Can WRS.STO 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.
WRS.STO
RISC-V WRS.STO Instruction Details
Instruction ManualI-typeWait on reservation set with short timeout
Instruction Syntax
wrs.sto
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
Instruction Behavior
Like WRS.NTO but stall duration bounded by implementation-defined short timeout (10-100x cache miss latency). Part of Zawrs.
Quick Understanding & Search Notes
WRS.STO 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.STO bounds the stall by an implementation-defined short timeout.
The instructions are available in all privilege modes, but WRS.NTO is affected by the mstatus.TW rule.
Common Usage Scenarios
Cache & Hints
Understand this scenario with real code like «lr.w x10, (x11) ; wrs.sto ; bne x10, x12, retry».
Device & IO
Understand this scenario with real code like «lr.w x10, (x11) ; wrs.sto ; bne x10, x12, retry».
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.
The short-timeout duration is implementation-defined and must not be documented as a fixed cycle count.
Returning from the wait does not mean the condition is satisfied; software must reload and test the shared state.
FAQ
Will WRS.STO 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.