ARM VERSION 1.2 - Manual (Page 74)

Writing ARM and Thumb Assembly Language
2-62 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
If you use the same technique for a section of memory containing memory-mapped I/O
(or whose absolute addresses must not change for other reasons), you must take care to
keep the code maintainable.
One method is to add comments to the code warning maintainers to take care when
modifying the definitions. A better method is to use definitions of the absolute addresses
to control the register-based definitions.
Using MAP offset,reg followed by label FIELD 0 makes label into a register-based
symbol with register part reg and numeric part offset . Example 2-25 shows this.
Example 2-25
StartOfIOArea EQU 0x1000000
SendFlag_Abs EQU 0x1000000
SendData_Abs EQU 0x1000004
RcvFlag_Abs EQU 0x1000008
RcvData_Abs EQU 0x100000C
IOAreaBase RN r11
MAP (SendFlag_Abs-StartOfIOArea),IOAreaBase
SendFlag FIELD 0
MAP (SendData_Abs-StartOfIOArea),IOAreaBase
SendData FIELD 0
MAP (RcvFlag_Abs-StartOfIOArea),IOAreaBase
RcvFlag FIELD 0
MAP (RcvData_Abs-StartOfIOArea),IOAreaBase
RcvData FIELD 0
Load the base address with LDR IOAreaBase,=StartOfIOArea . This allows the individual
locations to be accessed with statements like LDR R0,RcvFlag and STR R4,SendData .