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HINT

Hint instruction is for the instruction set space that is reserved for architectural hint instructions.

Some encodings described here are not allocated in this revision of the architecture, and behave as NOPs. These encodings might be allocated to other hint functionality in future revisions of the architecture and therefore must not be used by software.

313029282726252423222120191817161514131211109876543210
11010101000000110010CRmop211111

System

HINT #<imm>

SystemHintOp op;

case CRm:op2 of
    when '0000 000' op = SystemHintOp_NOP;
    when '0000 001' op = SystemHintOp_YIELD;
    when '0000 010' op = SystemHintOp_WFE;
    when '0000 011' op = SystemHintOp_WFI;
    when '0000 100' op = SystemHintOp_SEV;
    when '0000 101' op = SystemHintOp_SEVL;
    when '0000 111'
        SEE "XPACLRI";
    when '0001 xxx'
        SEE "PACIA1716, PACIB1716, AUTIA1716, AUTIB1716";
    when '0010 000'
        if !HaveRASExt() then EndOfInstruction();    // Instruction executes as NOP
        op = SystemHintOp_ESB;
    when '0010 001'
        if !HaveStatisticalProfiling() then EndOfInstruction();    // Instruction executes as NOP
        op = SystemHintOp_PSB;
    when '0010 010'
        if !HaveSelfHostedTrace() then EndOfInstruction();    // Instruction executes as NOP
        op = SystemHintOp_TSB;
    when '0010 100'
        op = SystemHintOp_CSDB;
    when '0011 xxx'
        SEE "PACIAZ, PACIASP, PACIBZ, PACIBSP, AUTIAZ, AUTIASP, AUTIBZ, AUTIBSP";
    when '0100 xx0'
        op = SystemHintOp_BTI;
        // Check branch target compatibility between BTI instruction and PSTATE.BTYPE
        - = BTypeCompatible_BTI(op2<2:1>);
    otherwise EndOfInstruction();

Assembler Symbols

<imm>

Is a 7-bit unsigned immediate, in the range 0 to 127 encoded in the "CRm:op2" field.

The encodings that are allocated to architectural hint functionality are described in the "Hints" table in the "Index by Encoding".

For allocated encodings of "CRm:op2":

  • A disassembler will disassemble the allocated instruction, rather than the HINT instruction.
  • An assembler may support assembly of allocated encodings using HINT with the corresponding <imm> value, but it is not required to do so.

Operation

case op of
    when SystemHintOp_YIELD
        Hint_Yield();

    when SystemHintOp_WFE
        if IsEventRegisterSet() then
            ClearEventRegister();
        else
            if PSTATE.EL == EL0 then
                // Check for traps described by the OS which may be EL1 or EL2.
                AArch64.CheckForWFxTrap(EL1, TRUE);
            if PSTATE.EL IN {EL0, EL1} && EL2Enabled() && !IsInHost() then
                // Check for traps described by the Hypervisor.
                AArch64.CheckForWFxTrap(EL2, TRUE);
            if HaveEL(EL3) && PSTATE.EL != EL3 then
                // Check for traps described by the Secure Monitor.
                AArch64.CheckForWFxTrap(EL3, TRUE);
            WaitForEvent();

    when SystemHintOp_WFI
        if !InterruptPending() then
            if PSTATE.EL == EL0 then
                // Check for traps described by the OS which may be EL1 or EL2.
                AArch64.CheckForWFxTrap(EL1, FALSE);
            if PSTATE.EL IN {EL0, EL1} && EL2Enabled() && !IsInHost() then
                // Check for traps described by the Hypervisor.
                AArch64.CheckForWFxTrap(EL2, FALSE);
            if HaveEL(EL3) && PSTATE.EL != EL3 then
                // Check for traps described by the Secure Monitor.
                AArch64.CheckForWFxTrap(EL3, FALSE);
            WaitForInterrupt();

    when SystemHintOp_SEV
        SendEvent();

    when SystemHintOp_SEVL
        SendEventLocal();

    when SystemHintOp_ESB
        if TSTATE.depth > 0 then
            FailTransaction(TMFailure_ERR, FALSE);
        SynchronizeErrors();
        AArch64.ESBOperation();
        if PSTATE.EL IN {EL0, EL1} && EL2Enabled() then AArch64.vESBOperation();
        TakeUnmaskedSErrorInterrupts();

    when SystemHintOp_PSB
        ProfilingSynchronizationBarrier();

    when SystemHintOp_TSB
        TraceSynchronizationBarrier();

    when SystemHintOp_CSDB
        ConsumptionOfSpeculativeDataBarrier();

    otherwise    // do nothing
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