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CHAPTER 3 CPU
3.4.5 Stack Operation at Interrupt Processing
This section describes how values in registers are saved and restored at interrupt
processing.
■ Stack Operation at the Beginning of Interrupt Processing
After accepting an interrupt, the CPU automatically saves the values in the program counter (PC) and
program status (PS) in the stack.
Figure 3.4-5 shows the stack operation at the beginning of interrupt processing.
Figure 3.4-5 Stack Operation at the Beginning of Interrupt Processing
■
Stack Operation at the End of Interrupt Processing
When the return instruction (RETI) is executed at the end of interrupt processing, the values in the program
status (PS) and the program counter (PC) are restored from the stack in that order (which is opposite to that
at the beginning of interrupt processing). This operation restores the values in the PS and PC to those
values used before interruption.
Note:
Values in the accumulator (A) and temporary accumulator (T) are not automatically saved in the stack.
Therefore, save and restore the values using the PUSHW and POPW instructions.
027C
H
XX
H
027D
H
XX
H
027E
H
XX
H
027F
H
XXH
0280
H
XX
H
0281
H
XX
H
027C
H
08
H
027D
H
70
H
027E
H
E0
H
027F
H
00
H
0280
H
XX
H
0281
H
XX
H
E000H
PC
0870H
PS
0280H
SP
E000
H
PC
0870
H
PS
027C
H
SP
PS
PC
Immediately before
interruption
Address
Memory
Address
Memory
Immediately after
interruption