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STREX

Store Register Exclusive calculates an address from a base register value and an immediate offset, stores a word from a register to the calculated address if the PE has exclusive access to the memory at that address, and returns a status value of 0 if the store was successful, or of 1 if no store was performed.

For more information about support for shared memory see Synchronization and semaphores. For information about memory accesses see Memory accesses.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 111100011000RnRd(1)(1)111001Rt
cond

A1

STREX{<c>}{<q>} <Rd>, <Rt>, [<Rn> {, {#}<imm>}]

d = UInt(Rd);  t = UInt(Rt);  n = UInt(Rn);  imm32 = Zeros(32); // Zero offset
if d == 15 || t == 15 || n == 15 then UNPREDICTABLE;
if d == n || d == t then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If d == t, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The store instruction executes but the value stored is unknown.

If d == n, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction performs the store to an unknown address.

T1

15141312111098765432101514131211109876543210
111010000100RnRtRdimm8

T1

STREX{<c>}{<q>} <Rd>, <Rt>, [<Rn> {, #<imm>}]

d = UInt(Rd);  t = UInt(Rt);  n = UInt(Rn);  imm32 = ZeroExtend(imm8:'00', 32);
if d == 15 || t == 15 || n == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13
if d == n || d == t then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If d == t, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The store instruction executes but the value stored is unknown.

If d == n, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction performs the store to an unknown address.

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

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rd>

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

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

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

<Rn>

Is the general-purpose base register, encoded in the "Rn" field.

<imm>

For encoding A1: the immediate offset added to the value of <Rn> to calculate the address. <imm> can only be 0 or omitted.

For encoding T1: the immediate offset added to the value of <Rn> to calculate the address. <imm> can be omitted, meaning an offset of 0. Values are multiples of 4 in the range 0-1020.

Aborts and alignment

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

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

A non word-aligned memory address causes an Alignment fault Data Abort exception to be generated, subject to the following rules:

  • If AArch32.ExclusiveMonitorsPass() returns TRUE, the exception is generated.
  • Otherwise, it is implementation defined whether the exception is generated.

If AArch32.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

if ConditionPassed() then
    EncodingSpecificOperations();
    address = R[n] + imm32;
    if AArch32.ExclusiveMonitorsPass(address,4) then
        MemA[address,4] = R[t];
        R[d] = ZeroExtend('0');
    else
        R[d] = ZeroExtend('1');

Operational information

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

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