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POST Checkpoint Codes
POST is performed by the BIOS when the computer is reset or booted. POST tests
and initializes key components. When a POST routine completes, a code is written
to I/O port address 80h. Display this code by attaching diagnostic equipment to I/O
port address 0080h.
Uncompressed Initialization Codes
Code Description
D0h The NMI is disabled. Power on delay is starting. Next, the initialization
code checksum will be verified.
D1h Initializing the DMA controller, performing the keyboard controller BAT
test, starting memory refresh, and entering 4 GB flat 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 id <Ctrl><Home> was pressed
and verifying the system BIOS checksum. If either <Ctrl><Home> was
press or the system BIOS checksum is bad, next will go to checksum
code E0h. Otherwise, going to checksum code D7h.
D7h Passing control to the interface module next.
D8h The main system BIOS routine code will be decompressed next.
D9h Passing control to the main system BIOS in shadow RAM next.
Bootable Recovery Codes
Code Description
E0h The onboard floppy controller if available is initialized. Next, beginning
the base 512KB memory test.