Program Register Set - NEC V850/SA1 mPD703015 Preliminary User's Manual

3.2.1 Program register set

The program register set includes general registers and a program counter.
(1) General registers
Thirty-two general registers, r0 to r31, are available. Any of these registers can be used as a data variable or
address variable.
However, r0 and r30 are implicitly used by instructions, and care must be exercised when using these registers.
Also, r1 to r5 and r31 are implicitly used by the assembler and C compiler. Therefore, before using these
registers, their contents must be saved so that they are not lost. The contents must be restored to the registers
after the registers have been used.
Name
r0 Zero register
r1 Assembler-reserved register
r2 Interrupt stack pointer
r3 Stack pointer
r4 Global pointer
r5 Text pointer
r6 to r29
r30 Element pointer
r31 Link pointer
PC Program counter
Note Area in which program code is mapped.
(2) Program counter
This register holds the address of the instruction under execution. The lower 24 bits of this register are valid, and
bits 31 to 24 are fixed to 0. If a carry occurs from bit 23 to 24, it is ignored.
Bit 0 is fixed to 0, and branching to an odd address cannot be performed.
After reset: 00000000H
Symbol 31
PC Fixed to 0
CHAPTER 3 CPU FUNCTIONS
Table 3-1. Program Registers
Usage
Always holds 0
Working register for generating 32-bit immediate
Stack pointer for interrupt handler
Used to generate stack frame when function is called
Used to access global variable in data area
Register to indicate the start of the text area
−
Address/data variable registers
Base pointer register when memory is accessed
Used by compiler when calling function
Holds instruction address during program execution
24 23
Instruction address under execution
Operation
Note
1 0
0
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