QBUS CPU ODT

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

• LSI-11 CPUs
• KDF11 CPUs
• KDJ11 CPUs