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B-1
Appendix B: BIOS POST Checkpoint Codes
Appendix B
BIOS POST Checkpoint Codes
When the AMIBIOS performs the Power On Self Test, it writes checkpoint codes to I/O
port 0080h. If the computer cannot complete the boot process, diagnostic equipment
can be attached to the computer to read I/O port 0080h.
B-1 Uncompressed Initialization Codes
The uncompressed initialization checkpoint codes are listed in order of execution:
Checkpoint Code Description
D0h The NMI is disabled. Power on delay is starting. Next, the initialization code check-
sum will be verifi ed.
D1h Initializing the DMA controller, performing the keyboard controller BAT test, starting
memory refresh and entering 4 GB fl at mode next.
D3h Starting memory sizing next.
D4h Returning to real mode. Executing any OEM patches and setting the Stack next.
D5h Passing control to the uncompressed code in shadow RAM at E000:0000h. The
initialization code is copied to segment 0 and control will be transferred to segment
0.
D6h Control is in segment 0. Next, checking if <Ctrl> <Home> was pressed and veri-
fying the system BIOS checksum. If either <Ctrl> <Home> was pressed or the
system BIOS checksum is bad, next will go to checkpoint code E0h. Otherwise,
going to checkpoint code D7h.