## Shifting |

The ARM processor incorporates a barrel shifter that can be used with the data processing instructions (ADC, ADD, AND, BIC, CMN, CMP, EOR, MOV, MVN, ORR, RSB, SBC, SUB, TEQ, TST). You can also use the barrel shifter to affect the index value in LDR/STR operations.

There are six mnemonics for the different shift types:

LSL Logical Shift Left ASL Arithmetic Shift Left LSR Logical Shift Right ASR Arithmetic Shift Right ROR Rotate Right RRX Rotate Right with Extend

`ASL`

and `LSL`

are the same, and may be freely interchanged.
You can specify a shift with an immediate value (from zero to thirty one), or by a register which contains a value between zero and thirty one.

Rx, LSL #n or Rx, ASL #n or Rx, LSL Rn or Rx, ASL RnThe contents of Rx will be taken and shifted to a more significant position by the amount specified by 'n' or in the register Rn. The least significant bits are filled with zeroes. The high bits shifted off to the left are discarded, except for the notional bit thirty three (i.e., the last bit to be shifted off) which becomes the value of the carry flag on exit from the barrel shifter.

Consider the following:

MOV R1, #12 MOV R0, R1, LSL#2On exit, R0 is 48. The instructions forming the sum

`R0 = #12, LSL#2`

is equivalent
to the BASIC `R0 = 12 << 2`

Rx, LSR #n or Rx, LSR RnThis does the notional opposite of shifting left. Everything is shifted to the right, to a less significant position. It is the same as

`register = value >>> shift`

.

Rx, ASR #n or Rx, ASR RnThis is similar to LSR, with the exception that the high bits are filled with the value of bit 31 of the register being shifted (Rx), in order to preserve the sign in 2's complement maths. It is the same as

`register = value >> shift`

.

Rx, ROR #n or Rx, ROR RnRotate Right is similar to a Logical Shift Right, except the bits which would normally be shifted off the right are replaced on the left, this the bits 'rotate'. An operation where the amount to be shifted by is 32 would result in the output being identical to the input as all bits would have been shifted by 32 places, i.e., right back to where they started!

Rx, RRXThis is a ROR#0 operation, which rotates one place to the right - the difference is that the processor's Carry flag is used to provide a 33 bit quantity to be shifted.

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Copyright © 2001 Richard Murray