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 Instruction 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

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 register. They can also operate on the value in a single register, or on a value in a register and a constant supplied within the instruction (an immediate value).

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

Single register load and store instructions

These instructions load or store the value of a single register from or to memory. They can load or store a 32-bit word or an 8-bit unsigned byte. In ARM architecture v4 and above they can also load or store a 16-bit unsigned halfword, or load and sign extend a 16-bit halfword or an 8-bit byte.

Multiple register load and store instructions

These instructions load or store any subset of the general-purpose registers from or to memory. Refer to Load and store multiple register instructions for a detailed description of these instructions.

Status register access instructions

These instructions move the contents of the CPSR or an SPSR to or from a general-purpose register.

Semaphore instructions

These instructions load and alter a memory semaphore.

Coprocessor instructions

These instructions support a general way to extend the ARM architecture.

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