Does ctz access memory?
No. It reads integer register operands and writes rd only.
CTZ
RISC-V CTZ Instruction Details
Instruction ManualI-type (OP-IMM)Count trailing zero bits
Instruction Syntax
ctz rd, rs1
Operand Breakdown
rd: x0–x31; receives the count, with writes to x0 discarded.
rs1: all XLEN bits participate; overlap with rd reads the source before writeback.
RV32/RV64, requiring Zbb. There is no immediate operand; imm[11:0]=011000000001 is a fixed instruction selector.
ZbbBit ManipulationInteger Operation
Related Search Terms
Instruction Behavior
ctz is an RV32/RV64 Zbb instruction that scans rs1 from bit 0 toward bit XLEN-1 and writes the number of zeros before the first 1 to rd.
CTZ Decode And Execute Animation
Starts with machine-code field decoding, then shows operand reads, instruction-specific execution, and ISA-visible state update.
rd
rs1
ctz
,
Execution Context
This example uses fixed initial register values and PC=0x2000; instruction fetch succeeds, with no asynchronous interrupt or extra extension checks. Exception branches stop ordinary commit; post-trap PC and handler state are EEI-dependent.
t1(x6)
0x00000014
31..20
19..15
14..12
11..7
6..0
011000000001
fixed imm[11:0]
00110
rs1
001
funct3
00101
rd
0010011
opcode
Execution Data Path
instruction
0x60131293
opcode
0010011 -> OP-IMM
funct3
001 -> CTZ
fixed imm[11:0]
011000000001 -> CTZ selector
rd / rs1
t0(x5) / t1(x6)=0x00000014
fixed imm[11:0]
011000000001 selects CTZ (not an arithmetic immediate)
zero-bit scan
Not computed
x5
t0(x5) = 0x00003001; pc = 0x00002000
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: 0x60131293
syntax : ctz t0(x5), t1(x6)
Pending result: Not computed
Current architectural state
t0(x5) = 0x00003001; pc = 0x00002000
This animation shows ISA-visible decode and state changes, not any specific CPU pipeline, cache, prediction, or timing implementation.
Quick Understanding & Search Notes
ctz is an RV32/RV64 Zbb instruction that scans rs1 from bit 0 toward bit XLEN-1 and writes the number of zeros before the first 1 to rd.
CTZ is an OP-IMM instruction in the Zbb basic bit-manipulation extension and operates on an XLEN-wide integer register value.
If the input is zero, the result is XLEN.
Common Usage Scenarios
ctz trailing-zero count
RV32/RV64 Zbb, x11=0x28 (binary 101000): ctz x10, x11 writes x10=3.
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is I-type (OP-IMM).
- 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
If the input is zero, the result is XLEN.
FAQ
How does ctz handle a zero input?
The count is XLEN. The entire XLEN-bit source register is examined.