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Member since:
2007-04-18
Can't speak for the HP2100, but the PDP-8 didn't really have ports, per se. It had a single opcode, IOT, that caused I/O to happen. The remainder of the instruction was used to select a device and an operation. A device could do any combination of:
- Reading the value from the accumulator
- Clearing the accumulator
- ORing a value into the accumulator
- Skipping the next instruction
On later models like the PDP-8/e, the processor essentially halts and allows the I/O device access to the control points within the processor. An IOT on those systems can do pretty much anything the processor can.