NEC 78K0 Series User Manual page 498

(5) Program status word (PSW)
The program status word is a register used to hold the instruction execution result and the current status for an
interrupt request. The IE flag that sets maskable interrupt enable/disable and the ISP flag that controls multiple
interrupt servicing are mapped to the PSW.
Besides 8-bit read/write, this register can carry out operations using bit manipulation instructions and dedicated
instructions (EI and DI). When a vectored interrupt request is acknowledged, if the BRK instruction is executed,
the contents of the PSW are automatically saved into a stack and the IE flag is reset to 0. If a maskable interrupt
request is acknowledged, the contents of the priority specification flag of the acknowledged interrupt are
transferred to the ISP flag. The PSW contents are also saved into the stack with the PUSH PSW instruction.
They are restored from the stack with the RETI, RETB, and POP PSW instructions.
Reset signal generation sets PSW to 02H.
<7> <6> <5>
PSW IE Z RBS1
498
CHAPTER 19 INTERRUPT FUNCTIONS
Figure 19-6. Format of Program Status Word
<4> <3> 2 <1>
AC RBS0 0 ISP CY
Preliminary User's Manual U17260EJ3V1UD
After reset
0
02H
Used when normal instruction is executed
ISP
Priority of interrupt currently being serviced
0 High-priority interrupt servicing (low-priority
interrupt disabled)
Interrupt request not acknowledged, or low-
1
priority interrupt servicing (all maskable
interrupts enabled)
IE
Interrupt request acknowledgment enable/disable
0 Disabled
1 Enabled