Index

RCWSWPP, RCWSWPPA, RCWSWPPL, RCWSWPPAL

Read Check Write Swap quadword in memory atomically loads a 128-bit quadword from a memory location, and conditionally stores the value held in a pair of registers back to the same memory location. Storing back to memory is conditional on RCW Checks. The value initially loaded from memory is returned in the same pair of registers. This instruction updates the condition flags based on the result of the update of memory.

Integer
(FEAT_D128 && FEAT_THE)

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
0 0 0 1 1 0 0 1 A R 1 Rt2 1 0 1 0 0 0 Rn Rt
S o3 opc

RCWSWPP (A == 0 && R == 0)

RCWSWPP <Xt1>, <Xt2>, [<Xn|SP>]

RCWSWPPA (A == 1 && R == 0)

RCWSWPPA <Xt1>, <Xt2>, [<Xn|SP>]

RCWSWPPAL (A == 1 && R == 1)

RCWSWPPAL <Xt1>, <Xt2>, [<Xn|SP>]

RCWSWPPL (A == 0 && R == 1)

RCWSWPPL <Xt1>, <Xt2>, [<Xn|SP>]

if !IsFeatureImplemented(FEAT_D128) || !IsFeatureImplemented(FEAT_THE) then UNDEFINED;
if Rt == '11111' then UNDEFINED;
if Rt2 == '11111' then UNDEFINED;
integer t = UInt(Rt);
integer t2 = UInt(Rt2);
integer n = UInt(Rn);

boolean acquire = A == '1';
boolean release = R == '1';
boolean tagchecked = n != 31;

boolean rt_unknown = FALSE;

if t == t2 then
    Constraint c = ConstrainUnpredictable(Unpredictable_LSE128OVERLAP);
    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();

For information about the CONSTRAINED UNPREDICTABLE behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors, and particularly CONSTRAINED UNPREDICTABLE behavior for A64 instructions.

Assembler Symbols

<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

if !IsD128Enabled(PSTATE.EL) then UNDEFINED;
bits(64) address;
bits(64) value1;
bits(64) value2;
bits(128) newdata;
bits(128) readdata;
bits(4) nzcv;

AccessDescriptor accdesc = CreateAccDescRCW(MemAtomicOp_SWP, FALSE, acquire, release, tagchecked);

if n == 31 then
    CheckSPAlignment();
    address = SP[];
else
    address = X[n, 64];

value1 = X[t, 64];
value2 = X[t2, 64];

newdata = if BigEndian(accdesc.acctype) then value1:value2 else value2:value1;

bits(128) compdata = bits(128) UNKNOWN;    // Irrelevant when not executing CAS
(nzcv, readdata) = MemAtomicRCW(address, compdata, newdata, accdesc);

PSTATE.<N,Z,C,V> = nzcv;
if rt_unknown then
    readdata = bits(128) UNKNOWN;

if BigEndian(accdesc.acctype) then
    X[t, 64] = readdata<127:64>;
    X[t2, 64] = readdata<63:0>;
else
    X[t, 64] = readdata<63:0>;
    X[t2, 64] = readdata<127:64>;

Operational information

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