Does ctzw access memory?
No. It reads integer register operands and writes rd only.
CTZW
RISC-V CTZW Instruction Details
Instruction ManualI-type (OP-IMM-32)Count trailing zero bits in word (RV64)
Instruction Syntax
ctzw rd, rs1
Operand Breakdown
This is a I-type (OP-IMM-32)-format instruction. Confirm operand positions based on the assembly syntax.
ZbbBit Manipulation
Instruction Behavior
ctzw on RV64 examines only the low 32 bits of rs1 and returns a count from 0 to 32.
CTZW Decode And Execute Animation
Starts with machine-code field decoding, then shows operand reads, instruction-specific execution, and ISA-visible state update.
rd
rs1
ctzw
,
Execution Context
t1(x6)
0x0000000000000014
31..20
19..15
14..12
11..7
6..0
011000000001
fixed imm[11:0]
00110
rs1
001
funct3
00101
rd
0011011
opcode
Execution Data Path
instruction
0x6013129B
opcode
0011011 -> OP-IMM-32
funct3
001 -> CTZW
fixed imm[11:0]
011000000001 -> CTZW selector
rd / rs1
t0(x5) / t1(x6)=0x00000014
fixed imm[11:0]
011000000001 selects CTZW (not an arithmetic immediate)
zero-bit scan
scan rs1[0] toward rs1[31] for the first 1 -> 2
x5
t0(x5) = 0x0000000000000002
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: 0x6013129B
syntax : ctzw t0(x5), t1(x6)
result : t0(x5) = 0x0000000000000002
Architectural Result
t0(x5) = 0x0000000000000002
This animation shows ISA-visible decode and state changes, not any specific CPU pipeline, cache, prediction, or timing implementation.
Quick Understanding & Search Notes
ctzw on RV64 examines only the low 32 bits of rs1 and returns a count from 0 to 32.
CTZW is an RV64 Zbb OP-IMM-32 instruction: it reads only rs1[31:0], and upper source bits do not participate in the count.
If the low 32 bits are zero, the result is 32.
Common Usage Scenarios
Bit Operations & Masks
Understand this scenario with real code like «ctzw x10, x11 ; x10 = trailing zeros in x11[31:0]».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is I-type (OP-IMM-32).
- 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 low 32 bits are zero, the result is 32.
FAQ
Does ctzw interpret signed numbers?
These bit operations work on bit patterns directly; aside from the specified word-width selection, arithmetic signed magnitude is not used.