ARM VERSION 1.2 manual Forcing correct alignment

Writing ARM and Thumb Assembly Language

2.10.5Forcing 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:3expression 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.

2-56 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B