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CPU Registers

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4.3.4 Constant Generator Registers (CG1 and CG2)

Six commonly-used constants are generated with the constant generator registers R2 (CG1) and R3

(CG2), without requiring an additional 16-bit word of program code. The constants are selected with the

source register addressing modes (As), as described in Table 4-2.

Table 4-2. Values of Constant Generators CG1, CG2

R2 00 – Register mode

R2 01 (0) Absolute address mode

R2 10 00004h +4, bit processing

R2 11 00008h +8, bit processing

R3 00 00000h 0, word processing

R3 01 00001h +1

R3 10 00002h +2, bit processing

R3 11 FFh, FFFFh, FFFFFh –1, word processing

The constant generator advantages are:

• No special instructions required

• No additional code word for the six constants

• No code memory access required to retrieve the constant

The assembler uses the constant generator automatically if one of the six constants is used as an

immediate source operand. Registers R2 and R3, used in the constant mode, cannot be addressed

explicitly; they act as source-only registers.

4.3.4.1 Constant Generator – Expanded Instruction Set

The RISC instruction set of the MSP430 has only 27 instructions. However, the constant generator allows

the MSP430 assembler to support 24 additional emulated instructions. For example, the single-operand

instruction:

CLR dst

is emulated by the double-operand instruction with the same length:

MOV R3,dst

where the #0 is replaced by the assembler, and R3 is used with As = 00.

INC dst

is replaced by:

ADD #1,dst

122

CPUX SLAU144J–December 2004–Revised July 2013

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