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LDRSB (register)

Load Register Signed Byte (register) calculates an address from a base register value and an offset register value, loads a byte from memory, sign-extends it to form a 32-bit word, and writes it to a register. The offset register value can be shifted left by 0, 1, 2, or 3 bits. 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
!= 1111000PU0W1RnRt(0)(0)(0)(0)1101Rm
cond

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

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

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

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

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

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

if P == '0' && W == '1' then SEE "LDRSBT";
t = UInt(Rt);  n = UInt(Rn);  m = UInt(Rm);
index = (P == '1');  add = (U == '1');  wback = (P == '0') || (W == '1');
(shift_t, shift_n) = (SRType_LSL, 0);
if t == 15 || m == 15 then UNPREDICTABLE;
if wback && (n == 15 || 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 as instruction, the base address might be corrupted so that the instruction cannot be repeated.

T1

1514131211109876543210
0101011RmRnRt

T1

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

t = UInt(Rt);  n = UInt(Rn);  m = UInt(Rm);
index = TRUE;  add = TRUE;  wback = FALSE;
(shift_t, shift_n) = (SRType_LSL, 0);

T2

15141312111098765432101514131211109876543210
111110010001!= 1111!= 1111000000imm2Rm
RnRt

T2

LDRSB{<c>}.W <Rt>, [<Rn>, {+}<Rm>] // (<Rt>, <Rn>, <Rm> can be represented in T1)

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

if Rt == '1111' then SEE "PLI";
if Rn == '1111' then SEE "LDRSB (literal)";
t = UInt(Rt);  n = UInt(Rn);  m = UInt(Rm);
index = TRUE;  add = TRUE;  wback = FALSE;
(shift_t, shift_n) = (SRType_LSL, UInt(imm2));
if m == 15 then UNPREDICTABLE; // 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

<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>

For encoding A1: is the general-purpose base register, encoded in the "Rn" field. The PC can be used in the offset variant.

For encoding T1 and T2: is the general-purpose base register, encoded in the "Rn" field.

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

Specifies the index register is added to the base register.

<Rm>

Is the general-purpose index register, encoded in the "Rm" field.

<imm>

If present, the size of the left shift to apply to the value from <Rm>, in the range 1-3. <imm> is encoded in imm2. If absent, no shift is specified and imm2 is encoded as 0b00.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    offset = Shift(R[m], shift_t, shift_n, PSTATE.C);
    offset_addr = if add then (R[n] + offset) else (R[n] - offset);
    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.

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