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
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 |
!= 1111 | 0 | 0 | 0 | P | U | 0 | W | 1 | Rn | Rt | (0) | (0) | (0) | (0) | 1 | 1 | 0 | 1 | Rm | ||||||||||||
cond |
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
15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
0 | 1 | 0 | 1 | 0 | 1 | 1 | Rm | Rn | Rt |
t = UInt(Rt); n = UInt(Rn); m = UInt(Rm); index = TRUE; add = TRUE; wback = FALSE; (shift_t, shift_n) = (SRType_LSL, 0);
T2
15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | != 1111 | != 1111 | 0 | 0 | 0 | 0 | 0 | 0 | imm2 | Rm | ||||||||||
Rn | Rt |
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> |
<q> |
<Rt> |
Is the general-purpose register to be transferred, encoded in the "Rt" field. |
+/- |
Specifies the index register is added to or subtracted from the base register, defaulting to + if omitted and
encoded in
U:
|
+ |
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.