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.
<< in C.
LSR – Logical Shift Right
Example: Logical Shift Right by 4.
>> in C. i.e. unsigned division by a power of 2.
ASR – Arithmetic Shift Right
Example: Arithmetic Shift Right by 4, positive value.
Example: Arithmetic Shift Right by 4, negative value.
>> in C. i.e. signed division by a power of 2.
ROR – Rotate Right
Example: Rotate Right by 4.
Bit rotate with wrap-around.
RRX – Rotate Right Extended
Example: Rotate Right Extended.
33-bit rotate with wrap-around through carry bit.
Examples of Barrel Shifting
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)
At the start I mentioned that certain C operations don’t map onto available CPU operations.
RRX are two of those operations.
Certain ARM instructions such as
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.)