I notice from the Zilog datasheet on the Z80 that with the I/O (IN and OUT) group of instructions, the contents of various registers are often placed in the top 8 bits of the address bus (depending on the instruction), with the lower 8 bits selecting one of up to 256 theoretically connected devices.
My question is what is the point of doing this with these upper 8 bits? I know some machines use this in someway related to decreasing decoding complexity, but are they seriously used for anything? I want to implement the instructions exactly as the Z80 suggests, but I don't see the point in implementing this behaviour as it is non-standard. This behaviour is described as undocumented, so on a 'Sega Master System' for example, will I get away with this? Many thanks.
Regards, Phil Potter
The behavior is fully documented by Zilog (pages 269-287).
I guess that some peripherials may use the upper bits A8..A15 of the address bus as a sort of 8-bit parameter.