The PC uses an irregular way of sending codes to ports 10 and 11, which makes it impractical to monitor them on a POST card. The XT, on the other hand, uses three methods; before initializing the display, it issues a few codes to port 60 (the 8255 controller for the keyboard) for critical system board errors. It beeps to indicate successful or unsuccessful POST, and displays error messages.
After initializing the display, it writes error codes to memory address 15, which are sent to the screen to make up part of other error messages.
00 or FF
CPU register test failed
BIOS ROM (ROS) checksum failed
Timer I failed
8237 DMA register write/read failed or unexpected timer 1 request for DMA ch 1
After enabling port 213 expansion box, base 32K memory write/read of AA, 55, FF, 01 and 00 test failed; POST output alternates between POST code and failing bit pattern.
Size memory, initialize the 8259 PlC, setup BIOS interrupt vectors in RAM, read the configuration switches, poll the manufacturing jumper. If installed, load the manufacturing test via the keyboard port and run it. If not, initialize the rest of the system.