8051 Rotate Accumulator

Bit Shifting

In many applications there is a need to perform a bitwise rotation of an operand. 8051 allow a program to rotate the accumulator right or left. In the 8051, to rotate a byte the operand must be in register A.

Rotate Accumulator Right – RR A
The eight bits in the Accumulator are rotated one bit to the right. Bit 0 is rotated into the bit 7 position. No flags are affected.

Encoding: 0 0 0 0 0 0 1 1

The Accumulator holds the value 0C5H (11000101B). The following instruction,

RR A

leaves the Accumulator holding the value 0E2H (11100010B) with the carry unaffected.

	ORG 0H
	MOV A, #0C5H	; A = 11000101B
	RR A	; A = 11100010B
	END	; end of asm source file

Rotate Accumulator Right through Carry flag – RRC A
The eight bits in the Accumulator and the carry flag are together rotated one bit to the right. Bit 0 moves into the carry flag; the original value of the carry flag moves into the bit 7 position. No other flags are affected.

Encoding: 0 0 0 1 0 0 1 1

The Accumulator holds the value 0C5H (11000101B), the carry is zero. The following instruction,

RRC A

leaves the Accumulator holding the value 62 (01100010B) with the carry set.

	ORG 0H
	MOV A, #0C5H	; A = 11000101B
	RRC A	; A = 01100010B, C = 1
	END	; end of asm source file

Rotate Accumulator Left – RL A
The eight bits in the Accumulator are rotated one bit to the left. Bit 7 is rotated into the bit 0 position. No flags are affected.

Encoding: 0 0 1 0 0 0 1 1

The Accumulator holds the value 0C5H (11000101B). The following instruction,

RL A

leaves the Accumulator holding the value 8BH (10001011B) with the carry unaffected.

	ORG 0H
	MOV A, #0C5H	; A = 11000101B
	RL A	; A = 10001011B
	END	; end of asm source file

Rotate Accumulator Left through the Carry flag – RLC A
The eight bits in the Accumulator and the carry flag are together rotated one bit to the left. Bit 7 moves into the carry flag; the original state of the carry flag moves into the bit 0 position. No other flags are affected.

Encoding: 0 0 1 1 0 0 1 1

The Accumulator holds the value 0C5H(11000101B), and the carry is zero. The following instruction,

RLC A

leaves the Accumulator holding the value 8AH (10001010B) with the carry set.

	ORG 0H
	MOV A, #0C5H	; A = 11000101B
	RLC A	; A = 10001010B, C = 1
	END	; end of asm source file

Serializing Data:
Serializing data is a way of sending a byte of data one bit at a time through a single pin of microcontroller. There are two ways to transfer a byte of data serially:

• Using serial port
• Transferring data one bit at a time through a single pin

Serializing data is one of the most widely used applications of the rotate instruction. Using that concept and the rotate instruction, we transfer a byte of data serially (one bit at a time).

	ORG 0H
	MOV A, #0C5H
	MOV R7, #8
LOOP :	RRC A	; move bit to carry
	MOV P0.1, C	; send bit on data pin
	DJNZ R7, LOOP
	END	; end of asm source file

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Related topics:
8051 Complement Accumulator and Bit   |   8051 Set Bit   |   8051 Clear Accumulator and Bit   |   8051 Swap Nibble   |   8051 Exchange Accumulator   |   8051 Exchange Digit   |   8051 Push and Pop   |   8051 No Operation

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List of topics: 8051

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