PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sat Jan 2 20:16:06 CST 2016

As a final diagnostic on the PDP-12 we tried to run FOCAL-69, but it
started executing instructions in non existent memory. FOCAL initializes
lots of peripherals and then tries different instructions to determine what
processor it is running on. It executed what it thought was a TC01 DECtape
IOT, but DEC had recycled the 6762 instruction for the KF12 Automatic
Priority Interrupt controller. This caused the KF12 to execute a hardware
PUSH instruction to save the processor state and then jump into memory
field 2. Since we only have 8k of core it executed 7777 instructions and
hung. Replacing the 6762 instruction at 4376 with a 7000 NOP let FOCAL
initialize and run OK.

Michael Thompson

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