Can WRS.NTO replace LR/SC?
No. It waits on a reservation set established by LR; it performs neither the LR load nor the SC conditional store.
WRS.NTO
RISC-V WRS.NTO Instruction Details
Instruction ManualI-typeWait on reservation set with no timeout
Instruction Syntax
wrs.nto
No explicit operands; requires Zawrs. To wait for a memory store, first use LR to reserve a set containing the target bytes.
32-bit SYSTEM I-type fields: funct12=000000001101, rs1=00000, funct3=000, rd=00000, opcode=1110011.
A stall is possible while the reservation is valid and no pending interrupt is observed; there is no built-in short timeout. The implementation may occasionally complete for any reason; software must recheck its condition.
Outside M, TW=1 causes an illegal-instruction exception on noncompletion within an implementation-specific bounded time. In VS/VU, TW=0 and VTW=1 cause a virtual-instruction exception on the same noncompletion condition.
ZawrsCache & Sync
Instruction Encoding
31..20
funct12
19..15
rs1=x0
14..12
funct3
11..7
rd=x0
6..0
opcode
WRS.NTO uses opcode 1110011 (0x73), funct3 000, funct12 000000001101. 32-bit SYSTEM I-type fields: funct12=000000001101, rs1=00000, funct3=000, rd=00000, opcode=1110011.
Format: SYSTEM I-type fields
opcode: 1110011 (0x73)
funct3: 000 (0x0)
funct12: 000000001101 (0x00d)
Instruction Behavior
May temporarily stall a hart while its LR-established reservation set is valid and no pending interrupt is observed, even if disabled. An implementation may occasionally end the stall for any reason.
Quick Understanding & Search Notes
WRS.NTO is a Zawrs wait-on-reservation-set instruction: a hart can temporarily stall while its reservation set is valid and no pending interrupt is observed; LR establishes that reservation set.
To wait for target bytes to be written, LR should first establish a reservation set containing them; 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. WRS.NTO has a bounded-time illegal-instruction rule outside M mode with mstatus.TW=1; in VS/VU modes, mstatus.TW=0 plus hstatus.VTW=1 instead gives a virtual-instruction exception.
Common Usage Scenarios
Polling an LR reservation set
In a polling loop, use LR to establish a reservation set covering the target bytes, then decide from the shared state whether to wait. Reload and test after returning; completion does not prove a target store occurred.
Waiting on shared state
In a polling loop, use LR to establish a reservation set covering the target bytes, then decide from the shared state whether to wait. Reload and test after returning; completion does not prove a target store occurred.
Pre-Use Checklist
Syntax Check
- No explicit operands; requires Zawrs. To wait for a memory store, first use LR to reserve a set containing the target bytes.
- 32-bit SYSTEM I-type fields: funct12=000000001101, rs1=00000, funct3=000, rd=00000, opcode=1110011.
Semantic Check
- A stall is possible while the reservation is valid and no pending interrupt is observed; there is no built-in short timeout. The implementation may occasionally complete for any reason; software must recheck its condition.
- Outside M, TW=1 causes an illegal-instruction exception on noncompletion within an implementation-specific bounded time. In VS/VU, TW=0 and VTW=1 cause a virtual-instruction exception on the same noncompletion condition.
Pitfalls / Common Confusions
To wait for a memory location to be written, first use LR to establish a reservation set that includes its bytes.
With mstatus.TW=1, WRS.NTO outside M mode raises an illegal-instruction exception if it does not complete within an implementation-specific bounded time. In VS/VU modes, with mstatus.TW=0 and hstatus.VTW=1, the same noncompletion causes a virtual-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
Will WRS.NTO always sleep until memory changes?
No. An implementation may end the stall for any reason; software must re-check the condition.