ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.10.1 Relative maps

To access data more than 4KB away from the current instruction, you can use a

LDR r4,[r9,#offset]

offset

is limited to 4096, so r9 must already contain a value within 4KB of the address

of the data.

Example2-16

MAP 0

consta FIELD 4 ; consta uses four bytes, located at offset 0

constb FIELD 4 ; constb uses four bytes, located at offset 4

x FIELD 8 ; x uses eight bytes, located at offset 8

y FIELD 8 ; y uses eight bytes, located at offset 16

string FIELD 256 ; string is up to 256 bytes long, starting at offset 24

Using the map in Example2-16, you can access the data structure using the following

instructions:

MOV r9,#4096

LDR r4,[r9,#constb]

The labels are relative to the start of the dat a structure. The reg ister used to hold the s tart

address of the map (r9 in this case) is called the base register.

There are likely to be many

LDR

STR

instructions acces sing data in this da ta structure.

This map does not contain the location of the data structure. The location of the

structure is determined by the value loaded into the base register at runtime.

The same map can be used to describe many instances of the data structure. These can

be located anywhere in memory.

There are restrictions on what addresses can be loaded into a register using the

MOV

instruction. Refer to Loading addresses into registers on page2-30 for details of how to

load arbitrary addresses.

Note

r9 is the static base register (sb) in the ARM-Thumb Procedure Call Standard. Refer to

the Using the Procedure Call Standard chapter in ADS Developer Guide for further

information.