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 set on error
   AH = status (00h OK, 01h list full, 80h invalid cmd, 86h unsupported)
   if function 00h:
      BX = number of error codes stored
      ES:DI -> error log

Note:  the log is a series of words, the first byte of which identifies the
     error code and the second the device.