ARM VERSION 1.2 - Manual (Page 332)

Directives Reference
7-50 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
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 2 0 to 2 31 .
offset can be any numeric expression.
The current location is aligned to the next address of the form:
offset + n * expr
If 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
page 7-13) if the data is to be accessed using LDRD or 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 specified differently (see AREA on page 7-52 and Examples on
page 7-51).