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FSTMDBX, FSTMIAX

FSTMX stores multiple SIMD&FP registers from the Advanced SIMD and floating-point register file to consecutive locations in using an address from a general-purpose register.

Arm deprecates use of FSTMDBX and FSTMIAX, except for disassembly purposes, and reassembly of disassembled code.

Depending on settings in the CPACR, NSACR, HCPTR, and FPEXC registers, and the Security state and PE mode in which the instruction is executed, an attempt to execute the instruction might be undefined, or trapped to Hyp mode. For more information see Enabling Advanced SIMD and floating-point support.

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

A1

313029282726252423222120191817161514131211109876543210
!= 1111110PUDW0RnVd1011imm8<7:1>1
condimm8<0>

Decrement Before (P == 1 && U == 0 && W == 1)

FSTMDBX{<c>}{<q>} <Rn>!, <dreglist>

Increment After (P == 0 && U == 1)

FSTMIAX{<c>}{<q>} <Rn>{!}, <dreglist>

if P == '0' && U == '0' && W == '0' then SEE "Related encodings";
if P == '1' && W == '0' then SEE "VSTR";
if P == U && W == '1' then UNDEFINED;
// Remaining combinations are PUW = 010 (IA without !), 011 (IA with !), 101 (DB with !)
single_regs = FALSE;  add = (U == '1');  wback = (W == '1');
d = UInt(D:Vd);  n = UInt(Rn);  imm32 = ZeroExtend(imm8:'00', 32);
regs = UInt(imm8) DIV 2;  // If UInt(imm8) is odd, see "FSTDBMX, FSTMIAX".
if n == 15 && (wback || CurrentInstrSet() != InstrSet_A32) then UNPREDICTABLE;
if regs == 0 || regs > 16 || (d+regs) > 32 then UNPREDICTABLE;
if imm8<0> == '1' && (d+regs) > 16 then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If regs == 0, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction operates as a VSTM with the same addressing mode but stores no registers.

If regs > 16 || (d+regs) > 16, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The memory locations specified by the instruction and the number of registers specified by the instruction become unknown. If the instruction specifies writeback, then that register becomes unknown. This behavior does not affect any other memory locations.

T1

15141312111098765432101514131211109876543210
1110110PUDW0RnVd1011imm8<7:1>1
imm8<0>

Decrement Before (P == 1 && U == 0 && W == 1)

FSTMDBX{<c>}{<q>} <Rn>!, <dreglist>

Increment After (P == 0 && U == 1)

FSTMIAX{<c>}{<q>} <Rn>{!}, <dreglist>

if P == '0' && U == '0' && W == '0' then SEE "Related encodings";
if P == '1' && W == '0' then SEE "VSTR";
if P == U && W == '1' then UNDEFINED;
// Remaining combinations are PUW = 010 (IA without !), 011 (IA with !), 101 (DB with !)
single_regs = FALSE;  add = (U == '1');  wback = (W == '1');
d = UInt(D:Vd);  n = UInt(Rn);  imm32 = ZeroExtend(imm8:'00', 32);
regs = UInt(imm8) DIV 2;  // If UInt(imm8) is odd, see "FSTDBMX, FSTMIAX".
if n == 15 && (wback || CurrentInstrSet() != InstrSet_A32) then UNPREDICTABLE;
if regs == 0 || regs > 16 || (d+regs) > 32 then UNPREDICTABLE;
if imm8<0> == '1' && (d+regs) > 16 then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If regs == 0, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction operates as a VSTM with the same addressing mode but stores no registers.

If regs > 16 || (d+regs) > 16, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The memory locations specified by the instruction and the number of registers specified by the instruction become unknown. If the instruction specifies writeback, then that register becomes unknown. This behavior does not affect any other memory locations.

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Related encodings: See Advanced SIMD and floating-point 64-bit move for the T32 instruction set, or Advanced SIMD and floating-point 64-bit move for the A32 instruction set.

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rn>

Is the general-purpose base register, encoded in the "Rn" field. If writeback is not specified, the PC can be used. However, Arm deprecates use of the PC.

!

Specifies base register writeback. Encoded in the "W" field as 1 if present, otherwise 0.

<dreglist>

Is the list of consecutively numbered 64-bit SIMD&FP registers to be transferred. The first register in the list is encoded in "D:Vd", and "imm8" is set to twice the number of registers in the list plus one. The list must contain at least one register, all registers must be in the range D0-D15, and must not contain more than 16 registers.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();  CheckVFPEnabled(TRUE);
    address = if add then R[n] else R[n]-imm32;
    for r = 0 to regs-1
        if single_regs then
            MemA[address,4] = S[d+r];  address = address+4;
        else
            // Store as two word-aligned words in the correct order for current endianness.
            MemA[address,4] = if BigEndian(AccType_ATOMIC) then D[d+r]<63:32> else D[d+r]<31:0>;
            MemA[address+4,4] = if BigEndian(AccType_ATOMIC) then D[d+r]<31:0> else D[d+r]<63:32>;
            address = address+8;
    if wback then R[n] = if add then R[n]+imm32 else R[n]-imm32;
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