STLTXR

Store-release unprivileged exclusive register

This instruction stores a 32-bit word or a 64-bit doubleword to memory if the PE has exclusive access to the memory address, and returns a status value of 0 if the store was successful, or of 1 if no store was performed. See Synchronization and semaphores. The memory access is atomic. The instruction also has memory ordering semantics as described in Load-Acquire, Store-Release.

Explicit Memory effects produced by the instruction behave as if the instruction was executed at EL0 if the Effective value of PSTATE.UAO is 0 and either:

The instruction is executed at EL1.

The instruction is executed at EL2 when the Effective value of HCR_EL2.{E2H, TGE} is {1, 1}.

Otherwise, the Explicit Memory effects operate with the restrictions determined by the Exception level at which the instruction is executed.

For the purposes of the Exclusives monitors, and the forward progress guarantees for Load-Exclusive and Store-Exclusive loops, STLTXR is equivalent to STXR.

For information about addressing modes, see Load/Store addressing modes.

Encoding: No offset

Variants: FEAT_LSUI (ARMv9.6)

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
1		0	0	1	0	0	1	0	0	0						1	(1)	(1)	(1)	(1)	(1)
	sz								L		Rs					o0	Rt2					Rn					Rt

32-bit (sz == 0)

STLTXR <Ws>, <Wt>, [<Xn|SP>{, #0}]

64-bit (sz == 1)

STLTXR <Ws>, <Xt>, [<Xn|SP>{, #0}]

Decoding algorithm

if !IsFeatureImplemented(FEAT_LSUI) then EndOfDecode(Decode_UNDEF);
constant integer s = UInt(Rs);
constant integer t = UInt(Rt);
constant integer n = UInt(Rn);

constant integer elsize = 32 << UInt(sz);
constant boolean acqrel = TRUE;
constant boolean tagchecked = n != 31;

boolean rt_unknown = FALSE;
boolean rn_unknown = FALSE;
if s == t then
    constant Constraint c = ConstrainUnpredictable(Unpredictable_DATAOVERLAP);
    assert c IN {Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP};
    case c of
        when Constraint_UNKNOWN    rt_unknown = TRUE;    // store UNKNOWN value
        when Constraint_UNDEF      EndOfDecode(Decode_UNDEF);
        when Constraint_NOP        EndOfDecode(Decode_NOP);
if s == n && n != 31 then
    constant Constraint c = ConstrainUnpredictable(Unpredictable_BASEOVERLAP);
    assert c IN {Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP};
    case c of
        when Constraint_UNKNOWN    rn_unknown = TRUE;    // address is UNKNOWN
        when Constraint_UNDEF      EndOfDecode(Decode_UNDEF);
        when Constraint_NOP        EndOfDecode(Decode_NOP);

Operation

bits(64) address;
bits(elsize) data;

constant integer dbytes = elsize DIV 8;
constant boolean privileged = AArch64.IsUnprivAccessPriv();
constant AccessDescriptor accdesc = CreateAccDescExLDST(MemOp_STORE, acqrel,
                                                        tagchecked, privileged);

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

if rt_unknown then
    data = bits(elsize) UNKNOWN;
else
    data = X[t, elsize];
bit status = '1';
// Check whether the Exclusives monitors are set to include the
// physical memory locations corresponding to virtual address
// range [address, address+dbytes-1].
// If AArch64.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.
// It is a limitation of this model that synchronous Data Aborts are never
// generated in this case, as Mem[] is not called.

// If FEAT_SPE is implemented, it is also IMPLEMENTATION DEFINED whether or not the
// physical address packet is output when permitted and when
// AArch64.ExclusiveMonitorPass() returns FALSE for a Store Exclusive instruction.
// This behavior is not reflected here due to the previously stated limitation.
if AArch64.ExclusiveMonitorsPass(address, dbytes, accdesc) then
    // This atomic write will be rejected if it does not refer
    // to the same physical locations after address translation.
    Mem[address, dbytes, accdesc] = data;
    status = ExclusiveMonitorsStatus();
X[s, 32] = ZeroExtend(status, 32);

Explanations

<Ws>: Is the 32-bit name of the general-purpose register into which the status result of the store exclusive is written, encoded in the "Rs" field. The value returned is:
<Wt>: Is the 32-bit name of the general-purpose register to be transferred, encoded in the "Rt" field.
<Xn|SP>: Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.
<Xt>: Is the 64-bit name of the general-purpose register to be transferred, encoded in the "Rt" field.

Operational Notes

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