ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.2.5 ARM instruction set overview

All ARM instructions are 32 bits long. Instructions are stored word-aligned, so the least

significant two bits of instruction addresses are always zero in ARM state. Some

instructions use the least significant bit to determine whether the code being branched

to is Thumb code or ARM code.

See Chapter 4 ARM Instruc tion Reference for detailed information on the syntax of the

ARM instruction set.

ARM instructions can be classified into a number of functional groups:

•Branch instructions

•Data processing instructions

•Single register load and store instructions on page2-7

•Multiple register load and store instructions on page2-7

•Status register access instructions on page2-7

•Semaphore instructions on page2-7

•Coprocessor instructions on page 2-7.

Branch instructions

These instructions are used to :

•branch backwards to form loops

•branch forward in conditional structures

•branch to subroutines

•change the processor from ARM state to Thumb state.

Data processing instructions

These instructions operate on the general-purpose registers. They can perform

operations such as addition, subtraction, or bitwise logic on the contents of two registers

and place the result in a third regis ter. They can also operate on the value in a single

immediate value).

Long multiply instructions (unavailable in some architectures) give a 64-bit result in

two registers.