LDURB

Load register byte (unscaled)

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

Encoding: Unscaled offset

313029282726252423222120191817161514131211109876543210
0011100001000
sizeVRopcimm9RnRt

LDURB <Wt>, [<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 = 8;
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, 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.
<simm>: Is the optional signed immediate byte offset, in the range -256 to 255, defaulting to 0 and encoded in the "imm9" 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.