FUJITSU Semiconductor Controller MB89950/950A - Manual (Page 55)

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CHAPTER 3 CPU
3.4.5 Stack Operation during Interrupt Processing
This section describes the saving of the register contents to the stack and restore
operation during interrupt processing.
■ Stack operation at start of interrupt processing
The CPU automatically saves the current contents of the program counter (PC) and program status (PS) to
the stack when an interrupt is accepted.
Figure 3.4-5 "Stack operation at start of interrupt processing" shows the stack operation at the start of
interrupt processing.
Figure 3.4-5 Stack operation at start of interrupt processing
■ Stack operation at interrupt return
On execution of the interrupt return instruction (RETI) at the completion of interrupt processing, the CPU
performs the opposite processing to interrupt initiation, restoring first the program status (PS) and then the
program counter (PC) from the stack. This returns the PS and PC to their states immediately prior to the
start of the interrupt.
Note:
The CPU does not automatically save the accumulator (A) or temporary accumulator (T) contents to the
stack. Use the PUSHW and POPW instructions to save and restore A and T contents to and from the
stack.
027C H
027D H
027E H
027F H
0280 H
0281 H
Address Memory
0870 H PS
E000 H PC
0280 H SP
Immediately before
interrupt
08 H
70 H
E0 H
00 H
XXH
XXH
027C H
027D H
027E H
027F H
0280 H
0281 H
Address Memory
027C H SP
0870 H PS
E000 H PC
Immediately after
interrupt
PS
PC
XXH
XXH
XXH
XXH
XXH
XXH