wrs.nto
Wait-on-Reservation-Set-with-No-Timeout
To mitigate the wasteful looping in such usages, a wrs.nto (WRS-with-no-timeout) instruction is provided. Instead of polling for a store to a specific memory location, software registers a reservation set that includes all the bytes of the memory location using the LR instruction. Then a subsequent wrs.nto instruction would cause the hart to temporarily stall execution in a low-power state until a store occurs to the reservation set or an interrupt is observed.
This instruction is not supported in a constrained LR/SC loop. While stalled, an implementation is permitted to occasionally terminate the stall and complete execution for any reason.
wrs.nto follows the rules of the WFI instruction for resuming execution on a pending interrupt.
When the TW (Timeout Wait) bit in mstatus is set and wrs.nto is executed in any privilege mode otherthan M mode, and it does not complete within an implementation-specific bounded time limit, the wrs.nto instruction will cause an illegal instruction exception.
When executing in VS or VU mode, if the VTW bit is set in hstatus, the TW bit in mstatus is clear, and the wrs.nto does not complete within an implementation-specific bounded time limit, the wrs.nto instruction will cause a virtual instruction exception.
Since wrs.nto can complete execution for reasons other than stores to the reservation set, software will likely need a means of looping until the required stores have occurred.
wrs.nto, unlike WFI, is not specified to cause an illegal instruction exception if executed in U-mode when the governing TW bit is 0. WFI is typically not expected to be used in U-mode and on many systems may promptly cause an illegal instruction exception if used at U-mode. Unlike WFI, wrs.nto is expected to be used by software in U-mode when waiting on memory but without a deadline for that wait.
Execution
if ((CSR[misa].S == 1) && (CSR[mstatus].TW == 1'b1)) {
if (mode() != PrivilegeMode::M) {
raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
}
if (CSR[misa].H == 1) {
if ((CSR[mstatus].TW == 1'b0) && (CSR[hstatus].VTW == 1'b1)) {
if ((mode() == PrivilegeMode::VS) || (mode() == PrivilegeMode::VU)) {
raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
}
}
}
if (!ZAWRS_NTO_IS_NOP) {
wrs_nto();
}Exceptions
This instruction may result in the following synchronous exceptions:
-
IllegalInstruction
-
VirtualInstruction