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.
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
- Confirm the current instruction format is R-type.
- 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
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.