233
CHAPTER 19 INTERRUPT FUNCTIONS
(6) Program status word (PSW)
The program status word is a register to hold the instruction execution result and the current status for an interrupt
request. The IE flag to set maskable interrupt enable/disable and the ISP flag to control multiple processing are
mapped.
Besides 8-bit 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. If a maskable interrupt
request is acknowledged, the contents of the priority specify 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 reset
from the stack with the RETI, RETB, and POP PSW instructions.
RESET input sets PSW to 02H.
Figure 19-7. Program Status Word Format
7
IE
6
Z
5
RBS1
4
AC
3
RBS0
2
0
1
ISP
0
CY
PSW
After Reset
02H
ISP
High-priority interrupt servicing (Low-priority
interrupt disable)
IE
0
1
Disable
Priority of Interrupt Currently Being Serviced
Interrupt Request Acknowledge Enable/Disable
Used when normal instruction is executed
Enable
Interrupt request not acknowledged, or low-
priority interrupt servicing. (all maskable
interrupts enable)
0
1
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