Barrel Shifter

Question 1

Barrel Shifter

Answer 1

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.)

Question 2

LSL – Logical Shift Left

Example: Logical Shift Left by 4.

Answer 2

Equivalent to << in C.

Question 3

LSR – Logical Shift Right

Example: Logical Shift Right by 4.

Answer 3

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

Question 4

ASR – Arithmetic Shift Right

Example: Arithmetic Shift Right by 4, positive value.

Question 5

ASR – Arithmetic Shift Right

Example: Arithmetic Shift Right by 4, negative value.

Answer 5

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

Question 7

ROR – Rotate Right

Example: Rotate Right by 4.

Answer 7

Bit rotate with wrap-around.

Question 8

RRX – Rotate Right Extended

Example: Rotate Right Extended.

Answer 8

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

Question 9

MOV r0, r0, LSL #1

Answer 9

Multiply R0 by two.

Question 10

MOV r1, r1, LSR #2

Answer 10

Divide R1 by four (unsigned).

Question 11

MOV r2, r2, ASR #2

Answer 11

Divide R2 by four (signed).

Question 12

MOV r3, r3, ROR #16

Answer 12

Swap the top and bottom halves of R3.

Question 13

ADD r4, r4, r4, LSL #4

Answer 13

Multiply R4 by 17. (N = N + N * 16)

Question 14

RSB r5, r5, r5, LSL #5

Answer 14

Multiply R5 by 31. (N = N * 32 – N)

Question 15

Certain C operations don’t map onto available CPU operations. \_\_\_ and\_\_\_ are two of those operations.

Answer 15

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

Question 16

Certain ARM instructions such as \_\_\_, ___ and \_\_\_ cannot use the barrel shifter.

Answer 16

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