Int 15 Fn 21  - System - Power-on Self-test Error Log (ps50+)              [B]

   AH = 21h
   AL = subfunction
       00h read POST log
       01h write POST log
       BH = device ID
       BL = error code

Return: CF clear if successful
   CF set on error
   AH = status
       00h OK
       01h list full
       02h unsupported subfunction
       80h invalid command
       86h unsupported function
   if function 00h:
      BX = number of error codes stored
      ES:DI -> error log

Notes: the log is a series of words, the first byte of which identifies the
     error code and the second the device.
   supported by AMI PCI BIOS
   IBM considers this a required BIOS function
   if the device ID is FFh, the "error code" is the actual device number
     minus 255 (thus these devices have no specific error codes)

See Also: AH=23h"SYSTEM",AH=24h"SYSTEM"