3 ms·

It does have a pin for that (#M1, "machine cycle 1"). Zilog actually designed their peripheral chips to handle the RETI (return from interrupt) opcode speciall

It does have a pin for that (#M1, "machine cycle 1"). Zilog actually designed their peripheral chips to handle the RETI (return from interrupt) opcode specially. On the Z80 itself it does the same thing as a normal RET, but other chips can detect it on the bus and treat it as the signal that their interrupt handler is finished. I also recall reading about some hobby project to add a PC-relative addressing mode to the 8080 or Z80. A redundant opcode like "MOV A,A" would be used as a prefix, which caused the external hardware to add the program counter to the immediate operand of the following instruction. Can't find it right now.

becurious 4y ago
Using M1 plus the address lines was used to trap on certain addresses on expansion devices for the Spectrum such as Interface 1 or the +D. Then they’d use a line on the expansion bus to disable the standard ROM and substitute their own ROM / static RAM until some other trigger such as an OUT to a port would page the ROM back in. I guess it’s a short step to looking up memory addresses against a bitmap for validation. I couldn’t tell from the video if this is what he did. Cutting the addresses into pages would let you save or add more levels and I guess you could implement a TLB similarly.