NEC PD78214 - Manual (Page 336)

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Chapter 12 Interrupt Functions
12
When a low-priority vectored interrupt is being handled, vectored interrupt requests with lower and higher
priorities are accepted for multiple-interrupt handling provided that interrupts are enabled. When a high-priority
interrupt is being handled, high-priority vectored interrupts are accepted for multiple-interrupt handling provided
that interrupts are enabled. Moreover, any interrupt requests specifying a macro service are accepted regardless
of their priority.
When the RESET signal is input, this register is set to FFFFH, thereby specifying that all interrupts be in the low-
priority group.
Fig. 12-6 Priority Specification Flag Register (PR0) Format
12.2.5 Interrupt Status Register (IST)
The IST register is an 8-bit register that controls multiple-interrupt handling for nonmaskable interrupt requests
(input to the NMI pin) and indicates whether a nonmaskable interrupt request has been accepted.
When a nonmaskable interrupt is being handled, another nonmaskable interrupt request may occur. In such a
case, the nonmaskable interrupt request is accepted if the NMIS bit is 0; it is not accepted if the NMIS bit is 1.
The NMIS bit is set to 1 when a nonmaskable interrupt request is accepted. It is reset to 0, when a return (execution
of the RETI instruction) from the interrupt handling for the nonmaskable interrupt request occurs.
Both an 8-bit manipulation instruction and bit manipulation instruction can be used to read data from and write
data to the IST register. The NMIS flag is set to 1 when a nonmaskable interrupt is accepted. It is reset to 0 by the
RETI instruction. Fig. 12-7 shows the format of the IST register.
When the RESET signal is input, the register is reset to 00H.
Fig. 12-7 Interrupt Status Register (IST) Format
Multiple interrupt processing by
nonmaskable interrupt request
NMIS
Enabled
Execution exits from nonmaskable
interrupt processing or nonmaskable
interrupt request is not accepted.
0
Disabled
Nonmaskable interrupt request is
accepted or being processed.
1
0
7
0
6
0
5
0
4
0
3
0
2
0
1
NMIS
0
IST
Accepting status for
nonmaskable interrupt request
CSIPR STPR SRPR SERPR CPR20 PPR5 PPR4 CPR21 PR0H
High priority 0
Low priority 1
CPR11 CPR10 CPR01 CPR00 PPR3 PPR2 PPR1 PPR0 PR0L (FFFFH when RESET signal is input)
Priority specification flags
7 6 5 4 3 2 1 0