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)Section 11 "Interrupts" mentions this.
== See also ==
* [[NMI]] - Non-maskable Interrupts (more commonly used by peripherals)
[[Category:Programming]]