PC-relative Branch Range
Conditional branches use the B-type immediate, a signed PC-relative offset in 2-byte units, giving an approximate reach of +/-4 KiB from the current PC.
RISC-V Unprivileged ISA, RV32I control transfer instructionsBLTU
RISC-V BLTU Instruction Details
Instruction ManualB-typeBranch if rs1 is less than rs2 (unsigned comparison)
Instruction Syntax
bltu rs1, rs2, offset
Operand Breakdown
Source rs1: first comparison operand.
Source rs2: second comparison operand.
Immediate offset: 13-bit signed branch offset (bit 0 is implicitly 0, ±4 KiB range).
RV32IControl TransferBranch
Instruction Encoding
31..25
imm[12|10:5]
24..20
rs2
19..15
rs1
14..12
funct3
11..7
imm[4:1|11]
6..0
opcode
BLTU uses opcode 1100011 (0x63), funct3 110. The rs1 and rs2 fields hold the comparison operands, and the 13-bit immediate encodes the branch offset (±4 KiB).
Format: B-type
opcode: 1100011 (0x63)
funct3: 110 (0x6)
Instruction Behavior
BLTU compares rs1 and rs2 as unsigned integers and branches if rs1 < rs2 (range ±4 KiB). A useful trick: signed array bounds checking can use a single BLTU, since any negative index (interpreted as a large unsigned value) will be greater than any nonnegative bound. BGTU can be synthesized by swapping operands.
Quick Understanding & Search Notes
BLTU is a conditional branch: when rs1 and rs2 satisfy the unsigned less than condition, the PC branches to the current PC plus the B-type offset; otherwise execution falls through.
The B-type branch offset is encoded in 2-byte units and reaches about +/-4 KiB from the current PC.
RISC-V conditional branches do not write rd and do not save a return address.
Official Spec Notes
These notes are checked against the RISC-V Unprivileged ISA manual and summarize operation semantics, immediate ranges, and edge behavior.
No Link Register Write
Conditional branches only update the PC based on the comparison result. They do not write rd or save a return address; use JAL/JALR forms for calls and returns.
RISC-V Unprivileged ISA, RV32I control transfer instructionsCommon Usage Scenarios
Comparison & Detection
Understand this scenario with real code like «bltu x9, x10, error # if x9 < x10 (unsigned), branch to error».
Type Conversion
Understand this scenario with real code like «bltu x9, x10, error # if x9 < x10 (unsigned), branch to error».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is B-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
Use BLT for signed comparison
Negative values become very large when interpreted as unsigned
FAQ
What is the BLTU branch range?
Conditional branches use a B-type immediate, giving an approximate reach of +/-4 KiB from the current PC.
What is the difference between signed and unsigned branches?
BLT/BGE compare as signed integers; BLTU/BGEU compare as unsigned integers. BEQ/BNE only test equality of the bit patterns.