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STXR

Store Exclusive Register stores a 32-bit word or a 64-bit doubleword 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. For information about memory accesses see Load/Store addressing modes.

313029282726252423222120191817161514131211109876543210
1x001000000Rs0(1)(1)(1)(1)(1)RnRt
sizeLo0Rt2

32-bit (size == 10)

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

64-bit (size == 11)

STXR <Ws>, <Xt>, [<Xn|SP>{,#0}]

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

integer elsize = 8 << UInt(size);
boolean tag_checked = n != 31;

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

Assembler Symbols

<Ws>

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:

0
If the operation updates memory.
1
If the operation fails to update memory.
<Xt>

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

<Wt>

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

<Xn|SP>

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

Aborts and alignment

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

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

Accessing an address that is not aligned to the size of the data being accessed causes an Alignment fault Data Abort exception to be generated, subject to the following rules:

  • If AArch64.ExclusiveMonitorsPass() returns TRUE, the exception is generated.
  • Otherwise, it is implementation defined whether the exception is generated.

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.

Operation

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

if HaveMTEExt() then
    SetNotTagCheckedInstruction(!tag_checked);

if s == t 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
    CheckSPAlignment();
    address = SP[];
elsif rn_unknown then
    address = bits(64) UNKNOWN;
else
    address = X[n];

if rt_unknown then
    data = bits(elsize) UNKNOWN;
else
    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_ATOMIC] = data;
    status = ExclusiveMonitorsStatus();
X[s] = ZeroExtend(status, 32);

Operational information

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

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