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LDXP

Load Exclusive Pair of Registers derives an address from a base register value, loads two 32-bit words or two 64-bit doublewords from memory, and writes them to two registers. A 32-bit pair requires the address to be doubleword aligned and is single-copy atomic at doubleword granularity. A 64-bit pair requires the address to be quadword aligned and is single-copy atomic for each doubleword at doubleword granularity. The PE marks the physical address being accessed as an exclusive access. This exclusive access mark is checked by Store Exclusive instructions. See Synchronization and semaphores. For information about memory accesses see Load/Store addressing modes.

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1sz001000011(1)(1)(1)(1)(1)0Rt2RnRt
LRso0

32-bit (sz == 0)

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

64-bit (sz == 1)

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

integer n = UInt(Rn);
integer t = UInt(Rt);
integer t2 = UInt(Rt2);

integer elsize = 32 << UInt(sz);
integer datasize = elsize * 2;
boolean tag_checked = n != 31;

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

Assembler Symbols

<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.

<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.

<Xn|SP>

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

Operation

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

if HaveMTEExt() then
    SetNotTagCheckedInstruction(!tag_checked);

if t == t2 then
    Constraint c = ConstrainUnpredictable(Unpredictable_LDPOVERLAP);
    assert c IN {Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP};
    case c of
        when Constraint_UNKNOWN rt_unknown = TRUE;    // result is UNKNOWN
        when Constraint_UNDEF   UNDEFINED;
        when Constraint_NOP     EndOfInstruction();

if n == 31 then
    CheckSPAlignment();
    address = SP[];
else
    address = X[n];

// Tell the Exclusives monitors to record a sequence of one or more atomic
// memory reads from virtual address range [address, address+dbytes-1].
// The Exclusives monitor will only be set if all the reads are from the
// same dbytes-aligned physical address, to allow for the possibility of
// an atomicity break if the translation is changed between reads.
AArch64.SetExclusiveMonitors(address, dbytes);

if rt_unknown then
    // ConstrainedUNPREDICTABLE case
    X[t] = bits(datasize) UNKNOWN;    // In this case t = t2
elsif elsize == 32 then
    // 32-bit load exclusive pair (atomic)
    data = Mem[address, dbytes, AccType_ATOMIC];
    if BigEndian() then
        X[t] = data<datasize-1:elsize>;
        X[t2] = data<elsize-1:0>;
    else
        X[t] = data<elsize-1:0>;
        X[t2] = data<datasize-1:elsize>;
else // elsize == 64
    // 64-bit load exclusive pair (not atomic),
    // but must be 128-bit aligned
    if address != Align(address, dbytes) then
        AArch64.Abort(address, AArch64.AlignmentFault(AccType_ATOMIC, FALSE, FALSE));
    X[t] = Mem[address, 8, AccType_ATOMIC];
    X[t2] = Mem[address+8, 8, AccType_ATOMIC];

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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