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:

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.


Internal version only: isa v01_31, pseudocode v2023-06_rel, sve v2023-06_rel ; Build timestamp: 2023-07-04T18:06

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