Arm VERSION 1.2 Manuale Utente Scaricare il pdf (Pagina 37)

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101

Writing ARM and Thumb Assembly Language

2.6 Loading constants into registers

You cannot load an arbitrary 32-bit immediate constant into a register in a single

instruction without performing a data load from memory. This is because ARM

instructions are only 32 bits long.

Thumb instructions have a similar limitation.

You can load any 32-bit value into a register with a data load, but there are more direct

and efficient ways to load many commonly-used constants. You can also include many

commonly-used constants directly as operands within data-processing instructions,

without a separate load operation at all.

The following sections describe:

• how to use the

MOV

and

MVN

instructions to load a range of immediate values, see

Direct loading with MOV and MVN on page 2-26

• how to use the

LDR

pseudo-instruction to load any 32-bit constant, see Loading

with LDR Rd, =const on page 2-27

• how to load floating-point constants, see Loading floating-point constants on

page 2-29.

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Arm VERSION 1.2 Manuale Utente Pagina 37

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