at a minimum, if the cpu, bios, and the i/o interface upon which the post card relies are all working, a post card can be used to monitor the system's power-on self test (post), or to diagnose problems with it. the system introduced on ibm pc computers sends 8-bit byte codes (usually displayed as two hexadecimal digits) 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 performed, is available.