INT 15 - SYSTEM - POWER-ON SELF-TEST ERROR LOG (PS50+)
        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.