logical_shifts_and_rotations

Logical Shifts and Rotations

Theory

In ARM assembly, logical shifts and rotations are operations that allow you to manipulate the bits of a register value.

LSL (Logical Shift Left)

The LSL instruction shifts all bits in a specified register to the left by a certain number of positions. Bits that are shifted off the end are lost, and zeros are shifted in from the right. This operation effectively multiplies the register value by 2 for each shift position.

LSR (Logical Shift Right)

The LSR instruction shifts all bits in a specified register to the right by a certain number of positions. Bits that are shifted off the end are lost, and zeros are shifted in from the left. This operation effectively divides the register value by 2 for each shift position, rounding towards zero.

ROR (Rotate Right)

The ROR instruction rotates all bits in a specified register to the right by a certain number of positions. The bits that are rotated off the end are reintroduced on the left. This is different from LSR as it preserves all bits and simply changes their positions.

Code Explanation

.global _start
_start:
	MOV R0, #15      ; Load literal value 15 into R0.
	LSL R0, #1       ; Perform logical shift left on R0 by 1 position.
	LSR R0, #1       ; Perform logical shift right on R0 by 1 position.
	
	MOV R1, R0, LSL #1 ; Move the value of R0 shifted left by 1 into R1.
	
	ROR R0, #1       ; Rotate right R0 by 1 position.

Line-by-Line Explanation

• MOV R0, #15: This instruction moves the literal value 15 into register R0.
• LSL R0, #1: This instruction shifts all bits in R0 to the left by one position. The value in R0 becomes 30 (15 * 2).
• LSR R0, #1: This instruction shifts all bits in R0 to the right by one position. The value in R0 becomes 15 (30 / 2).
• MOV R1, R0, LSL #1: This instruction moves into R1 the value of R0 shifted left by one position. The value in R1 becomes 30.
• ROR R0, #1: This instruction rotates all bits in R0 to the right by one position. If we consider that before this operation, R0 had a value of 15 (00001111 in binary), after rotation, it would be (10000111 in binary), which is 135.

Output Explanation

After executing the above lines of code:

• The final value in R0 is 135, due to the rotate right operation.
• The final value in R1 is 30, as it was moved from R0 after performing a logical shift left.