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STR (register)

Store Register (register) calculates an address from a base register value and an offset register value, stores a word from a register to memory. The offset register value can optionally be shifted. 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
!= 1111011PU0W0RnRtimm5stype0Rm
cond

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

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

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

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

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

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

if P == '0' && W == '1' then SEE "STRT";
t = UInt(Rt);  n = UInt(Rn);  m = UInt(Rm);
index = (P == '1');  add = (U == '1');  wback = (P == '0') || (W == '1');
(shift_t, shift_n) = DecodeImmShift(stype, imm5);
if 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 store instruction executes but the value stored is unknown.

If wback && n == 15, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction executes without writeback of the base address.
  • The instruction uses the addressing mode described in the equivalent immediate offset instruction.

T1

1514131211109876543210
0101000RmRnRt

T1

STR{<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
111110000100!= 1111Rt000000imm2Rm
Rn

T2

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

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

if Rn == '1111' then UNDEFINED;
t = UInt(Rt);  n = UInt(Rn);  m = UInt(Rm);
index = TRUE;  add = TRUE;  wback = FALSE;
(shift_t, shift_n) = (SRType_LSL, UInt(imm2));
if t == 15 || m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13

CONSTRAINED UNPREDICTABLE behavior

If t == 15, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The store instruction performs the store using the specified addressing mode but the value corresponding to R15 is unknown.

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>

For encoding A1: is the general-purpose register to be transferred, encoded in the "Rt" field. The PC can be used, but this is deprecated.

For encoding T1 and T2: 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, but this is deprecated.

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.

<shift>

The shift to apply to the value read from <Rm>. If absent, no shift is applied. Otherwise, see Shifts applied to a register.

<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];
    if t == 15 then  // Only possible for encoding A1
        data = PCStoreValue();
    else
        data = R[t];
    MemU[address,4] = data;
    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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