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When 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
D0h
D1h
D3h
D4h
D5h
D6h
Appendix B
BIOS POST Checkpoint Codes
Code Description
The NMI is disabled. Power on delay is starting. Next, the initialization
code checksum will be verified.
Initializing the DMA controller, performing the keyboard controller
BAT test, starting memory refresh, and entering 4 GB flat mode next.
Starting memory sizing next.
Returning to real mode. Executing any OEM patches and setting the
Stack next.
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.
Control is in segment 0. Next, checking if <Ctrl> <Home> was pressed
and verifying 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.
Appendix B: BIOS POST Checkpoint Codes
B-1
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