Working Principle
If at least the CPU, BIOS, and the I/O interface the POST card relies on are working, the system sends two-hexadecimal-digit codes to a specified I/O port (usually 80 hex) during startup, some indicating a stage in the startup procedure, others identifying errors. The description for each code must be looked up in a table for the particular BIOS. For example, for the 1984 IBM PC/AT code 1D is issued when about to Determine Memory Size Above 1024K, and code 2D in the event of 8042 Keyboard Controller Failure, 105 System Error. If startup does not complete successfully, either an error code, or the code of the last operation, is available.
POST cards provide information even when a standard display is not available, either because connecting a monitor is impractical, or because the failure occurs before the video subsystem is operational.
Read more about this topic: POST Card
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