LDRSH (register)

Load register signed halfword (register)

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

Encoding: Integer

313029282726252423222120191817161514131211109876543210
011110001x110
sizeVRopcRmoptionSRnRt

32-bit (opc == 11)

LDRSH <Wt>, [<Xn|SP>, (<Wm>|<Xm>){, <extend> {<amount>}}]

64-bit (opc == 10)

LDRSH <Xt>, [<Xn|SP>, (<Wm>|<Xm>){, <extend> {<amount>}}]

Decoding algorithm

if option<1> == '0' then EndOfDecode(Decode_UNDEF);             // sub-word index
constant ExtendType extend_type = DecodeRegExtend(option);
constant integer shift = if S == '1' then 1 else 0;

Operation

constant integer t = UInt(Rt);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant integer regsize = 64 >> UInt(opc<0>);
constant boolean nontemporal = FALSE;
constant boolean tagchecked = TRUE;
constant bits(64) offset = ExtendReg(m, extend_type, shift, 64);
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(16) data = Mem[address, 2, 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.
<Wm>: When option<0> is set to 0, is the 32-bit name of the general-purpose index register, encoded in the "Rm" field.
<Xm>: When option<0> is set to 1, is the 64-bit name of the general-purpose index register, encoded in the "Rm" field.
<extend>: <amount>: <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.