I/O on the #Datapoint 2200 (as I'm understanding it) :
The instructions are similar to the i8008. Bits 4 to 1 of the bytecode code the I/O port. 32 addresses, but the first 8 are inputs, mapped to instructions IN 0..7. The remaining 24 are outputs mapped to instructions OUT 0..23.
The Datapoint uses the instruction EX for outputs (short for external IIRC)). Rather than port numbers it uses names. The first 8 are generic:
EX ADR
EX STATUS
EX DATA
EX WRITE
EX COM1 though EX COM4
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To poll a device for input data:
Set A to device address
EX ADR ;Device selected
EX STATUS ;Prepare to read status byte
INPUT ;Read status byte
Check value. If data available:
EX DATA ;Prepare to read data
INPUT ;Read data
Data is now in A register.
However, the output port is decoded on the decoder board using bits 4..1 of the instruction register. They're exposed on pins on the processor boards edge connector.
The instruction decoding for the INPUT command looks to be partial enough that it is issued irrespective of the port address. If so another board could be decoding I4..I1 to give an input port address.
(But the pinouts for every board are unique to that board. It's possible no other board has access to them).
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