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39. IOU, the Input/Output Unit

This is a set of 74LS374 octal D-type latches, 40-bits wide, one bank for the input and on e for the output. Using these the host Z80 can send data to and from the CPU.

When the CPU executes an IN instruction, a Z80 interrupt gets generated. The IOU is then marked as busy on the unit scoreboard until the Z80 has loaded all 5 8-bit registers, so a complete 40-bit CPU word is ready. Only then does the IOU send a "finish" signal to the Result Arbitration circuit.

When the CPU executes an OUT instruction, a Z80 interrupt is also generated. The IOU busy scoreboard bit remains set until the Z80 has read all the 5 8-bit chunks of the 40-bit word.