ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.10.2 Register-based maps

In many cases, you can use the same register as the base register every time you access

a data structure. You can include the name of the register in the base address of the map.

Example2-17 shows such a register-b ased map. The labels defined in the map include

the register.

Example2-17

MAP 0,r9

consta FIELD 4 ; consta uses four bytes, located at offset 0 (from r9)

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 Example2-17, you can access the data structure wherever it is:

ADR r9,datastart

LDR r4,constb ; => LDR r4,[r9,#4]

constb

contains the offset of the data item from the start of the data structure, and also

includes the base register. In this case the base register is r9, defined in the

MAP

directive.