Texas Instruments MSP50C6xx manual AC Register #, Points to Offset

Computation Unit

When writing an accumulator-referenced instruction, therefore, the working accumulator address is stored in one of AP0 to AP3. The C6xx instruction set provides a two-bit field for all accumulator referenced instructions. The two-bit field serves as a reference to the accumulator pointer which, in turn, stores the address of the actual 16-bit accumulator. Some MOV instructions store the contents of the APn directly to memory or load from memory to the APn register. Other instructions can add or load 5-bit constants to the current APn register contents. A full description of the C6xx instruction set is given in Chapter 4, Assembly Language Instructions.

Figure 2–5. Overview of the Accumulators

Accumulator Block:		32, 16-bit registers		AC(0)	. . .	AC(31)
Accumulator Block Pointers:		4, 5-bit registers		AP(0)	. . .	AP(3)
	The accumulator block pointers may assume values in one of two forms:
	1) DIRECT REFERENCE:	0 . . .	31
					AC Register #
	2) INDIRECT REFERENCE:	0 . . .	15	points to:	0	. . . 15
		0 . . .	15 OFFSET	points to:	16	. . . 31
		15 . .	. 31 OFFSET	points to:	0	. . . 15

–AP registers are served by a 5-bit processor for sequencing addresses or repetitive operations.

–Selection between the 4 AP’s is made in the 2-bit An field in all accumulator-referenced instructions

2.2.2.3String Operations

The AP registers are served by a 5-bit processor that provides efficient sequencing of accumulator addresses. The design automates repetitive operations like long data strings or repeated operations on a list of data.

When operating on a multiword data string, the address is copied from the AP register to fetch the least significant word of the string. This copy is then consecutively incremented to fetch the next n words of the string. At the completion of the consecutive operations, the actual address stored in the AP register is left unchanged; its value still points to the least significant location. The AP register, therefore, is loaded and ready for the next repeatable operation.

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