "41" display on the POST code LEDs. If you have 4 LEDs (a four digit value), then use AX instead of AL or use port 81h and a second write.

mov ax, 5150h ;5150h, the value to output
out 80h, al   ;send the value to IO port 80h

Note: as I recall in/out instructions are protected instructions and will generate an exeception when the CPU is in protected mode (e.g. from the Windows command line)