Fujitsu CM71-00101-5E User Manual
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CHAPTER 3 REGISTER DESCRIPTIONS
3.3.4
Return Pointer (RP)
The return pointer (RP) is a register used to contain the program counter (PC) value
during execution of call instructions, in order to assure return to the correct address
after the call instruction has executed.
during execution of call instructions, in order to assure return to the correct address
after the call instruction has executed.
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Overview of the Return Pointer
The contents of the return pointer (RP) depend on the type of instruction. For a call instruction with a delay
slot, the value is the address stored +4, and for a call instruction with no delay slot, the value is the address
stored +2. The save data is returned from the "RP" pointer to the "PC" counter by execution of a "RET"
instruction.
Figure 3.3-9 shows a sample operation of the "RP" pointer in the execution of a "CALL" instruction with
no delay slot, and Figure 3.3-10 shows a sample operation of the "RP" pointer in the execution of a "RET"
instruction.
Figure 3.3-9 Sample Operation of "RP" in Execution of a "CALL" Instruction with No Delay Slot
Figure 3.3-10 Sample Operation of "RP" in Execution of a "RET" Instruction
Memory space
CALL SUB1
RET
Before execution
12345678
H
????????
H
PC
RP
Memory space
CALL SUB1
RET
After execution
SUB1
1234567A
H
PC
RP
SUB1
SUB1
Memory space
CALL:D SUB
RET
After execution
1234567A
H
1234567A
H
PC
RP
Memory space
CALL SUB1
RET
Before execution
SUB1
1234567A
H
PC
RP
SUB1
SUB1
ADD #1,R00
ADD #1,R00