NEC mPD178054 Series User Manual page 165

(5) Program status word (PSW)
The program status word is a register that holds the instruction execution result and the current status for
interrupt request. The IE flag to set maskable interrupt enable/disable and the ISP flag to control multiple
interrupt servicing are mapped.
Besides 8-bit unit read/write, this register can carry out operations with a bit manipulation instruction and
dedicated instructions (EI and DI). When a vectored interrupt request is acknowledged, if the BRK instruction
is executed, the contents of PSW are automatically saved into a stack and the IE flag is reset to 0. When
a maskable interrupt request is acknowledged, the contents of the priority specification flag of the acknowledged
interrupt are transferred to the ISP flag. The acknowledged interrupt is also saved into the stack with the PUSH
PSW instruction. It is restored from the stack with the RETI, RETB, and POP PSW instructions.
Reset input sets PSW to 02H.
Figure 12-6. Configuration of Program Status Word (PSW)
7 6 5
PSW
IE Z RBS1 AC
CHAPTER 12 INTERRUPT FUNCTIONS
4 3 2 1 0
RBS0 0 ISP CY
User's Manual U15104EJ2V0UD
After reset
02H
Used when normal instruction is executed
ISP Priority of interrupt currently being received
0 High-priority interrupt servicing
(low-priority interrupt disable)
Interrupt request not acknowledged or low-priority
interrupt servicing
1
(all-maskable interrupts enable)
IE Interrupt request acknowledge enable/disable
0 Disabled
1 Enabled
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