Arm VERSION 1.2 Uživatelský manuál stáhnout pdf (Strana 73)

100

101

Writing ARM and Thumb Assembly Language

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

LDR DataAreaBase,=StartOfData

statement

provides the absolute position of the entire data section.

1 2 ... 68 69 70 71 72 73 74 75 76 77 78 ... 359 360

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