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LDRB (immediate)

Load Register Byte (immediate) 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. For information about memory accesses, see Load/Store addressing modes.

It has encodings from 3 classes: Post-index , Pre-index and Unsigned offset



LDRB <Wt>, [<Xn|SP>], #<simm>

boolean wback = TRUE;
boolean postindex = TRUE;
bits(64) offset = SignExtend(imm9, 64);



LDRB <Wt>, [<Xn|SP>, #<simm>]!

boolean wback = TRUE;
boolean postindex = FALSE;
bits(64) offset = SignExtend(imm9, 64);

Unsigned offset


LDRB <Wt>, [<Xn|SP>{, #<pimm>}]

boolean wback = FALSE;
boolean postindex = FALSE;
bits(64) offset = LSL(ZeroExtend(imm12, 64), 0);

For information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors, and particularly LDRH (immediate).

Assembler Symbols


Is the 32-bit name of the general-purpose register to be transferred, encoded in the "Rt" field.


Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.


Is the signed immediate byte offset, in the range -256 to 255, encoded in the "imm9" field.


Is the optional positive immediate byte offset, in the range 0 to 4095, defaulting to 0 and encoded in the "imm12" field.

Shared Decode

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

boolean tag_checked = wback || n != 31;


if HaveMTEExt() then

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

boolean wb_unknown = FALSE;

if wback && n == t && n != 31 then
    c = ConstrainUnpredictable(Unpredictable_WBOVERLAPLD);
    assert c IN {Constraint_WBSUPPRESS, Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP};
    case c of
        when Constraint_WBSUPPRESS wback = FALSE;    // writeback is suppressed
        when Constraint_UNKNOWN    wb_unknown = TRUE;    // writeback is UNKNOWN
        when Constraint_UNDEF      UNDEFINED;
        when Constraint_NOP        EndOfInstruction();

if n == 31 then
    address = SP[];
    address = X[n];

if !postindex then
    address = address + offset;

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

if wback then
    if wb_unknown then
        address = bits(64) UNKNOWN;
    elsif postindex then
        address = address + offset;
    if n == 31 then
        SP[] = address;
        X[n] = address;

Operational information

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