The extended memory controller on the PDP-8 (the original one, sometimes called the Straight-8) apparently had difficulty when selecting a datafield that didn't exist:

On machines with 8K or more, an attempt to change the data field to a non-existant field caused a bizarre double-indirect and skip instruction execution that must be accounted for in memory diagnostics.

What is this instruction execution and why did it happen, and what is it supposed to achieve? How can it be double indirect, when that's an addressing mode that does not exist on the PDP-8?

  • 3
    The data field is only used for indirect addressing to start with. And accessing non existent momroy is somethat tricky on any machine. So this is not supposed to archive anything, but rather a side effect. Keep in mind, this was at a time when hardware was not meant to pamper for programmers errors :))
    – Raffzahn
    Commented Dec 9, 2019 at 12:02
  • It's likely just like the "undocumented" 6502 instructions: Unintentional effects that happen when a bit combination that was never taken into account appears at the decoding/control logic, which by chance produce something that doesn't completely crash the system.
    – dirkt
    Commented Dec 9, 2019 at 17:48


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