ARM VERSION 1.2

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 t he error:

Immediate n out of range for this operation

2.6.2 Loading 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

MVN

instruction, the assembler

generates the appropriate instruction.

•If the constant cannot be constructed with a

MOV

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 th e

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 page2-28 for more

information.

Refer to LDR ARM pseudo-instruction on page4-82 for a description of the syntax of

the

LDR

pseudo-instruction.