LDRSHT
Load Register Signed Halfword Unprivileged
Load Register Signed Halfword Unprivileged loads a halfword from memory, sign-extends it to form a 32-bit word, and writes it to a register. For information about memory accesses see Memory accesses.
The memory access is restricted as if the PE were running in User mode. This makes no difference if the PE is actually running in User mode.
LDRSHT is unpredictable in Hyp mode.
The T32 instruction uses an offset addressing mode, that calculates the address used for the memory access from a base register value and an immediate offset, and leaves the base register unchanged.
The A32 instruction uses a post-indexed addressing mode, that uses a base register value as the address for the memory access, and calculates a new address from a base register value and an offset and writes it back to the base register. The offset can be an immediate value or a register value.
For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.
If CPSR.DIT is 1, the timing of this instruction is insensitive to the value of the data being loaded or stored.
It has encodings from the following instruction sets:
A32 (
A1
and
A2
)
and
T32 (
T1
)
.
!= 1111
0
0
0
0
1
1
1
1
1
1
1
LDRSHT{<c>}{<q>} <Rt>, [<Rn>] {, #{+/-}<imm>}
t = UInt(Rt); n = UInt(Rn); postindex = TRUE; add = (U == '1');
register_form = FALSE; imm32 = ZeroExtend(imm4H:imm4L, 32);
if t == 15 || n == 15 || n == t then UNPREDICTABLE;
n == 15
The instruction uses post-indexed addressing with the base register as PC. This is handled as described in Using R15.
The instruction is treated as if bit[24] == '1' and bit[21] == '0'. The instruction uses immediate offset addressing with the base register as PC, without writeback.
n == t && n != 15
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.
!= 1111
0
0
0
0
0
1
1
(0)
(0)
(0)
(0)
1
1
1
1
LDRSHT{<c>}{<q>} <Rt>, [<Rn>], {+/-}<Rm>
t = UInt(Rt); n = UInt(Rn); m = UInt(Rm); postindex = TRUE; add = (U == '1');
register_form = TRUE;
if t == 15 || n == 15 || n == t || m == 15 then UNPREDICTABLE;
n == t && n != 15
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.
1
1
1
1
1
0
0
1
0
0
1
1
!= 1111
1
1
1
0
LDRSHT{<c>}{<q>} <Rt>, [<Rn> {, #{+}<imm>}]
if Rn == '1111' then SEE "LDRSH (literal)";
t = UInt(Rt); n = UInt(Rn); postindex = FALSE; add = TRUE;
register_form = FALSE; imm32 = ZeroExtend(imm8, 32);
if t == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13
<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 encoding A1: specifies the offset is added to or subtracted from the base register, defaulting to + if omitted and
+/-
For encoding A2: specifies the index register is added to or subtracted from the base register, defaulting to + if omitted and
<Rm>
Is the general-purpose index register, encoded in the "Rm" field.
+
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.
<imm>
For encoding T1: is an optional 8-bit unsigned immediate byte offset, in the range 0 to 255, defaulting to 0 and encoded in the "imm8" field.
if ConditionPassed() then
EncodingSpecificOperations();
if PSTATE.EL == EL2 then UNPREDICTABLE; // Hyp mode
offset = if register_form then R[m] else imm32;
offset_addr = if add then (R[n] + offset) else (R[n] - offset);
address = if postindex then R[n] else offset_addr;
data = MemU_unpriv[address,2];
if postindex then R[n] = offset_addr;
R[t] = SignExtend(data, 32);
PSTATE.EL == EL2
The instruction executes as LDRSH (immediate).