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LDRB (literal)

Load Register Byte (literal) calculates an address from the PC value and an immediate offset, loads a byte from memory, zero-extends it to form a 32-bit word, and writes it to a register. For information about memory accesses see Memory accesses.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .


!= 1111010PU1W11111Rtimm12

A1 (!(P == 0 && W == 1))

LDRB{<c>}{<q>} <Rt>, <label> // (Normal form)

LDRB{<c>}{<q>} <Rt>, [PC, #{+/-}<imm>] // (Alternative form)

if P == '0' && W == '1' then SEE "LDRBT";
t = UInt(Rt);  imm32 = ZeroExtend(imm12, 32);
add = (U == '1');  wback = (P == '0') || (W == '1');
if t == 15 || wback then UNPREDICTABLE;


If wback, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction executes with the additional decode: wback = FALSE;.
  • The instruction treats bit[24] as the P bit, and bit[21] as the writeback (W) bit, and uses the same addressing mode as described in LDRB (immediate). The instruction uses post-indexed addressing when P == '0' and uses pre-indexed addressing otherwise. The instruction is handled as described in Using R15.


11111000U0011111!= 1111imm12


LDRB{<c>}{<q>} <Rt>, <label> // (Preferred syntax)

LDRB{<c>}{<q>} <Rt>, [PC, #{+/-}<imm>] // (Alternative syntax)

if Rt == '1111' then SEE "PLD";
t = UInt(Rt);  imm32 = ZeroExtend(imm12, 32);  add = (U == '1');
// Armv8-A removes UNPREDICTABLE for R13

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols


See Standard assembler syntax fields.


See Standard assembler syntax fields.


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


The label of the literal data item that is to be loaded into <Rt>. The assembler calculates the required value of the offset from the Align(PC, 4) value of the instruction to this label. Permitted values of the offset are -4095 to 4095.

If the offset is zero or positive, imm32 is equal to the offset and add == TRUE, encoded as U == 1.

If the offset is negative, imm32 is equal to minus the offset and add == FALSE, encoded as U == 0.

+/- Specifies the offset is added to or subtracted from the base register, defaulting to + if omitted and encoded in U:
U +/-
0 -
1 +

For encoding A1: is the 12-bit unsigned immediate byte offset, in the range 0 to 4095, defaulting to 0 if omitted, and encoded in the "imm12" field.

For encoding T1: is a 12-bit unsigned immediate byte offset, in the range 0 to 4095, encoded in the "imm12" field.

The alternative syntax permits the addition or subtraction of the offset and the immediate offset to be specified separately, including permitting a subtraction of 0 that cannot be specified using the normal syntax. For more information, see Use of labels in UAL instruction syntax.


if ConditionPassed() then
    base = Align(PC,4);
    address = if add then (base + imm32) else (base - imm32);
    R[t] = ZeroExtend(MemU[address,1], 32);

Operational information

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

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