Shifts

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Revision as of 18:09, 3 June 2011

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The barrel shifter

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, or by a register which holds the value to be shifted by.

Logical or Arithmetic Shift Left

  Rx, LSL #n    or
  Rx, ASL #n    or
  Rx, LSL Rn    or
  Rx, ASL Rn

The 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 shift amount can be from zero to thirty one. The least significant bits introduced are 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.

Diagram of LSL

Consider the following:

  MOV    R1, #12
  MOV    R0, R1, LSL#2

On exit, R0 is 48. The instructions forming the sum R0 = #12, LSL#2 is equivalent to the BASIC R0 = 12 << 2, or in maths form, R0 = 12 × 4.

Logical Shift Right

  Rx, LSR #n    or
  Rx, LSR Rn

This 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.

Diagram of LSR

The shift amount can be from one to thirty two, and the carry-out is the value of the last bit shifted out.

Arithmetic Shift Right

  Rx, ASR #n    or
  Rx, ASR Rn

This 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 otherwise the same as register = value >> shift.

Diagram of ASR

Rotate Right

  Rx, ROR #n    or
  Rx, ROR Rn

Rotate Right is similar to a Logical Shift Right, except the bits which would normally be shifted off the right are replaced on the left, thus the bits 'rotate'.

Diagram of ROR

If it was possible to specify the amount to be shifted as 32, the result would be that all the bits would be rotated by 32 places, right back to their original positions.

The carry-out is the value of the last bit shifted.

Rotate Right with Extend

  Rx, RRX

This is a ROR #1 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.

Diagram of RRX
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