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CPU Registers
3-5
RISC 16-Bit CPU
3.2.2 Stack Pointer (SP)
The stack pointer (SP/R1) is used by the CPU to store the return addresses
of subroutine calls and interrupts. It uses a predecrement, postincrement
scheme. In addition, the SP can be used by software with all instructions and
addressing modes. Figure 3−3 shows the SP. The SP is initialized into RAM
by the user, and is aligned to even addresses.
Figure 3−4 shows stack usage.
Figure 3−3. Stack Pointer
0
15 0
Stack Pointer Bits 15 to 1
1
MOV 2(SP),R6 ; Item I2 −> R6
MOV R7,0(SP) ; Overwrite TOS with R7
PUSH #0123h ; Put 0123h onto TOS
POP R8 ; R8 = 0123h
Figure 3−4. Stack Usage
I3
I1
I2
I3
0xxxh
0xxxh − 2
0xxxh − 4
0xxxh − 6
0xxxh − 8
I1
I2
SP
0123h
SP
I1
I2
I3
SP
PUSH #0123h
POP R8Address
0123h
The special cases of using the SP as an argument to the PUSH and POP
instructions are described and shown in Figure 3−5.
Figure 3−5. PUSH SP - POP SP Sequence
SP
1
SP
old
SP
1
PUSH SP
The stack pointer is changed after
a PUSH SP instruction.
SP
1
SP
2
POP SP
The stack pointer is not changed after a POP SP
instruction. The POP SP instruction places SP1 into the
stack pointer SP (SP2=SP1)