ARM VERSION 1.2 manual Direct loading with MOV in Thumb state, Loading with LDR Rd, =const

Writing ARM and Thumb Assembly Language

Direct loading with MOV in Thumb state

In Thumb state you can use the MOV instruction to load constants in the range 0-255. You cannot generate constants outside this range because:

•The Thumb MOV instruction does not provide inline access to the barrel shifter. Constants cannot be right-rotated as they can in ARM state.

•The Thumb MVN instruction can act only on registers and not on constant values. Bitwise complements cannot be directly loaded as they can in ARM state.

If you attempt to use a MOV instruction with a value outside the range 0-255, the assembler reports the error:

Immediate n out of range for this operation.

2.6.2Loading with LDR Rd, =const

The LDR Rd,=const pseudo-instruction can construct any 32-bit numeric constant in a single instruction. Use this pseudo-instruction to generate constants that are out of range of the MOV and MVN instructions.

The LDR pseudo-instruction generates the most efficient code for a specific constant:

•If the constant can be constructed with a MOV or MVN instruction, the assembler generates the appropriate instruction.

•If the constant cannot be constructed with a MOV or MVN instruction, the assembler:

—places the value in a literal pool (a portion of memory embedded in the code to hold constant values)

—generates an LDR instruction with a program-relative address that reads the constant from the literal pool.

For example:

LDR	rn, [pc, #offset to		literal pool]
	;	load	register n with one word
	;	from	the address [pc + offset]

You must ensure that there is a literal pool within range of the LDR instruction generated by the assembler. Refer to Placing literal pools on page 2-28 for more information.

Refer to LDR ARM pseudo-instructionon page 4-82 for a description of the syntax of the LDR pseudo-instruction.

ARM DUI 0068B Copyright © 2000, 2001 ARM Limited. All rights reserved. 2-27