3.3.3. Flexible second operand

Many general data processing instructions have a flexible second operand. This is shown as Operand2 in the descriptions of the syntax of each instruction.

Operand2 can be a:

Constant

You specify an Operand2 constant in the form:

#constant

where constant can be:

  • Any constant that can be produced by shifting an 8‑bit value left by any number of bits within a 32‑bit word.

  • Any constant of the form 0x00XY00XY.

  • Any constant of the form 0xXY00XY00.

  • Any constant of the form 0xXYXYXYXY.

Note

In these constants, X and Y are hexadecimal digits.

In addition, in a small number of instructions, constant can take a wider range of values. These are described in the individual instruction descriptions.

When an Operand2 constant is used with the instructions MOVS, MVNS, ANDS, ORRS, ORNS, EORS, BICS, TEQ or TST, the carry flag is updated to bit[31] of the constant, if the constant is greater than 255 and can be produced by shifting an 8-bit value. These instructions do not affect the carry flag if Operand2 is any other constant.

Instruction substitution

Your assembler might be able to produce an equivalent instruction in cases where you specify a constant that is not permitted. For example, an assembler might assemble the instruction CMP Rd, #0xFFFFFFFE as the equivalent instruction CMN Rd, #0x2.

Register with optional shift

You specify an Operand2 register in the form:

Rm {, shift}

Where:

Rm

Is the register holding the data for the second operand.

shift

Is an optional 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.

-

If omitted, no shift occurs, equivalent to LSL #0.

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 in the register Rm remain unchanged. Specifying a register with shift also updates the carry flag when used with certain instructions. For information on the shift operations and how they affect the carry flag, see Shift Operations.

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