LDURSH

Load register signed halfword (unscaled)

This instruction calculates an address from a base register and an immediate offset, loads a signed halfword from memory, sign-extends it, and writes it to a register. For information about addressing modes, see Load/Store addressing modes.

Encoding: Unscaled offset

313029282726252423222120191817161514131211109876543210
011110001x000
sizeVRopcimm9RnRt

32-bit (opc == 11)

LDURSH <Wt>, [<Xn|SP>{, #<simm>}]

64-bit (opc == 10)

LDURSH <Xt>, [<Xn|SP>{, #<simm>}]

Decoding algorithm

constant bits(64) offset = SignExtend(imm9, 64);

Operation

constant integer n = UInt(Rn);
constant integer t = UInt(Rt);
constant integer datasize = 16;
constant integer regsize = 64 >> UInt(opc<0>);
constant boolean nontemporal = FALSE;
constant boolean tagchecked = n != 31;
bits(64) address;

constant boolean privileged = PSTATE.EL != EL0;
constant AccessDescriptor accdesc = CreateAccDescGPR(MemOp_LOAD, nontemporal, privileged,
                                                     tagchecked);

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

address = AddressAdd(address, offset, accdesc);

constant bits(datasize) data = Mem[address, datasize DIV 8, accdesc];
X[t, regsize] = SignExtend(data, regsize);

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.
<simm>: Is the optional signed immediate byte offset, in the range -256 to 255, defaulting to 0 and encoded in the "imm9" 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.