ARM/Instruction format
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< ARM
The ARM instruction format is split into three types of instructions:
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.
