NEC PD750008 - Manual (Page 220)

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µPD750008 USER'S MANUAL
6.7 MACHINE CYCLES FOR STARTING INTERRUPT PROCESSING
With the µPD750008 series, the following machine cycles are used to start the execution of the interrupt
service routine after an interrupt request flag (IRQn) is set.
(1) When IRQn is set during execution of an interrupt control instruction
When IRQn is set during execution of an interrupt control instruction, an instruction preceded by that
instruction 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 according to the instruction).
C: Interrupt processing (3 machine cycles)
D: Interrupt service routine is executed.
Remarks 1. An interrupt control instruction manipulates hardware (address FBxH in data memory) which
handles interrupt processings. There are two types of interrupt control instruction, a DI
instruction and an EI instruction.
2. Three machine cycles required for the interrupt processing include the time to manipulate
the stack when an interrupt is accepted.
Cautions 1. When interrupt control instructions are contiguous these interrupt control instructions
are executed up to the last one. An instruction preceded by the interrupt control
instruction executed last is executed, and an interrupt processing of three machine
cycles is executed, then the interrupt service routine is started.
2. When a DI instruction is executed in the period during which IRQn is set (A in the
figure), or in the immediately following period, the interrupt request of the set IRQn
is held until an EI instruction is executed.
A B C D
Interrupt control
instruction