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Store Exclusive Register Byte stores a byte from a register to memory if the PE has exclusive access to the memory address, and returns a status value of 0 if the store was successful, or of 1 if no store was performed. See Synchronization and semaphores. The memory access is atomic.

For information about memory accesses see Load/Store addressing modes.


No offset

STXRB <Ws>, <Wt>, [<Xn|SP>{,#0}]

integer n = UInt(Rn);
integer t = UInt(Rt);
integer t2 = UInt(Rt2); // ignored by load/store single register
integer s = UInt(Rs);   // ignored by all loads and store-release

AccType acctype = if o0 == '1' then AccType_ORDEREDATOMIC else AccType_ATOMIC;
boolean pair = FALSE;
MemOp memop = if L == '1' then MemOp_LOAD else MemOp_STORE;
integer elsize = 8 << UInt(size);
integer regsize = if elsize == 64 then 64 else 32;
integer datasize = if pair then elsize * 2 else elsize;
boolean tag_checked = n != 31;

For information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors, and particularly STXRB.

Assembler Symbols


Is the 32-bit name of the general-purpose register into which the status result of the store exclusive is written, encoded in the "Rs" field. The value returned is:

If the operation updates memory.
If the operation fails to update memory.

Is the 32-bit name of the general-purpose register to be transferred, encoded in the "Rt" field.


Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.


If a synchronous Data Abort exception is generated by the execution of this instruction:

  • Memory is not updated.
  • <Ws> is not updated.

If AArch64.ExclusiveMonitorsPass() returns FALSE and the memory address, if accessed, would generate a synchronous Data Abort exception, it is implementation defined whether the exception is generated.


bits(64) address;
bits(datasize) data;
constant integer dbytes = datasize DIV 8;
boolean rt_unknown = FALSE;
boolean rn_unknown = FALSE;

if HaveMTEExt() then

if memop == MemOp_LOAD && pair && t == t2 then
    Constraint c = ConstrainUnpredictable(Unpredictable_LDPOVERLAP);
    assert c IN {Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP};
    case c of
        when Constraint_UNKNOWN    rt_unknown = TRUE;    // result is UNKNOWN
        when Constraint_UNDEF      UNDEFINED;
        when Constraint_NOP        EndOfInstruction();

if memop == MemOp_STORE then
    if s == t || (pair && s == t2) then
        Constraint c = ConstrainUnpredictable(Unpredictable_DATAOVERLAP);
        assert c IN {Constraint_UNKNOWN, Constraint_NONE, Constraint_UNDEF, Constraint_NOP};
        case c of
            when Constraint_UNKNOWN    rt_unknown = TRUE;    // store UNKNOWN value
            when Constraint_NONE       rt_unknown = FALSE;   // store original value
            when Constraint_UNDEF      UNDEFINED;
            when Constraint_NOP        EndOfInstruction();
    if s == n && n != 31 then
        Constraint c = ConstrainUnpredictable(Unpredictable_BASEOVERLAP);
        assert c IN {Constraint_UNKNOWN, Constraint_NONE, Constraint_UNDEF, Constraint_NOP};
        case c of
            when Constraint_UNKNOWN    rn_unknown = TRUE;    // address is UNKNOWN
            when Constraint_NONE       rn_unknown = FALSE;   // address is original base
            when Constraint_UNDEF      UNDEFINED;
            when Constraint_NOP        EndOfInstruction();

if n == 31 then
    address = SP[];
elsif rn_unknown then
    address = bits(64) UNKNOWN;
    address = X[n];

case memop of
    when MemOp_STORE
        if rt_unknown then
            data = bits(datasize) UNKNOWN;
        elsif pair then
            bits(datasize DIV 2) el1 = X[t];
            bits(datasize DIV 2) el2 = X[t2];
            data = if BigEndian() then el1 : el2 else el2 : el1;
            data = X[t];

        bit status = '1';
        // Check whether the Exclusives monitors are set to include the
        // physical memory locations corresponding to virtual address
        // range [address, address+dbytes-1].
        if AArch64.ExclusiveMonitorsPass(address, dbytes) then
            // This atomic write will be rejected if it does not refer
            // to the same physical locations after address translation.
            Mem[address, dbytes, acctype] = data;
            status = ExclusiveMonitorsStatus();
        X[s] = ZeroExtend(status, 32);

    when MemOp_LOAD
        // Tell the Exclusives monitors to record a sequence of one or more atomic
        // memory reads from virtual address range [address, address+dbytes-1].
        // The Exclusives monitor will only be set if all the reads are from the
        // same dbytes-aligned physical address, to allow for the possibility of
        // an atomicity break if the translation is changed between reads.
        AArch64.SetExclusiveMonitors(address, dbytes);

        if pair then
            if rt_unknown then
                // ConstrainedUNPREDICTABLE case
                X[t]  = bits(datasize) UNKNOWN;        // In this case t = t2
            elsif elsize == 32 then
                // 32-bit load exclusive pair (atomic)
                data = Mem[address, dbytes, acctype];
                if BigEndian() then
                    X[t]  = data<datasize-1:elsize>;
                    X[t2] = data<elsize-1:0>;
                    X[t]  = data<elsize-1:0>;
                    X[t2] = data<datasize-1:elsize>;
            else // elsize == 64
                // 64-bit load exclusive pair (not atomic),
                // but must be 128-bit aligned
                if address != Align(address, dbytes) then
                    iswrite = FALSE;
                    secondstage = FALSE;
                    AArch64.Abort(address, AArch64.AlignmentFault(acctype, iswrite, secondstage));
                X[t]  = Mem[address + 0, 8, acctype];
                X[t2] = Mem[address + 8, 8, acctype];
            data = Mem[address, dbytes, acctype];
            X[t] = ZeroExtend(data, regsize);

Operational information

If PSTATE.DIT is 1, the timing of this instruction is insensitive to the value of the data being loaded or stored.