Texas Instruments MSP430x1xx Constant Generator Registers CG1 and CG2, Register Constant Remarks

CPU Registers

3.2.4Constant Generator Registers CG1 and CG2

Six commonly-used constants are generated with the constant generator registers R2 and R3, 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 3−2.

Table 3−2. Values of Constant Generators CG1, CG2

Register	As	Constant	Remarks
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	0FFFFh	−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.

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:

CLRdst

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

MOVR3,dst

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

INCdst

is replaced by:

ADD0(R3),dst

RISC 16-Bit CPU 3-7