Does bext access memory?
No. It reads integer register operands and writes rd only.
BEXT
RISC-V BEXT Instruction Details
Instruction ManualR-typeExtract single bit by register index
Instruction Syntax
bext 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.
BZbsBit ManipulationSingle-Bit Operation
Related Search Terms
Instruction Behavior
bext is a Zbs single-bit extract instruction. The bit index comes from the low log2(XLEN) bits of rs2.
BEXT Decode And Execute Animation
Starts with machine-code field decoding, then shows operand reads, instruction-specific execution, and ISA-visible state update.
rd
rs1
rs2
bext
,
,
Execution Context
a0(x10)
0x00001000
a6(x16)
0xFFFFFFF0
31..25
24..20
19..15
14..12
11..7
6..0
0100100
funct7
10000
rs2
01010
rs1
101
funct3
00101
rd
0110011
opcode
Execution Data Path
instruction
0x490552B3
opcode
0110011 -> OP
funct3
101 -> BEXT
funct7
0100100 -> single-bit operation selector
rd / rs1 / rs2
t0(x5) / a0(x10)=0x00001000 / a6(x16)=0xFFFFFFF0
rs2[4:0]
0xFFFFFFF0 & 31 -> index=16
single-bit extract
(0x00001000 >> 16) & 1 = 0
x5
t0(x5) = 0x00000000
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: 0x490552B3
syntax : bext t0(x5), a0(x10), a6(x16)
result : t0(x5) = 0x00000000
Architectural Result
t0(x5) = 0x00000000
This animation shows ISA-visible decode and state changes, not any specific CPU pipeline, cache, prediction, or timing implementation.
Quick Understanding & Search Notes
bext is a Zbs single-bit extract instruction. The bit index comes from the low log2(XLEN) bits of rs2.
These instructions are part of a B-extension subset and operate on XLEN-wide integer register values.
The register-index form uses only the low log2(XLEN) bits of rs2 to select the bit.
bext/bexti returns 0 or 1; it does not preserve the original bit position.
Common Usage Scenarios
Address & Pointer
Understand this scenario with real code like «bext x10, x11, x12 ; x10 = (x11 >> (x12 & (XLEN-1))) & 1».
Bit Operations & Masks
Understand this scenario with real code like «bext x10, x11, x12 ; x10 = (x11 >> (x12 & (XLEN-1))) & 1».
Comparison & Detection
Understand this scenario with real code like «bext x10, x11, x12 ; x10 = (x11 >> (x12 & (XLEN-1))) & 1».
Pre-Use Checklist
Syntax Check
- Verify rd, rs1, rs2 (and rs3) are valid GPRs.
- Confirm funct3 and funct7 encoding is correct.
Semantic Check
- Check if the result affects subsequent branches or address calculations.
- Ensure the rd register is not overwritten by another instruction.
Pitfalls / Common Confusions
The register-index form uses only the low log2(XLEN) bits of rs2 to select the bit.
bext/bexti returns 0 or 1; it does not preserve the original bit position.
FAQ
How is the bit index selected for bext?
The low log2(XLEN) bits of rs2 select the bit, so higher bits do not extend the index range.