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STRBT

Store Register Byte Unprivileged stores a byte from a register to memory. 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.

STRBT 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 an optionally-shifted register value.

It has encodings from the following instruction sets: A32 ( A1 and A2 ) and T32 ( T1 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 11110100U110RnRtimm12
cond

A1

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

t = UInt(Rt);  n = UInt(Rn);  postindex = TRUE;  add = (U == '1');
register_form = FALSE;  imm32 = ZeroExtend(imm12, 32);
if t == 15 || n == 15 || n == t then UNPREDICTABLE;

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.

If 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 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.

A2

313029282726252423222120191817161514131211109876543210
!= 11110110U110RnRtimm5type0Rm
cond

A2

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

t = UInt(Rt);  n = UInt(Rn);  m = UInt(Rm);  postindex = TRUE;  add = (U == '1');
register_form = TRUE;  (shift_t, shift_n) = DecodeImmShift(type, imm5);
if t == 15 || n == 15 || n == t || m == 15 then UNPREDICTABLE;

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.

If 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 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

15141312111098765432101514131211109876543210
111110000000!= 1111Rt1110imm8
Rn

T1

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

if Rn == '1111' then UNDEFINED;
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

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 A2 and T1: 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 encoded in U:
U +/-
0 -
1 +
For encoding A2: specifies the index register is added to or subtracted from the base register, defaulting to + if omitted and encoded in U:
U +/-
0 -
1 +
<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.

+

Specifies the offset is added to the base register.

<imm>

For encoding A1: is the 12-bit unsigned immediate byte offset, in the range 0 to 4095, defaulting to 0 if omitted, and encoded in the "imm12" field.

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.

Operation

if ConditionPassed() then
    if PSTATE.EL == EL2 then UNPREDICTABLE;               // Hyp mode
    EncodingSpecificOperations();
    offset = if register_form then Shift(R[m], shift_t, shift_n, PSTATE.C) else imm32;
    offset_addr = if add then (R[n] + offset) else (R[n] - offset);
    address = if postindex then R[n] else offset_addr;
    MemU_unpriv[address,1] = R[t]<7:0>;
    if postindex then R[n] = offset_addr;

CONSTRAINED UNPREDICTABLE behavior

If PSTATE.EL == EL2, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction executes as STRB (immediate).

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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