LDTRB

Load Register Byte (unprivileged) loads a byte from memory, zero-extends it, and writes the result to a register. The address that is used for the load is calculated from a base register and an immediate offset.

Memory accesses made by the instruction behave as if the instruction was executed at EL0 if the Effective value of PSTATE.UAO is 0 and either:

The instruction is executed at EL1.
The instruction is executed at EL2 when the Effective value of HCR_EL2.{E2H, TGE} is {1, 1}.

Otherwise, the memory access operates with the restrictions determined by the Exception level at which the instruction is executed. For information about memory accesses, see Load/Store addressing modes.

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	0	1	1	1	0	0	0	0	1	0	imm9									1	0	Rn					Rt
size								opc

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

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

Assembler Symbols

<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.

Shared Decode

integer n = UInt(Rn);
integer t = UInt(Rt);

boolean tagchecked = n != 31;

Operation

bits(64) address;
bits(8) data;

boolean privileged = AArch64.IsUnprivAccessPriv();
AccessDescriptor accdesc = CreateAccDescGPR(MemOp_LOAD, FALSE, privileged, tagchecked);

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

address = GenerateAddress(address, offset, accdesc);

data = Mem[address, 1, accdesc];
X[t, 32] = ZeroExtend(data, 32);

Operational information

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