UPD70732 datasheet(13/66 Pages) NEC

13 / 66 page

µPD70732

2.1 Program Register Set

The program register set is composed of general-purpose registers and a program counter.

(1)

General-purpose registers

Thirty-two general-purpose registers, r0 to r31, are available. All these registers can be used as data

registers or address registers.

Of these registers, r0 and r26 through r30 are implicitly used by some instructions, and r1 through r5 and

r31 are implicitly used by the assembler and C compiler. Therefore, when using these registers, it is

necessary to take special care such as saving these registers’ contents to different areas before using

these registers and restoring the contents after using them.

Table 2-1. Program Registers

Application

Operation

Zero register

Always holds zeros.

Used as a working register to generate a 32-bit immediate data.

Handler stack pointer

Used as the stack pointer for the handler.

Stack pointer

Used to generate a stack frame at a function call.

Global pointer

Used to access a global variable in the data area.

Text pointer

Points the start address of the text area.

r6 to r25

—

Stores address or data variables.

r26

String destination bit offset

Used in a bit-string instruction execution.

r27

String source bit offset

r28

String length register

r29

String destination address register

r30

String address register

r31

Link pointer

Stores the return address at execution of a JAL instruction.

(2)

Program Counter

The program counter (PC) indicates the address of the instruction currently executed by the program.

Bit 0 of the PC is fixed to 0, and execution cannot branch to an odd address. The contents of the PC

are initialized to FFFFFFF0H at reset.

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