QBUS CPU ODT

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Revision as of 18:38, 19 August 2019 by Jnc (talk | contribs) (Break halts)
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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, start the CPU, etc.

The command set is:

  • '/' - open location
  • '<CR>' - close open location
  • '<LF>' - open next location
  • '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.)

See also