Is it a normal user-mode instruction?
No. These instructions are constrained by privilege level and extension support; user-mode legality must follow the privileged rules.
WFI
RISC-V WFI Instruction Details
Instruction ManualI-typeWait-for-interrupt hint: an implementation may stall the hart until an interrupt may need service; a legal implementation may also treat it as NOP.
Instruction Syntax
wfi
Operand Breakdown
Destination rd: general-purpose register receiving the result.
Source rs1: register holding the first operand.
Immediate imm: 12-bit signed value, sign-extended before operation with rs1.
PrivilegedSystem & Privileged
Related Search Terms
Instruction Encoding
31..20
imm[11:0]
19..15
rs1
14..12
funct3
11..7
rd
6..0
opcode
WFI uses opcode 1110011 (0x73), funct3 000, funct12 000100000101. The rs1 field selects the source register, the 12-bit immediate provides the second operand, and rd selects the destination.
Format: I-type
opcode: 1110011 (0x73)
funct3: 000 (0x0)
funct12: 000100000101 (0x105)
Instruction Behavior
WFI informs the implementation that the current hart may be stalled until an interrupt might need service. If a locally enabled interrupt is already pending or later becomes pending, the trap is taken on a following instruction; implementations may also resume for any reason, so software must handle WFI returning as if it were a NOP. WFI is available in all privileged modes; U-mode availability and trapping when TW=1 are governed by the privileged specification.
Quick Understanding & Search Notes
WFI is a system-level instruction defined by the privileged architecture. This page covers architectural semantics only, not platform firmware policy.
Execution permission, trap type, and state restoration depend on privilege level, relevant CSR bits, and extension presence.
Executing at an unsupported privilege level or without the required extension raises the specified exception.
Common Usage Scenarios
Idle loop power saving
Understand this scenario with real code like «wfi # Wait for interrupt».
Low-power waiting
Understand this scenario with real code like «wfi # Wait for interrupt».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is I-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
WFI is a hint; an implementation may avoid stalling and return as a NOP.
WFI can resume for locally enabled pending interrupts even when global interrupt-enable bits are clear.
If the wake event does not cause a trap, execution continues after WFI; software usually rechecks conditions and loops.
FAQ
Does it replace ordinary memory fences?
No. Address translation, instruction fetch, and data-memory ordering use different fence/invalidation instructions.