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TSTART

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 non-zero value that encodes the cause of the failure.

System
(TME)

313029282726252423222120191817161514131211109876543210
110101010010001100110000011Rt

System

TSTART <Xt>

if !HaveTME() then UNDEFINED;
integer t = UInt(Rt);

Assembler Symbols

<Xt>

Is the 64-bit name of the general-purpose destination register, encoded in the "Rt" field.

Operation

CheckTMEEnabled();

boolean IsEL1Regime;
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();

enable = tme == '1';
trivial = tmt == '1';

if !enable then
    TransactionStartTrap(t);
elsif trivial then
    FailTransaction(TMFailure_TRIVIAL, FALSE);
elsif TSTATE.depth == 255 then
    FailTransaction(TMFailure_NEST, FALSE);
elsif TSTATE.depth == 0 then
    TSTATE.nPC = NextInstrAddr();
    TSTATE.Rt = t;
    ClearExclusiveLocal(ProcessorID());
    TakeTransactionCheckpoint();
    StartTrackingTransactionalReadsWrites();

TSTATE.depth = TSTATE.depth + 1;
X[t] = Zeros(64);
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