Power-On Self Test (Post) - Compaq 100 Series Manual

2.2 Power-On Self Test (Post)

When the computer starts, the system BIOS runs a series of internal
checks on the hardware. This allows the computer to detect problems as
early as the power-on stage. The POST alerts you to problems by
displaying error messages.
If POST detects an error, the system displays an error message on the
display. If the error occurs before the display comes up, error codes or
system beeps indicate the POST error.
The value for diagnostic POST (378H) is written at the beginning of the
test. Therefore, if the test fails, you can determine where the problem
occurred by reading the last value written to POST 378H by the PIO
Debug Board Plug at the PIO port. The following table lists error codes
in sequential order on the PIO Debug Board.
Code Description
01h Start of boot loader sequence.
02h Initialize chipset.
03h Memory sizing.
04h Perform conventional RAM (1st 640K) test with crossed
pattern R/W.
05h Move boot loader to the RAM.
06h Start point of execution of boot loader in RAM.
07h Shadow screen BIOS.
08h Initialize clock synthesizer.
09h Initialize audio controller.
0Ah Detect internal ISA modem.
0Bh Proceed with normal boot.
0Ch Proceed with crisis boot.
0Fh DRAM sizing
10h Initial L1, L2 cache, make stack and diagnose CMOS.
11h Turn off fast A20 for post, Reset GDT's, 8259's quickly.
2-2 Troubleshooting
Table 2-1
Error Codes
continued