Introduction to ARM

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

LSL

e.g. multiplication by a power of 2.
Equivalent to << in C.

LSR – Logical Shift Right

LSR

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

ASR – Arithmetic Shift Right

ASR +ve

ASR -ve

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

ROR – Rotate Right

ROR

Bit rotate with wrap-around.

RRX – Rotate Right Extended

RRX

Bit rotate with wrap-around including 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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