Restore - NEC UPD703116 User Manual

7.2.2 Restore

Execution is restored from the non-maskable interrupt servicing by the RETI instruction.
When the RETI instruction is executed, the CPU performs the following processing, and transfers control to the
address of the restored PC.
(1) Restores the values of the PC and the PSW from FEPC and FEPSW, respectively, because the EP bit of the
PSW is 0 and the NP bit of the PSW is 1.
(2) Transfers control back to the address of the restored PC and PSW.
Figure 7-3 illustrates how the RETI instruction is processed.
1
Caution When the PSW.EP bit and PSW.NP bit are changed by the LDSR instruction during non-
maskable interrupt servicing, in order to restore the PC and PSW correctly during recovery
by the RETI instruction, it is necessary to set PSW.EP back to 0 and PSW.NP back to 1 using
the LDSR instruction immediately before the RETI instruction.
Remark The solid lines show the CPU processing flow.
CHAPTER 7 INTERRUPT/EXCEPTION PROCESSING FUNCTION
Figure 7-3. RETI Instruction Processing
RETI instruction
PSW.EP
0
PSW.NP
0
PC EIPC
PSW EIPSW
Original processing restored
User's Manual U14492EJ5V0UD
1
PC FEPC
PSW FEPSW
163