ARM VERSION 1.2 - Manual (Page 68)

Writing ARM and Thumb Assembly Language
2-56 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
2.10.5 Forcing correct alignment
You are likely to have problems if you include some character variables in the data
structure, as in Example 2-20. This is because a lot of words are misaligned.
Example 2-20
StartOfData EQU 0x1000
EndOfData EQU 0x2000
MAP StartOfData
Char FIELD 1
Char2 FIELD 1
Char3 FIELD 1
Integer FIELD 4 ; alignment = 3
Integer2 FIELD 4
String FIELD MaxStrLen
Array FIELD ArrayLen*8
BitMask FIELD 4
EndOfUsedData FIELD 0
ASSERT EndOfUsedData <= EndOfData
You cannot use the ALIGN directive, because the ALIGN directive aligns the current
location within memory. MAP and FIELD directives do not allocate any memory for the
structures they define.
You could insert a dummy FIELD 1 after Char3 FIELD 1 . However, this makes
maintenance difficult if you change the number of character variables. You must
recalculate the right amount of padding each time.
Example 2-21 on page 2-57 shows a better way of adjusting the padding. The example
uses a
FIELD directive with a 0 operand to label the end of the character data. A second
FIELD directive inserts the correct amount of padding based on the value of the label. An
:AND: operator is used to calculate the correct value.
The (-EndOfChars):AND:3 expression calculates the correct amount of padding:
0 if EndOfChars is 0 mod 4;
3 if EndOfChars is 1 mod 4;
2 if EndOfChars is 2 mod 4;
1 if EndOfChars is 3 mod 4.
This automatically adjusts the amount of padding used whenever character variables are
added or removed.