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WRS.NTO

RISC-V WRS.NTO Instruction Details

Instruction ManualI-type

Wait on reservation set with no timeout

Instruction Syntax

wrs.nto
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.
ZawrsCache & Sync
Related Search Terms
wrs.nto instructionZawrs wait reservation setwrs.nto LRwrs.nto no timeoutlow-power polling

Instruction Behavior

Stalls the hart in a low-power state until a store occurs to the reservation set or a pending interrupt is observed. Must be used with LR. Part of Zawrs for energy-efficient polling.

Quick Understanding & Search Notes

WRS.NTO is a Zawrs wait-on-reservation-set instruction used with a prior LR reservation, allowing the hart to temporarily stall while waiting for a store to the reserved location.

A prior LR must establish a reservation set containing the target bytes; the instruction has no address operand.
WRS.NTO has no short timeout, but can end on a store to the reservation set, an observed interrupt, or an implementation choice.
The instructions are available in all privilege modes, but WRS.NTO is affected by the mstatus.TW rule.
RISC-V Unprivileged ISA Manual: Zawrs wait-on-reservation-set extension

Common Usage Scenarios

Cache & Hints

Understand this scenario with real code like «lr.w x10, (x11) ; wrs.nto».

Comparison & Detection

Understand this scenario with real code like «lr.w x10, (x11) ; wrs.nto».

Concurrency & Multi-core

Understand this scenario with real code like «lr.w x10, (x11) ; wrs.nto».

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

Must be used with a reservation set established by a prior LR.
mstatus.TW can cause a long wait in lower privilege modes to raise an illegal-instruction exception.
There is no short-timeout guarantee, but the implementation can still end the wait because of a store to the reservation set, an interrupt, or its own choice; software must re-check the condition in a loop.

FAQ

Can WRS.NTO replace LR/SC?

No. It only waits for events related to an existing reservation set; synchronization algorithms still need LR/SC, AMO, or CAS operations to update state.

Will WRS.NTO always sleep until memory changes?

No. The spec allows an implementation to occasionally terminate the stall for any reason, so software must re-check the condition in a loop.