Shifts
There are eleven types of shift possible in Data Processing instructions where a "shift" is available. This document describes them.
The I bit, bit 25, of the instruction determines if the shift type is Immediate or Register. With this selection made, the lower twelve bits of the instruction define the shift.
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Non shifts
There are two non-shift types - the immediate and a register reference.
Immediate values are values that are directly encoded into the instruction, such as:
ADD R0, R1, #123
There is, however, a slight complication in that not all values can be represented. The immediate works by optionally shifting an eight bit value. Thus, any value up to &FF is valid. Likewise, it is possible to shift that around, so &F000000F is valid, as is &000AD000. However &00000123 is not (for &123 cannot be described by rotating an eight bit value).
In the case of requiring a larger value, you should ADD or SUB until you have the desired number, or LDR it from a data word.
This is encoded into the instruction as:
Register values are references to a register, for example:
ADD R0, R1, R2
As a technical note, this is actually stored in memory as a Logical Shift Left by immediate with the shift being zero.
This is encoded into the instruction as:
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.
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
.
This shift is encoded for immediate shifts as:
and for register shifts as:
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
.
The shift amount can be from one to thirty two, and the carry-out is the value of the last bit shifted out.
This shift is encoded for immediate shifts as:
and for register shifts as:
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
.
This shift is encoded for immediate shifts as:
and for register shifts as:
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'.
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.
It is not possible to ROR #0, as that encoding is used to signify RRX.
The carry-out is the value of the last bit shifted.
This shift is encoded for immediate shifts as:
and for register shifts as:
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.
This shift is encoded for immediate shifts as:
It is encoded into the space that would be used for ROR, #0.
How to Rotate Left with Extend
It is possible to mimic the behaviour of RRX in the leftwards direction using the ADC instruction:
ADCS R0, R0, R0