User guide

The benefits of using a scatter description file are:

• All the (target-specific) absolute addresses chosen for your devices, code, and data are located in one file and

maintenance is simplified.

• If you decide to change your memory map (for example if peripherals are moved), you do not have to rebuild

your entire project but only to re-link the existing objects.

For a description of scatter loading, see the Linker and Utilities Guide

. For a description of how to specify code and

data sections from C and C++, see the section on pragmas in the Compiler and Libraries Guide

6.9.5 Code efficiency

The ARM compiler normally uses a base register plus the immediate offset field available in the load/store instruction

to compile struct member or specific array element access.

The ARM instruction set, LDR/STR word/byte have a 4Kbyte range, but LDRH/STRH has a smaller immediate offset of

256bytes.

The Thumb instruction set is much more restricted in addressing range than the ARM instructions. The Thumb

LDR/STR has a range of 32 words, LDRH/STRH has a range of 32 halfwords, LDRB/STRB has a range of 32 bytes.

You must group related peripheral registers near to each other if possible. The compiler will generally do a good job

of minimizing the number of instructions required to access the array elements or structure members by using base

registers.

There is a choice between one big C struct/array for the whole I/O space and smaller per-peripheral structs. There is

not much difference in efficiency. The big struct might be a benefit if you are using ARM code where a base pointer

can have a 4Kbyte range (for word/byte access) and the entire I/O space is less than 4KB. Smaller structs for each

peripheral are more maintainable.

Writing Code for ROM