Philips CEM2100/00, Service Manual

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Figure 3-4 Saving/Restoring General-purpose Registers under Interrupt Processing
3.3.3 Interrupt return
Interrupt return instructions [RETI]/[RETN] perform as follows.
As for address trap interrupt (INTATRAP), it is required to alter stacked data for program counter (PC) to
restarting address, during interrupt service program.
Note:If [RETN] is executed with the above data unaltered, the program returns to the address trap area and
INTATRAP occurs again.When interrupt acceptance processing has completed, stacked data for PCL and
PCH are located on address (SP + 1) and (SP + 2) respectively.
Example :Save/store register using data transfer instructions
PINTxx: LD (GSAVA), A ; Save A register
(interrupt processing)
LD A, (GSAVA) ; Restore A register
RETI ; RETURN
[RETI]/[RETN] Interrupt Return
1. Program counter (PC) and program status word
(PSW, includes IMF) are restored from the stack.
2. Stack pointer (SP) is incremented by 3.
Example 1 :Returning from address trap interrupt (INTATRAP) service program
PINTxx: POP WA ; Recover SP by 2
LD WA, Return Address ;
PUSH WA ; Alter stacked data
(interrupt processing)
RETN ; RETURN
Interrupt
acceptance
Interrupt
service task
Restoring
registers
Saving
registers
Interrupt return
Saving/Restoring general-purpose registers using PUSH/POP data transfer instruction
Main task