Does PACKW access memory?
No. It only rearranges bit fields or performs register lookup within integer registers.
PACKW
RISC-V PACKW Instruction Details
Instruction ManualR-typeRV64 pack low 16-bit fields (Zbkb), RV64-only.
Instruction Syntax
packw 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.
ZbkbBit Manipulation
Instruction Behavior
PACKW (Zbkb, RV64) places rs1[15:0] into result bits [15:0], rs2[15:0] into bits [31:16], then sign-extends bit 31 into the upper 32 bits of rd.
PACKW Decode & Execute Animation
Shows only specification-defined encodings, conditions, and architectural state; it does not model pipelines, cache coherence, or platform policy.
packw a0, a1, a2
Execution Context
Instruction Encoding
31..25
funct7
0000100
24..20
rs2
a2
19..15
rs1
a1
14..12
funct3
100
11..7
rd
a0
6..0
opcode
0111011 (OP-32)
Decode fixed fields
Fixed encoding fields identify the instruction and operand roles.
x[a0] <- 0xFFFFFFFFFF807788
Architectural state has not been committed.
Quick Understanding & Search Notes
PACKW is a Zbkb cryptographic bit-manipulation instruction; this page follows the official B-extension operation.
The official W form is defined only for RV64.
These instructions rearrange bit/byte fields and do not access memory.
Common Usage Scenarios
Crypto & Security
Understand this scenario with real code like «packw a0, a1, a2».
Logical Operations
Understand this scenario with real code like «packw a0, a1, a2».
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
PACKW is defined only for RV64.
The result is the sign-extension of the packed 32-bit value, not zero-extension.
PACKW is RV64-only and sign-extends the packed 32-bit value.
FAQ
Which extension contains PACKW?
It belongs to the Zbkb cryptographic bit-manipulation extension.