What offset range does LBU use?
Base load instructions use a sign-extended 12-bit byte offset, typically -2048 through 2047.
LBU
RISC-V LBU Instruction Details
Instruction ManualI-typeLoad a byte from memory, zero-extend to 32 bits, and write to rd
Instruction Syntax
lbu rd, offset(rs1)
Operand Breakdown
Destination rd: general-purpose register receiving the result.
Source rs1: register holding the first operand.
Immediate imm: 12-bit signed value, sign-extended before operation with rs1.
RV32IMemory Load
Related Search Terms
Instruction Encoding
31..20
imm[11:0]
19..15
rs1
14..12
funct3
11..7
rd
6..0
opcode
LBU uses opcode 0000011 (0x03), funct3 100. The rs1 field selects the source register, the 12-bit immediate provides the second operand, and rd selects the destination.
Format: I-type
opcode: 0000011 (0x03)
funct3: 100 (0x4)
Instruction Behavior
LBU (I-type, opcode=0000011, funct3=100) loads an 8-bit byte from memory at the effective address, zero-extends it to 32 bits, and writes to rd. Unlike LB (which sign-extends), LBU zero-extends. Commonly used for unsigned byte data and characters.
Quick Understanding & Search Notes
LBU reads a 8-bit byte from the effective address formed by rs1 plus a signed 12-bit offset, then zero-extends the value into rd.
All base loads form an effective byte address as rs1 plus a sign-extended 12-bit offset.
Naturally aligned accesses should not raise address-misaligned exceptions; misaligned behavior depends on the execution environment interface.
Common Usage Scenarios
Data Loading
Understand this scenario with real code like «lbu x7, 0(x10) # x7 = zero_ext(mem[x10+0][7:0])».
Network & Byte Order
Understand this scenario with real code like «lbu x7, 0(x10) # x7 = zero_ext(mem[x10+0][7:0])».
Type Conversion
Understand this scenario with real code like «lbu x7, 0(x10) # x7 = zero_ext(mem[x10+0][7:0])».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is I-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
vs LB: zero-extension vs sign-extension
Use LB if signed value is actually needed
FAQ
What is the difference between signed and unsigned loads?
LB/LH/LW sign-extend the loaded value to XLEN; LBU/LHU zero-extend. On RV64, use LWU when a 32-bit word should be zero-extended.