10.5 Syntax of Operand2 as a register with optional shift

When you use an Operand2 register in an instruction, you can optionally also specify a shift value.

Syntax

Rm {, shift}
where:
Rm
is the register holding the data for the second operand.
shift
is an optional constant or register-controlled shift to be applied to Rm. It can be one of:
ASR #n
arithmetic shift right n bits, 1 ≤ n ≤ 32.
LSL #n
logical shift left n bits, 1 ≤ n ≤ 31.
LSR #n
logical shift right n bits, 1 ≤ n ≤ 32.
ROR #n
rotate right n bits, 1 ≤ n ≤ 31.
RRX
rotate right one bit, with extend.
type Rs
register-controlled shift is available in ARM code only, where:
type
is one of ASR, LSL, LSR, ROR.
Rs
is a register supplying the shift amount, and only the least significant byte is used.
-
if omitted, no shift occurs, equivalent to LSL #0.

Usage

If you omit the shift, or specify LSL #0, the instruction uses the value in Rm.
If you specify a shift, the shift is applied to the value in Rm, and the resulting 32-bit value is used by the instruction. However, the contents of the register Rm remain unchanged. Specifying a register with shift also updates the carry flag when used with certain instructions.
Related concepts
10.6 Shift operations
Related reference
10.3 Flexible second operand (Operand2)
10.4 Syntax of Operand2 as a constant
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