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CHAPTER 16 INTERRUPT AND TEST FUNCTIONS
User’s Manual U11302EJ4V0UM
(6) Program status word (PSW)
The program status word is a register used to hold the instruction execution result and the current status for
interrupt requests. The IE flag used to set maskable interrupt enable/disable and the ISP flag used to control
multiple interrupt servicing are mapped to the PSW.
Besides 8-bit unit read/write, this register can carry out operations using bit manipulation and dedicated
instructions (EI and DI). When a vectored interrupt request is acknowledged or when the BRK instruction is
executed, the contents of the PSW are automatically saved into the 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 contents of the PSW are also saved to the stack by the PUSH
PSW instruction. They are reset from the stack with the RETI, RETB, and POP PSW instructions.
RESET input sets PSW to 02H.
Figure 16-8. Format of Program Status Word
7
IE
Symbol
PSW
6
Z
5
RBS1
4
AC
3
RBS0
2
0
1
ISP
0
CY
02H
After reset
ISP
0
Used when normal instruction is executed
Priority of interrupt currently being serviced
High-priority interrupt servicing
(low-priority interrupts disabled)
1
Interrupt request not acknowledged or low-priority
interrupt servicing
(all maskable interrupts enabled)
IE
Interrupt request acknowledgment enable/disable
0
Disabled
1
Enabled