ARM: Introduction to ARM: Barrel Shifter

by David Thomas on

Barrel Shifter

The barrel shifter is a functional unit which can be used in a number of different circumstances. It provides five types of shifts and rotates which can be applied to Operand2. (These are not operations themselves in ARM mode.)

LSL – Logical Shift Left

Example: Logical Shift Left by 4.

LSL diagram.

Equivalent to << in C.

LSR – Logical Shift Right

Example: Logical Shift Right by 4.

LSR diagram.

Equivalent to >> in C. i.e. unsigned division by a power of 2.

ASR – Arithmetic Shift Right

Example: Arithmetic Shift Right by 4, positive value.

ASR shifting in zero diagram.

Example: Arithmetic Shift Right by 4, negative value.

ASR shifting in one diagram.

Equivalent to >> in C. i.e. signed division by a power of 2.

ROR – Rotate Right

Example: Rotate Right by 4.

ROR diagram.

Bit rotate with wrap-around.

RRX – Rotate Right Extended

Example: Rotate Right Extended.

RRX diagram.

33-bit rotate with wrap-around through carry bit.

Examples

  • MOV r0, r0, LSL #1
    • Multiply R0 by two.
  • MOV r1, r1, LSR #2
    • Divide R1 by four (unsigned).
  • MOV r2, r2, ASR #2
    • Divide R2 by four (signed).
  • MOV r3, r3, ROR #16
    • Swap the top and bottom halves of R3.
  • ADD r4, r4, r4, LSL #4
    • Multiply R4 by 17. (N = N + N * 16)
  • RSB r5, r5, r5, LSL #5
    • Multiply R5 by 31. (N = N * 32 - N)

Remarks

At the start I mentioned that certain C operations don’t map onto available CPU operations. ROR and RRX are two of those operations.

Certain ARM instructions such as MUL, CLZ and QADD cannot use the barrel shifter.

Note that right shifting negative signed quantities is strictly implementation defined behaviour in C. The compiler is allowed to choose whether it performs a logical or an arithmetic shift. (That said, it’s always implemented as an arithmetic shift in all of the ARM toolchains the author has used.)

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