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## STXP

Store Exclusive Pair of registers stores two 32-bit words or two 64-bit doublewords from two registers to a memory location 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 on single-copy atomicity and alignment requirements, see Requirements for single-copy atomicity and Alignment of data accesses. If a 64-bit pair Store-Exclusive succeeds, it causes a single-copy atomic update of the 128-bit memory location being updated. For information about addressing modes, see Load/Store addressing modes.

31 | 30 | 29 | 28 | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0
1 | sz | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | Rs | 0 | Rt2 | Rn | Rt
|  |  | L |  |  | o0 |  |  |

#### 32-bit (sz == 0)

STXP <Ws>, <Wt1>, <Wt2>, [<Xn|SP>{, #0}]

#### 64-bit (sz == 1)

STXP <Ws>, <Xt1>, <Xt2>, [<Xn|SP>{, #0}]

integer s = UInt(Rs); integer t = UInt(Rt); integer t2 = UInt(Rt2); integer n = UInt(Rn); constant integer elsize = 32 << UInt(sz); constant integer datasize = elsize * 2; boolean tagchecked = n != 31; boolean rt_unknown = FALSE; boolean rn_unknown = FALSE; if s == t || (s == t2) then Constraint c = ConstrainUnpredictable(Unpredictable_DATAOVERLAP); assert c IN {Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP}; case c of when Constraint_UNKNOWN rt_unknown = TRUE; // store UNKNOWN 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_UNDEF, Constraint_NOP}; case c of when Constraint_UNKNOWN rn_unknown = TRUE; // address is UNKNOWN when Constraint_UNDEF UNDEFINED; when Constraint_NOP EndOfInstruction();

For information about the CONSTRAINED UNPREDICTABLE behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors, and particularly STXP.

### 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.

<Wt1> | Is the 32-bit name of the first general-purpose register to be transferred, encoded in the "Rt" field.

<Wt2> | Is the 32-bit name of the second general-purpose register to be transferred, encoded in the "Rt2" field.

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

<Xt1> | Is the 64-bit name of the first general-purpose register to be transferred, encoded in the "Rt" field.

<Xt2> | Is the 64-bit name of the second general-purpose register to be transferred, encoded in the "Rt2" 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(datasize) data; constant integer dbytes = datasize DIV 8; AccessDescriptor accdesc = CreateAccDescExLDST(MemOp_STORE, FALSE, tagchecked); if n == 31 then CheckSPAlignment(); address = SP[]; elsif rn_unknown then address = bits(64) UNKNOWN; else address = X[n, 64]; if rt_unknown then data = bits(datasize) UNKNOWN; else bits(datasize DIV 2) el1 = X[t, datasize DIV 2]; bits(datasize DIV 2) el2 = X[t2, datasize DIV 2]; data = if BigEndian(accdesc.acctype) then el1:el2 else el2:el1; 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() returns FALSE and the memory address, // if accessed, would generate a synchronous Data Abort exception, it is // IMPLEMENTATION DEFINED whether the exception is generated. // It is a limitation of this model that synchronous Data Aborts are never // generated in this case, as Mem[] is not called. // If FEAT_SPE is implemented, it is also IMPLEMENTATION DEFINED whether or not the // physical address packet is output when permitted and when // AArch64.ExclusiveMonitorPass() returns FALSE for a Store Exclusive instruction. // This behavior is not reflected here due to the previously stated limitation. if AArch64.ExclusiveMonitorsPass(address, dbytes, accdesc) then // This atomic write will be rejected if it does not refer // to the same physical locations after address translation. Mem[address, dbytes, accdesc] = data; status = ExclusiveMonitorsStatus(); X[s, 32] = 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.

Base Instructions | SIMD&FP Instructions | SVE Instructions | SME Instructions | Index by Encoding | Shared Pseudocode | Proprietary Notice

Internal version only: aarchmrs v2024-03_relA, pseudocode v2024-03_rel, sve v2024-03_rel ; Build timestamp: 2024-03-26T09:45

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