Part 6 – Overview of the Linux ARM assembly instruction set

ARM instruction set can be divided into jump instruction, data processing instruction, program state register transfer instruction, Load/Store instruction, coprocessor instruction and abnormal interrupt generation instruction.

The ARM command length is a fixed 32 bits. Typical ARM instruction encoding format is as follows:

Opcode: Instruction operator encoding

Cond is the conditional encoding of instruction execution

S is the value that determines whether the operation of the instruction affects the CPSR

Rd Target register code

Rn contains the register code of the first operand

Shifter_operand: indicates the second operand.

Condition code

The conditional code cond is four bits.

Reference:

Developer.arm.com/docs/den002…

Addressing mode of ARM instruction

The main types are as follows:

• The way operands of data processing instructions are addressed
• Address for word and unsigned byte Load/Store instructions
• Addressing for miscellaneous Load/Store instructions
• Batch Load/Store instruction addressing mode
• The addressing mode of the Load/Store instruction of the coprocessor.

Storage access instruction:

LDR

Load data from memory into registers.

STR

Stores data to a specified storage unit.

LDM

Loads multiple data from the specified storage unit into a register list.

STM

Stores the data of a register list of data to the specified storage location.

PUSH

Push the register onto the full decrement stack.

POP

Eject data from decrement stack to register.

SWP

Used for data exchange between registers and memory.

ADD the sample

This works on 32 if it’s 64 bits if bx LR needs to be replaced.

as -g -o add.o add.s

gcc -o add add.o

64-bit bits:

as -g -o add.o add.s

gcc -o add add.o

Jump instruction

There are two ways to jump programs in ARM:

• Jump instruction
• Write the destination address value directly to the PC register (R15).

Directly write the target address value into the PC register, you can realize arbitrary jump in the 4GB address space, this jump instruction is also called long jump.

You can also jump from the current instruction forward or backward 32MB of address space. There are four types of jump commands:

B: Jump instruction

BL: jump instruction to be returned

BLX: jump instruction with return and state switching

BX: indicates the jump instruction to be switched

System instructions