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LDRH (immediate)

Load Register Halfword (immediate) calculates an address from a base register value and an immediate offset, loads a halfword from memory, zero-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 , T2 and T3 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 1111000PU1W1!= 1111Rtimm4H1011imm4L
condRn

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

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

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

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

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

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

if Rn == '1111' then SEE "LDRH (literal)";
if P == '0' && W == '1' then SEE "LDRHT";
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

1514131211109876543210
10001imm5RnRt

T1

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

t = UInt(Rt);  n = UInt(Rn);  imm32 = ZeroExtend(imm5:'0', 32);
index = TRUE;  add = TRUE;  wback = FALSE;

T2

15141312111098765432101514131211109876543210
111110001011!= 1111!= 1111imm12
RnRt

T2

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

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

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

T3

15141312111098765432101514131211109876543210
111110000011!= 1111Rt1PUWimm8
Rn

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

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

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

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

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

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

if Rn == '1111' then SEE "LDRH (literal)";
if Rt == '1111' && P == '1' && U == '0' && W == '0' then SEE "PLDW (immediate)";
if P == '1' && U == '1' && W == '0' then SEE "LDRHT";
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>

For encoding A1, T2 and T3: is the general-purpose base register, encoded in the "Rn" field. For PC use see LDRH (literal).

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

+/- 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 the optional positive unsigned immediate byte offset, a multiple of 2, in the range 0 to 62, defaulting to 0 and encoded in the "imm5" field as <imm>/2.

For encoding T2: 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 T3: 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 CurrentInstrSet() == InstrSet_A32 then
    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];
        data = MemU[address,2];
        if wback then R[n] = offset_addr;
        R[t] = ZeroExtend(data, 32);
else
    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];
        data = MemU[address,2];
        if wback then R[n] = offset_addr;
        R[t] = ZeroExtend(data, 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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