tstart

Transaction Start

TSTART <Xd>

Starts a memory transaction. Returns 0 if successful.

Pseudocode Operation

status ← AttemptTransactionStart()
Xd ← status
if status == 0 then
  EnterTransactionMode()
else
  Transaction not started, handle abort reason in status value
endif

Example

TSTART x0

Encoding

Binary Layout

1101010100100

31:19

011

18:16

0011

15:12

0000

11:8

011

7:5

Rt

4:0

 

Format System

Opcode 0xD5233060

Extension TME (Transactional)

Operands

• Xd
  Destination 64-bit integer register

Related

More in TME (Transactional)

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0xD5233060	TSTART <Xt>		A64		1101010100100 | 011 | 0011 | 0000 | 011 | Rt

Description

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.

Operation

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();

boolean enable = tme == '1';
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);