What happens when I8008 CPU reads the "undefined" opcode (22h, 2Ah, 32h, 38h, 39h, 3Ah; or in octal 042, 052, 062, 070, 071, and 072)? Are these opcodes evaluated as a NOP instruction, or lead to some side effects (RESET, etc). How many T cycles those instruction takes?

  • Worth noting that almost all of these instructions are also undefined in the Datapoint 2200, which has the same instruction set architecture. The only exception is 38h = 070, which is PUSH on the Datapoint. There is no PUSH on the 8008, as the stack is on-chip instead of in memory.
    – DrSheldon
    Commented Jan 21, 2023 at 7:46
  • 1
    Also, Ken Shirriff has done several die analyses of the 8008, but none that indicate what the undefined opcodes do.
    – DrSheldon
    Commented Jan 21, 2023 at 7:56


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