You copied the Doc URL to your clipboard.

LDRSB (immediate)

Load Register Signed Byte (immediate) calculates an address from a base register value and an immediate offset, loads a byte from memory, sign-extends it to form a 32-bit word, and writes it to a register. It can use offset, post-indexed, or pre-indexed addressing. For information about memory accesses see Memory accesses.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 and T2 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 1111000PU1W1!= 1111Rtimm4H1101imm4L
condRn

Offset (P == 1 && W == 0)

LDRSB{<c>}{<q>} <Rt>, [<Rn> {, #{+/-}<imm>}]

Post-indexed (P == 0 && W == 0)

LDRSB{<c>}{<q>} <Rt>, [<Rn>], #{+/-}<imm>

Pre-indexed (P == 1 && W == 1)

LDRSB{<c>}{<q>} <Rt>, [<Rn>, #{+/-}<imm>]!

if Rn == '1111' then SEE "LDRSB (literal)";
if P == '0' && W == '1' then SEE "LDRSBT";
t = UInt(Rt);  n = UInt(Rn);  imm32 = ZeroExtend(imm4H:imm4L, 32);
index = (P == '1');  add = (U == '1');  wback = (P == '0') || (W == '1');
if t == 15 || (wback && n == t) then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If wback && n == t, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction performs all of the loads using the specified addressing mode and the content of the register that is written back is UNKNOWN. In addition, if an exception occurs during such an instruction, the base address might be corrupted so that the instruction cannot be repeated.

T1

15141312111098765432101514131211109876543210
111110011001!= 1111!= 1111imm12
RnRt

T1

LDRSB{<c>}{<q>} <Rt>, [<Rn> {, #{+}<imm>}]

if Rt == '1111' then SEE "PLI";
if Rn == '1111' then SEE "LDRSB (literal)";
t = UInt(Rt);  n = UInt(Rn);  imm32 = ZeroExtend(imm12, 32);
index = TRUE;  add = TRUE;  wback = FALSE;
// Armv8-A removes UNPREDICTABLE for R13

T2

15141312111098765432101514131211109876543210
111110010001!= 1111Rt1PUWimm8
Rn

Offset (Rt != 1111 && P == 1 && U == 0 && W == 0)

LDRSB{<c>}{<q>} <Rt>, [<Rn> {, #-<imm>}]

Post-indexed (P == 0 && W == 1)

LDRSB{<c>}{<q>} <Rt>, [<Rn>], #{+/-}<imm>

Pre-indexed (P == 1 && W == 1)

LDRSB{<c>}{<q>} <Rt>, [<Rn>, #{+/-}<imm>]!

if Rt == '1111' && P == '1' && U == '0' && W == '0' then SEE "PLI";
if Rn == '1111' then SEE "LDRSB (literal)";
if P == '1' && U == '1' && W == '0' then SEE "LDRSBT";
if P == '0' && W == '0' then UNDEFINED;
t = UInt(Rt);  n = UInt(Rn);  imm32 = ZeroExtend(imm8, 32);
index = (P == '1');  add = (U == '1');  wback = (W == '1');
if (t == 15 && W == '1') || (wback && n == t) then UNPREDICTABLE;
// Armv8-A removes UNPREDICTABLE for R13

CONSTRAINED UNPREDICTABLE behavior

If wback && n == t, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction performs all of the loads using the specified addressing mode and the content of the register that is written back is UNKNOWN. In addition, if an exception occurs during such an instruction, the base address might be corrupted so that the instruction cannot be repeated.

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

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rt>

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

<Rn>

Is the general-purpose base register, encoded in the "Rn" field. For PC use see LDRSB (literal).

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

Specifies the offset is added to the base register.

<imm>

For encoding A1: is the 8-bit unsigned immediate byte offset, in the range 0 to 255, defaulting to 0 if omitted, and encoded in the "imm4H:imm4L" field.

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

For encoding T2: is an 8-bit unsigned immediate byte offset, in the range 0 to 255, defaulting to 0 if omitted, and encoded in the "imm8" field.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    offset_addr = if add then (R[n] + imm32) else (R[n] - imm32);
    address = if index then offset_addr else R[n];
    R[t] = SignExtend(MemU[address,1], 32);
    if wback then R[n] = offset_addr;

Operational information

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

Was this page helpful? Yes No