During power up, if you press a key too early (during the “ABCDEFGH” portion of the display), you will get a “KEY xx y” display (XX being the key code of the key pressed and y some kind of icon associated with that code). The startup sequence then seems to halt, so press the RESET switch.

Following this initial power up sequence, you then see the system cycling/scanning through the available ROS (sort of like boot up BIOS) checking for any errors. During this portion of the startup, you can press CMD-ATTN keys (at the same time) to enter a built-in ROS diagnostic tool. The pictures below show some of the diagnostic features in action. A summary of the command options is as follows: (full description is in the IBM MIM 4-8 DIAGNOSTIC CONTROL PROGRAM).

  • Type “A xxxx“: alter address xxxx (display 32 bytes, and allow user to edit contents of each byte offset, use arrow keys to scroll through addresses up and down)
  • BE xxxx yyyy” BRANCH to Executable ROS (halt on yyyy)
  • BR xxxx yyyy” BRANCH to read/write storage (halt on yyyy)
  • BX xxxx yyyy” BRANCH to extended ROS (halt on yyyy)
  • Type “C” to enter a diagnostic menu (test I/O, OP codes, ROS, DCP I/O)
  • Type “D xxxx“: display address xxxx (use arrow keys to cycle addresses)

Other ways to enter diagnostic modes: (these were mentioned in the IBM manual, but haven’t confirmed them yet myself)

  • Hold-CMD, press HOLD. Press MINUS key on keypad. Release CMD.
  • Press ATTN. Hold-CMD. Press “x” (ASTERICK) on keypad.
  • Exit DCP: Press ATTN. Hold-CMD. Press PLUS on keypad.

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