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.


1521 - SYSTEM - POWER-ON SELF-TEST ERROR LOG (PS50+)