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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. r0 is a register that always holds 0, and is used for operations using 0 and offset 0
addressing. r30 is used, by means of the SLD and SST instructions, as a base pointer for when
memory is accessed. 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. The
contents must be restored to the registers after the registers have been used.
Name
r0
Zero register
r1
Assembler-reserved register Working register for generating 32-bit immediate data
r2
Address/data variable registers
r3
Stack pointer
r4
Global pointer
r5
Text pointer
r6 to r29
Address/data variable registers
r30
Element pointer
r31
Link pointer
PC
Program counter
(2)
Program counter
This register holds the instruction address during program execution. The lower 26 bits of this regis-
ter are valid, and bits 31 to 26 are fixed to 0. If a carry occurs from bit 25 to 26, it is ignored.
Bit 0 is fixed to 0, and branching to an odd address cannot be performed.
31
PC
Fixed to 0
Chapter 3 CPU Function
Table 3-1: Program Registers
Usage
Always holds 0
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 (where pro-
gram code is located)
Base pointer when memory is accessed
Used by compiler when calling function
Holds instruction address during program execution
Figure 3-2: Program Counter (PC)
26 25
Instruction address during execution
Preliminary User's Manual U14913EE1V0UM00
Operation
1 0
0
After reset
00000000H
55
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