CHAPTER 3 CPU FUNCTIONS
3.2.1 Program register set
The program register set includes general-purpose registers and a program counter.
(1) General-purpose registers (r0 to r31)
Thirty-two general-purpose registers, r0 to r31, are available. All 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. r0 always holds 0 and is used for operations that use 0 and offset 0 addressing. r30 is used as a
base pointer when performing memory access with the SLD and SST instructions.
Also, r1, r3 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, and they must be restored to the registers
after the registers have been used. There are cases when r2 is used by the real-time OS. If r2 is not used by
the real-time OS, r2 can be used as a variable register.
Table 3-1. Program Registers
Name
Usage
Zero register
Assembler-reserved register
Address/data variable register (when r2 is not used by the real-time OS to be used)
Operation
r0
r1
r2
r3
r4
r5
Always holds 0
Working register for generating 32-bit immediate
Stack pointer
Used to generate stack frame when function is called
Used to access global variable in data area
Global pointer
Text pointer
Register to indicate the start of the text area (area for placing program code)
r6 to r29
r30
Address/data variable register
Element pointer
Link pointer
Base pointer when memory is accessed
Used by compiler when calling function
r31
PC
Program counter
Holds instruction address during program execution
(2) Program counter (PC)
This register holds the address of the instruction under execution. The lower 26 bits of this register are valid,
and bits 31 to 26 are fixed to 0. If a carry occurs from bit 25 to bit 26, it is ignored.
Bit 0 is fixed to 0, and branching to an odd address cannot be performed.
31
2625
1 0
0
After reset
00000000H
PC
Fixed to 0
Instruction address under execution
User’s Manual U15862EJ3V0UD
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NEC [ NEC ]
