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README.md
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README.md

IBM PCjr BIOS

IBM PCjr BIOS dump

Comments were taken from the IBM PCjr Technical Reference Guide. Only the comments between brackets and the comments in the BASIC section are from my own.

Includes the the code located from f000:0000 to f000:ffff.

This is WIP. Far from complete. I do it in a let's see how this code works basis.

Using IDA PRO freeware version

Hidden diagnostic modes

Detailed description:

TL;DR;:

  • Connect two joysticks
  • Press Ctrl + Alt + Insert keys to enter into diagnostics mode
  • Immediately after that, do:
    • Press: Joy 1 button B, and Joy 2 buttons A & B for Manufacturing burn-in mode: enters diag loop mode without the diag screen
    • Press: Joy 1 button A, and Joy 2 buttons A & B for Manufacturing system test mode: enters boot loop. Keeps booting
    • Press: Joy 1 buttons A & B, and Joy 2 button B for Service loop-post mode: displays all diagnostics options, even if the hardware is not present
    • Press: Joy 1 buttons A & B, and Joy 2 button A for Service system-test mode
    • Press: Joy 1 buttons A & B, and joy 2 buttons A & B: enters boot loop with sound test