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
