ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.10.3 Program-relative maps

You can use the program counter (r15) as the base register for a map. In this case, each

STM

LDM

instruction must be within 4KB of the data item it addresses, because the

offset is limited to 4KB. The data structure must be in the same section as the

instructions, because otherwise there is no guarantee that the data items will be within

range after linking.

Example2-18 shows a program fragment with such a map. It includes a directive which

allocates space in memory for the data structure, and an instruction which accesses it.

Example2-18

datastruc SPACE 280 ; reserves 280 bytes of memory for datastruc

MAP datastruc

consta FIELD 4

constb FIELD 4

x FIELD 8

y FIELD 8

string FIELD 256

code LDR r2,constb ; => LDR r2,[pc,offset]

In this case, there is no need to load the base register before loading the data as the

program counter already holds the correct address. (This is not actually the same as the

address of the

LDR

instruction, because of pipelining in the processor. However, the

assembler takes care of this for you.)