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• 4. Assembly Language Programming
• Data Transfer and Addressing Modes
  • Data Transfer Introduction
  • Register-Indirect Addressing
  • PC-Relative Addressing
  • Pre-indexed Addressing
  • Post-indexed Addressing
  • Byte Load and Store
• Shift and Rotate Instructions
  • Shift and Rotate Introduction
  • Logical Shift Operations
  • Arithmetic Shift Operations
  • Rotate Operations
• Putting It All Together
  • Flowcharts for Assembly Language Programming
  • Example Program

Logical Shift Operations

ARM has two logical shift operations, namely LSL (Logical Shift Left) and LSR (Logical Shift Right).

LSL is a logical shift left by 0 to 31 places. The vacated bits at the least significant end of the word are filled with zeros.

LSR is a logical shift right by 0 to 32 places. The vacated bits at the most significant end of the word are filled with zeros.

Consider the following ARM instruction with r1 = 3 and r2 = 5

ADD

r0, r1, r2

; r0 := r1 + r2 which is r0 := 3 + 5 = 8

Now try the same instruction with a LSL operand, say LSL #3 (logical shift left 3 places which is equivalent to multiplying by 8 (2^3)):

ADD

r0, r1, r2, LSL #3

; r0 := r1 + (8 x r2) which is r0:= 3 +(8x5) =43

The value of a LSL or a LSR does not have to be a literal. It can instead be sourced from a register (but must still be in the ranges 0 to 31 or 0 to 32 respectively).

Consider a MOV instruction with r1 = 168 and r2 = 3:

MOV	r0, r1, LSR r2	; shift the binary value of 168 3 places to the right
		; 168 = 0000 0000 0000 0000 0000 0000 1010 1000
		; shifted 3 places,
		; becomes 0000 0000 0000 0000 0000 0000 0001 0101
		; r0 := 21

For positive numbers, LSR 3 is the same as dividing by 2 ^ 3 (8).