Does clzw access memory?
No. It reads integer register operands and writes rd only.
CLZW
RISC-V CLZW Instruction Details
Instruction ManualI-type (OP-IMM-32, RV64)Count leading zero bits in word (RV64)
Instruction Syntax
clzw rd, rs1
Operand Breakdown
rd: x0–x31; receives the count, with writes to x0 discarded.
rs1: only its low 32 bits participate; overlap with rd reads the source before writeback.
RV64-only, requiring Zbb. There is no immediate operand; imm[11:0]=011000000000 is a fixed instruction selector.
ZbbBit Manipulation
Related Search Terms
Instruction Behavior
clzw is an RV64 Zbb instruction that scans rs1 from bit 31 toward bit 0 and writes the number of zeros before the first 1 to rd.
CLZW Decode And Execute Animation
Starts with machine-code field decoding, then shows operand reads, instruction-specific execution, and ISA-visible state update.
rd
rs1
clzw
,
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)
0x0000000000000014
31..20
19..15
14..12
11..7
6..0
011000000000
fixed imm[11:0]
00110
rs1
001
funct3
00101
rd
0011011
opcode
Execution Data Path
instruction
0x6003129B
opcode
0011011 -> OP-IMM-32
funct3
001 -> CLZW
fixed imm[11:0]
011000000000 -> CLZW selector
rd / rs1
t0(x5) / t1(x6)=0x00000014
fixed imm[11:0]
011000000000 selects CLZW (not an arithmetic immediate)
zero-bit scan
Not computed
x5
t0(x5) = 0x0000000000003001; pc = 0x0000000000002000
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: 0x6003129B
syntax : clzw t0(x5), t1(x6)
Pending result: Not computed
Current architectural state
t0(x5) = 0x0000000000003001; pc = 0x0000000000002000
This animation shows ISA-visible decode and state changes, not any specific CPU pipeline, cache, prediction, or timing implementation.
Quick Understanding & Search Notes
clzw is an RV64 Zbb instruction that scans rs1 from bit 31 toward bit 0 and writes the number of zeros before the first 1 to rd.
CLZW is an RV64 Zbb OP-IMM-32 instruction and examines only the low 32 bits of rs1.
If the low 32 bits are zero, the result is 32, not XLEN.
Common Usage Scenarios
clzw leading-zero count
RV64 Zbb, x11=0xffffffff00000010: clzw x10, x11 examines only the low 32 bits and writes x10=27.
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is I-type (OP-IMM-32, RV64).
- 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, not XLEN.
FAQ
How does clzw handle a zero input?
The count is 32. Only the low 32 bits are examined; upper 32 bits do not participate.