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CHAPTER 3 CPU AND CONTROL UNITS
3.8.3
SSP (System Stack Pointer)
The system stack pointer (SSP) serves as the pointer that points to the stack for saving
data to upon receipt of an EIT or for restoring data from upon recovery from the EIT.
The pointer value is decremented by "8" when an EIT is serviced and incremented by "8" when the RETI
instruction is executed to return from the EIT.
31
SSP
The SSP serves also as the general-purpose register R15 when the S-flag in the CCR (condition code
register) contains "0".
The initial value after a reset is "00000000
■ Interrupt Stack
The interrupt stack is the area pointed to by the SSP, to/from which the PC and PS values are saved/
restored. After an interrupt occurs, the PC and PS values are stored at the address held in the SSP and at
the address of (SSP + 4), respectively.
Example:
SSP
80000000 H
7FFFFFFC H
7FFFFFF8 H
52
Figure 3.8-1 Interrupt Stack
[Before interrupt]
80000000
H
Memory
0
[Initial value]
00000000
H
".
H
[After interrupt]
SSP
80000000 H
7FFFFFFC H
7FFFFFF8 H
7FFFFFF8
H
PS
PC
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