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Writing ARM and Thumb Assembly Language
ARM DUI 0068B Copyright © 2000, 2001 ARM Limited. All rights reserved. 2-61
Example 2-23 on page 2-60 contains separate
LDR
pseudo-instructions to load the
address of each of the data items. Each
LDR
pseudo-instruction is converted to a separate
instruction by the assembler. However, it is possible to access the entire data section
with a single
LDR
pseudo-instruction. Example 2-24 shows how to do this. Both speed
and code size are improved.
Example 2-24
AREA data, DATA
StartOfData EQU 0x1000
EndOfData EQU 0x2000
DataAreaBase RN r11
MAP 0,DataAreaBase
StartOfUsedData FIELD 0
Integer FIELD 4
String FIELD MaxStrLen
Array FIELD ArrayLen*8
BitMask FIELD 4
EndOfUsedData FIELD 0
UsedDataLen EQU EndOfUsedData - StartOfUsedData
ASSERT UsedDataLen <= (EndOfData - StartOfData)
AREA code, CODE
LDR DataAreaBase,=StartOfData
MOV r0,#1
STR r0,Integer
MOV r0,#0
STRB r0,String
MOV r0,#0xA000000A
STRB r0,BitMask
Note
In this example, the
MAP
directive is:
MAP 0, DataAreaBase
not:
MAP StartOfData,DataAreaBase
The
MAP
and
FIELD
directives give the position of the data relative to the
DataAreaBase
register, not the absolute position. The
LDR DataAreaBase,=StartOfData
statement
provides the absolute position of the entire data section.