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LDREX

Load Register Exclusive calculates an address from a base register value and an immediate offset, loads a word from memory, writes it to a register and:

  • If the address has the Shared Memory attribute, marks the physical address as exclusive access for the executing PE in a global monitor.
  • Causes the executing PE to indicate an active exclusive access in the local monitor.

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
!= 111100011001RnRt(1)(1)111001(1)(1)(1)(1)
cond

A1

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

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

T1

15141312111098765432101514131211109876543210
111010000101RnRt(1)(1)(1)(1)imm8

T1

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

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

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.

<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.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    address = R[n] + imm32;
    AArch32.SetExclusiveMonitors(address,4);
    R[t] = MemA[address,4];

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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