NEC PD75402A - Manual (Page 144)

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CHAPTER 6. INTERRUPT FUNCTIONS
(4) Interrupt status flag
The interrupt status flag (IST0) is the flag which shows the status of the processing currently being executed by
the CPU, and is contained in the PSW.
The interrupt priority control circuit performs interrupt control according to the contents of this flag as shown
in Table 6-3.
IST0 cannot be modified by 4-bit handling instructions or bit-handling instructions. IST0 is always set to 1 during
interrupt servicing. Therefore, it is not possible to write 0 to IST in the interrupt service routine which would result
in multiple interrupt.
After being saved to stack memory together with the rest of the PSW when an interrupt is acknowledged, IST0
is automatically set to 1. When an RETI instruction is executed, the original IST0 value (0) is restored.
A RESET input clears (0) the flag contents.
Table 6-3 IST0 Interrupt Servicing Status
Executing
Processing Status
Status 0
Status 1
IST0
0
1
CPU Processing
Normal program
processing in
progress
Interrupt servicing in
progress
Interrupt Requests of which
Acknowledgment is Possible
Acknowledgment of all interrupts
possible
Acknowledgment of all interrupts
disabled
After Interrupt
Acknowledgment
IST0
1
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