NEC PD750008 - Manual (Page 221)

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CHAPTER 6 INTERRUPT AND TEST FUNCTIONS
(2) When IRQn is set during an instruction other than that described in (1)
(a) When IRQn is set at the last machine cycle of the instruction being executed
In this case, an instruction preceded by the instruction being executed is executed, and an interrupt
processing of three machine cycles is executed, then the interrupt service routine is started.
A: IRQn is set.
B: The next instruction is executed (1 to 3 machine cycles to the instruction).
C: Interrupt processing (3 machine cycles)
D: Interrupt service routine is executed.
Caution When one or more interrupt control instructions follow, an instruction preceded by the
interrupt control instructions is executed, and an interrupt processing of three machine
cycles is executed, then the interrupt service routine is started. When an instruction to
be executed after setting IRQn is a DI instruction, the interrupt request of the set IRQn
is held.
(b) When IRQn is set earlier than the last machine cycle of the instruction being executed
In this case, after executing the instruction being executed, an interrupt processing of three machine
cycles is executed, then the interrupt service routine is started.
A: IRQn is set.
C: Interrupt processing (3 machine cycles)
D: Interrupt service routine is executed.
A B C D
An instruction other than
interrupt control instruction
A C D
An instruction other than
interrupt control instruction