How is SUB encoded differently from ADD?
They share opcode and funct3. SUB uses funct7=0100000, while ADD uses funct7=0000000.
SUB
RISC-V SUB Instruction Details
Instruction ManualR-typeSubtract rs2 from rs1, place result in rd
Instruction Syntax
sub rd, rs1, rs2
Operand Breakdown
Destination rd: register receiving the operation result.
Source rs1: register holding the first operand.
Source rs2: register holding the second operand.
RV32IArithmetic
Instruction Behavior
SUB (R-type) subtracts rs2 from rs1 (overflow ignored), writing to rd. funct7=0100000, funct3=000. Bit30=1 in funct7 distinguishes SUB from ADD (same funct3/opcode). There is no SUBI; use ADDI with a negated immediate instead.
SUB Decode And Execute Animation
Uses the same rhythm as the ADDI page: machine-code fields, fixed-field identification, operand reads, instruction-specific execution, and ISA-visible state update.
rd
rs1
rs2
sub
,
,
Execution Context
31..25
24..20
19..15
14..12
11..7
6..0
0100000
funct7
00111
rs2
00110
rs1
000
funct3
00101
rd
0110011
opcode
Execution Data Path
instruction
0x407302B3
opcode
0110011 -> OP
funct3
000 -> SUB
funct7
0100000 -> alternate ALU op
rd / rs1 / rs2
t0(x5) / t1(x6) / t2(x7)
immediate
not used
ALU
0x00000014 - 0x00000007 = 0x0000000D
x5
t0(x5) = 0x0000000D
Current Step
Concept Step: receive the 32-bit instruction encoding
The machine code is split by the current instruction format; the animation starts from encoding/decode.
encoding: 0x407302B3
syntax : sub t0(x5), t1(x6), t2(x7)
result : t0(x5) = 0x0000000D
This animation shows ISA-visible decode and state changes, not any specific CPU pipeline, cache, prediction, or timing implementation.
Quick Understanding & Search Notes
SUB subtracts rs2 from rs1 and writes rd. It is register subtraction; the base integer ISA has no immediate subtract, so subtracting a small constant is normally ADDI with a negative immediate.
SUB shares opcode/funct3 with ADD and is distinguished by funct7=0100000.
Integer subtraction overflow does not raise an exception.
Common Usage Scenarios
Address & Pointer
Understand this scenario with real code like «sub x5, x6, x7 # x5 = x6 - x7».
Basic Arithmetic
Understand this scenario with real code like «sub x5, x6, x7 # x5 = x6 - x7».
Numerical Computing
Understand this scenario with real code like «sub x5, x6, x7 # x5 = x6 - x7».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is R-type.
- Confirm the operand order matches the example.
Semantic Check
- Ensure the destination register usage is compatible with the calling convention.
- Confirm this is not the lower-level form of a pseudo-instruction expansion.
Pitfalls / Common Confusions
Overflow silently ignored
No immediate subtraction; use ADDI with negated immediate
funct7 bit30=1 distinguishes SUB from ADD; wrong funct7 changes operation
FAQ
How do I subtract an immediate?
Use ADDI with a negative immediate, for example addi rd, rs1, -8 to subtract 8.