Load Exclusive Register Halfword derives an address from a base register value, loads a halfword from memory, zero-extends it and writes it to a register. The memory access is atomic. 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.


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

integer n = UInt(Rn); integer t = UInt(Rt); integer t2 = UInt(Rt2); // ignored by load/store single register integer s = UInt(Rs); // ignored by all loads and store-release AccType acctype = if o0 == '1' then AccType_ORDEREDATOMIC else AccType_ATOMIC; boolean pair = FALSE; MemOp memop = if L == '1' then MemOp_LOAD else MemOp_STORE; integer elsize = 8 << UInt(size); integer regsize = if elsize == 64 then 64 else 32; integer datasize = if pair then elsize * 2 else elsize; boolean tag_checked = n != 31; boolean rt_unknown = FALSE; boolean rn_unknown = FALSE; if memop == MemOp_LOAD && pair && 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 memop == MemOp_STORE then if s == t || (pair && s == t2) then 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 UNDEFINED; when Constraint_NOP EndOfInstruction(); if s == n && n != 31 then 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 UNDEFINED; when Constraint_NOP EndOfInstruction();

Assembler Symbols


Is the 32-bit name of the general-purpose register to be transferred, encoded in the "Rt" field.


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


bits(64) address; bits(datasize) data; constant integer dbytes = datasize DIV 8; if HaveMTE2Ext() then SetTagCheckedInstruction(tag_checked); if n == 31 then CheckSPAlignment(); address = SP[]; elsif rn_unknown then address = bits(64) UNKNOWN; else address = X[n]; case memop of when MemOp_STORE if rt_unknown then data = bits(datasize) UNKNOWN; elsif pair then bits(datasize DIV 2) el1 = X[t]; bits(datasize DIV 2) el2 = X[t2]; data = if BigEndian(acctype) then el1 : el2 else el2 : el1; else data = X[t]; 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(address, dbytes) then // This atomic write will be rejected if it does not refer // to the same physical locations after address translation. Mem[address, dbytes, acctype] = data; status = ExclusiveMonitorsStatus(); X[s] = ZeroExtend(status, 32); when MemOp_LOAD // 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 pair then 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]; if BigEndian(acctype) 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 iswrite = FALSE; secondstage = FALSE; AArch64.Abort(address, AlignmentFault(acctype, iswrite, secondstage)); X[t] = Mem[address + 0, 8, acctype]; X[t2] = Mem[address + 8, 8, acctype]; else data = Mem[address, dbytes, acctype]; X[t] = ZeroExtend(data, regsize);

Operational information

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

Internal version only: isa v33.11seprel, AdvSIMD v29.05, pseudocode v2021-09_rel, sve v2021-09_rc3d ; Build timestamp: 2021-10-06T11:41

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