Does cpopw access memory?
No. It reads integer register operands and writes rd only.
CPOPW
RISC-V CPOPW Instruction Details
Instruction ManualI-type (OP-IMM-32, RV64)Count set bits in word (RV64)
Instruction Syntax
cpopw 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]=011000000010 is a fixed instruction selector.
ZbbBit Manipulation
Related Search Terms
Instruction Behavior
cpopw is an RV64 Zbb instruction that counts the set bits in the low 32 bits of rs1 and writes the count to rd.
CPOPW Decode And Execute Animation
Starts with machine-code field decoding, then shows operand reads, instruction-specific execution, and ISA-visible state update.
rd
rs1
cpopw
,
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
011000000010
fixed imm[11:0]
00110
rs1
001
funct3
00101
rd
0011011
opcode
Execution Data Path
instruction
0x6023129B
opcode
0011011 -> OP-IMM-32
funct3
001 -> CPOPW
fixed imm[11:0]
011000000010 -> CPOPW selector
rd / rs1
t0(x5) / t1(x6)=0x00000014
fixed imm[11:0]
011000000010 selects CPOPW (not an arithmetic immediate)
population count
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: 0x6023129B
syntax : cpopw 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
cpopw is an RV64 Zbb instruction that counts the set bits in the low 32 bits of rs1 and writes the count to rd.
CPOPW is an RV64 Zbb OP-IMM-32 instruction and counts only the low 32 bits of rs1.
The result range is 0..32; upper bits do not participate.
Common Usage Scenarios
cpopw population count
RV64 Zbb, x11=0xffffffff00000015: cpopw x10, x11 ignores the upper 32 bits and writes x10=3.
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
The result range is 0..32; upper bits do not participate.
FAQ
How does cpopw handle a zero input?
The count is 0. Only the low 32 bits are examined; upper 32 bits do not participate.