ARM/Instruction format

The ARM instruction format is split into three types of instructions:

• ARM/Data-processing instruction
• ARM/Memory instruction
• ARM/Branch instruction

This page will go through the instruction format of each type.

Data-processing instruction format

Data-processing instructions take in the following operands

• Rn: the first source register
• Src2: the second source, which may be register or immediate
• Rd: destination register

and the following is used to control the instruction

• cond: specifies conditional execution (31:28)
• op: the opcode, which is 0b00 for data processing instructions (27:26)
• funct: the function to perform (25:20)

Control fields

For all data processing instructions, the opcode is 0b00. The exact operation is specified by the funct.

The funct field is further split into three subfields:

• I is a one-bit field that specifies whether src2 is an immediate.
• cmd specifies which DP operation is performed
• S is a one-bit field that specifies whether to set condition flags.

Src2

This 12-bit field can represent an immediate, a constant-shifted register, or a register-shifted register.

• Immediate: rot4 imm8
• Register: shamt5 sh2 0 Rm
• Register shifted register: Rn 0 sh2 1 Rm

Src2

Allows for immediate (a constant with rotation), register (with a static shift amount), or register-shifted register.

The immediate is rotated imm8 ROR rot*2, limiting precision but allowing for a greater range of values.

Memory instruction format

Memory instructions have the following fields

• op: 01
• Rn: base register
• Rd: destination (load), source (store). Holds the actual loaded/stored value.
• Src2: offset
• funct: 6 control bits

Control bits

Control bits determine which memory instruction is being executed

• I bar: immediate bar, determine offset type
• P: Preindex (offset is added to address before accessing)
• U: Add (determine whether offset is added or subtracted)
• B: Operate with Bytes
• W: Writeback (address accessed is wrote to register)
• L: Operation is load instruction (store if 0)

Branch instruction

It's literally just an L and some immediates. L is for linking and jumping back.