QBUS CPU ODT

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ODT (Octal Debugging Technique) on QBUS PDP-11s is effectively their front panel. These CPUs do not have a front panel to control them; instead, as a cost-reduction measure, when the CPU is halted, specialized microcode uses the main serial line as a operating console. There are commands to read and write main memory, registers in the CPU, start the CPU, etc.

The command set is:

  • '/' - open word
  • '<CR>' - close open word
  • '<LF>' - open next word
  • 'G' - start CPU
  • 'P' - proceed CPU after halt

The main serial interface is normally configured so that when the CPU is running, sending a break on the console serial line halts the CPU. (Depending on which interface card is used, some can be set to bootstrap the machine instead.)

LSI-11s

The LSI-11 CPUs, using the LSI-11 chip set, have an extended ODT syntax with extra commands:

  • '^' - open previous word
  • '@' - indirect; open word at location given by current word
  • '_' - indexed; open word at location given by current word plus the address of the current word
  • 'M' - maintenance; indicates how the CPU got to ODT
  • 'L' - start microcode PDP-11 Bootstrap Loader

See also