11.4 Syntax of Operand2 as a constant

An Operand2 constant in an instruction has a limited range of values.

Syntax

#constant

where constant is an expression evaluating to a numeric value.

Usage

In ARM instructions, constant can have any value that can be produced by rotating an 8-bit value right by any even number of bits within a 32-bit word.

In Thumb instructions, 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 listed 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

If the value of an Operand2 constant is not available, but its logical inverse or negation is available, then the assembler produces an equivalent instruction and inverts or negates the constant.

For example, an assembler might assemble the instruction CMP Rd, #0xFFFFFFFE as the equivalent instruction CMN Rd, #0x2.

Be aware of this when comparing disassembly listings with source code.

You can use the --diag_warning 1645 assembler command line option to check when an instruction substitution occurs.

Non-ConfidentialPDF file icon PDF versionARM DUI0473M
Copyright © 2010-2016 ARM Limited or its affiliates. All rights reserved.