Peripherals that do use external INTs:
* [[Music Machine]] - accordingly to the manual: It can "disable" the internal interrupt, and replace it by external MIDI/ACIA interrupt. ''Unknown if/how it is really doing that, "disabling" the internal interrupt could be probably done only "shortcutting" the INT signal to 5V, which would be a rather aggressive solution - so maybe the manual only meant that software "ignores" the internal interrupt (?) - also, it referred to IRQ, which could eventually also mean NMI rather than INT...? - the Music Machine software isn't in the internet yet, so one can't disassemble/verify this.''
When the Z80 acknowledges a maskable interrupt, it will disables acknowledgement of maskable interrupts.
It will then perform the appropiate interrupt method (as defined by IM0, IM1 or IM2). Interrupts should end with EI:RETI to re-enable maskable interrupts.
RETI is the special instruction recognised by Z80 compatible peripheral ICs. It is used to control the daisy-chain interrupt mechanism that they have.
The CPC operating system interrupt handler briefly re-enables interrupts. It does this to effectively detect other interrupt sources. After this the external interrupt vector is jumped to (&003B). This is normally just a RET.
The gate-array interrupt is automatically acknowledged and cleared, so external maskable interrupts should only be cleared explicitly by the Z80 performing an I/O instruction, otherwise they would conflict with the gate-array interrupt and you would not know the source of the interrupt.
(This information is mentioned in SOFT968).
== See also ==
* [[NMI]] - Non-maskable Interrupts (more commonly used by peripherals)