ARM VERSION 1.2 - page 332

Directives Reference

7.7.1 ALIGN

The

ALIGN

directive aligns the current location to a specified boundary by padding with

zeroes.

Syntax

ALIGN {expr{,offset}}

where:

expr

is a numeric expression evaluating to any power of 2 from 20 to 231.

offset

can be any numeric expression.

The current location is aligned to the next address of the form:

offset + n * expr

expr

is not specified,

ALIGN

sets the current location to the next word (four byte)

boundary.

Usage

Use

ALIGN

to ensure that your data and code is aligned to appropriate boundaries. This

is typically required in the following circumstances:

•The

ADR

Thumb pseudo-instruction can only load addresses that are word aligned,

but a label within Thumb code might not be word aligned. Use

ALIGN 4

to ensure

4-byte alignment of an address within Thumb code.

•Use

ALIGN

to take advantage of caches on some ARM processors. For example,

the ARM940T has a cache with 16-byte lines. Use

ALIGN 16

to align function

entries on 16-byte boundaries and maximize the efficiency of the cache.

•

LDRD

and

STRD

double-word data transfers must be 8-byte aligned. Use

ALIGN 8

before memory allocation directives such as

DCQ

(see Data definition directives on

page7-13) if the data is to be accessed using

LDRD

STRD

•A label on a line by itself can be arbitrarily aligned. Following ARM code is

word-aligned (Thumb code is half-word aligned). The label therefore does not

address the code correctly. Use

ALIGN 4

(or

ALIGN 2

for Thumb) before the label.

Alignment is relative to the start of the ELF section where the routine is located. The

section must be aligned to the same, or coarser, boundaries. The

ALIGN

attribute on the

AREA

directive is specifie d differently (see AREA on page 7-52 and Examples on

page7-51).