LDARH

Load-acquire register halfword

This instruction derives an address from a base register value, loads a halfword from memory, zero-extends it, and writes it to a register. The instruction also has memory ordering semantics as described in Load-Acquire, Store-Release. For information about addressing modes, see Load/Store addressing modes.

For this instruction, if the destination is WZR/XZR, it is impossible for software to observe the presence of the acquire semantic other than its effect on the arrival at endpoints.

Encoding: No offset

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

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

Decoding algorithm

constant integer t = UInt(Rt);
constant integer n = UInt(Rn);
constant boolean tagchecked = n != 31;

Operation

bits(64) address;

constant AccessDescriptor accdesc = CreateAccDescAcqRel(MemOp_LOAD, tagchecked);

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

constant bits(16) data = Mem[address,  2, accdesc];
X[t, 32] = ZeroExtend(data, 32);

Explanations

<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.

Operational Notes

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