TSTART Start transaction This instruction starts a new transaction. If the transaction started successfully, the destination register is set to zero. If the transaction failed or was canceled, then all state modifications that were performed transactionally are discarded and the destination register is written with a nonzero value that encodes the cause of the failure. 1 1 0 1 0 1 0 1 0 0 1 0 0 0 1 1 0 0 1 1 0 0 0 0 0 1 1 TSTART <Xt> if !IsFeatureImplemented(FEAT_TME) then UNDEFINED; constant integer t = UInt(Rt); <Xt> Is the 64-bit name of the general-purpose destination register, encoded in the "Rt" field. if !IsTMEEnabled() then UNDEFINED; boolean IsEL1Regime; bit tme; bit tmt; case PSTATE.EL of when EL0 IsEL1Regime = S1TranslationRegime() == EL1; if IsEL1Regime then tme = SCTLR_EL1.TME0; tmt = SCTLR_EL1.TMT0; else tme = SCTLR_EL2.TME0; tmt = SCTLR_EL2.TMT0; when EL1 tme = SCTLR_EL1.TME; tmt = SCTLR_EL1.TMT; when EL2 tme = SCTLR_EL2.TME; tmt = SCTLR_EL2.TMT; when EL3 tme = SCTLR_EL3.TME; tmt = SCTLR_EL3.TMT; otherwise Unreachable(); constant boolean enable = tme == '1'; constant boolean trivial = tmt == '1'; if !enable then TransactionStartTrap(t); elsif trivial then TSTATE.nPC = NextInstrAddr(64); TSTATE.Rt = t; FailTransaction(TMFailure_TRIVIAL, FALSE); elsif IsFeatureImplemented(FEAT_SME) && PSTATE.SM == '1' then FailTransaction(TMFailure_ERR, FALSE); elsif TSTATE.depth == 255 then FailTransaction(TMFailure_NEST, FALSE); elsif TSTATE.depth == 0 then TSTATE.nPC = NextInstrAddr(64); TSTATE.Rt = t; ClearExclusiveLocal(ProcessorID()); TakeTransactionCheckpoint(); StartTrackingTransactionalReadsWrites(); TSTATE.depth = TSTATE.depth + 1; X[t, 64] = Zeros(64);