490CHAPTER 21 INTERRUPT AND TEST FUNCTIONS(6) Program status word (PSW)The program status word is a register to hold the instruction execution result and the current status for interruptrequest. The IE flag to set maskable interrupt enable/disable and the ISP flag to control multiple interruptprocessing are mapped.In addition to being able to perform read and write operations in 8 bit units, operations using bit operationcommands and special commands (EI, DI) can be performed. Also, when a vectored interrupt request isreceived or when a BRK command is executed, the contents of the PSW are automatically saved to the stackand the IE flag is set at (0). Also, when a maskable interrupt request is received, the contents of the receivedinterrupt priority order specification flag are transferred to the ISP flag. The contents of the PSW are also savedto the stack by the PUSH PSW command. The stack contents are recovered by the RETI, RETB and POPPSW commands.RESET input sets PSW to 02H.Figure 21-9. Program Status Word Format7IEPSW6Z5RBS14AC3RBS0201ISP0CY 02HAfterResetISP0Used when normal instruction is executedPriority of Interrupt Currently Being ReceivedHigh-priority interrupt servicing(low-priority interrupt disable)1Interrupt request not acknowledged or low-priorityinterrupt servicing(all-maskable interrupts enable)IE Interrupt request Acknowledge Enable/Disable0 Disable1 Enable
