FSTMDBX, FSTMIAX
FSTMX
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.
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.
This instruction is a data-independent-time instruction as described in About the DIT bit.
It has encodings from the following instruction sets:
A32 (
A1
)
and
T32 (
T1
)
.
!= 1111
1
1
0
0
1
0
1
1
x
x
x
x
x
x
x
1
1
0
1
FSTMDBX{<c>}{<q>} <Rn>!, <dreglist>
0
1
FSTMIAX{<c>}{<q>} <Rn>{!}, <dreglist>
if P == '0' && U == '0' && W == '0' then See("Related encodings"); end;
if P == '1' && W == '0' then See("VSTR"); end;
if P == U && W == '1' then Undefined(); end;
// Remaining combinations are PUW = 010 (IA without !), 011 (IA with !), 101 (DB with !)
let single_regs : boolean = FALSE;
let add : boolean = (U == '1');
let wback : boolean = (W == '1');
let d : integer = UInt(D::Vd);
let n : integer = UInt(Rn);
let imm32 : bits(32) = ZeroExtend{}(imm8::'00');
let regs : integer = UInt(imm8) DIVRM 2; // If UInt(imm8) is odd, see "FSTDBMX, FSTMIAX".
if n == 15 && (wback || CurrentInstrSet() != InstrSet_A32) then UnpredictableProcedure(); end;
if regs == 0 || regs > 16 || (d+regs) > 32 then UnpredictableProcedure(); end;
if imm8[0] == '1' && (d+regs) > 16 then UnpredictableProcedure(); end;
regs == 0
The instruction operates as a VSTM with the same addressing mode but stores no registers.
regs > 16 || (d+regs) > 16
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.
1
1
1
0
1
1
0
0
1
0
1
1
x
x
x
x
x
x
x
1
1
0
1
FSTMDBX{<c>}{<q>} <Rn>!, <dreglist>
0
1
FSTMIAX{<c>}{<q>} <Rn>{!}, <dreglist>
if P == '0' && U == '0' && W == '0' then See("Related encodings"); end;
if P == '1' && W == '0' then See("VSTR"); end;
if P == U && W == '1' then Undefined(); end;
// Remaining combinations are PUW = 010 (IA without !), 011 (IA with !), 101 (DB with !)
let single_regs : boolean = FALSE;
let add : boolean = (U == '1');
let wback : boolean = (W == '1');
let d : integer = UInt(D::Vd);
let n : integer = UInt(Rn);
let imm32 : bits(32) = ZeroExtend{}(imm8::'00');
let regs : integer = UInt(imm8) DIVRM 2; // If UInt(imm8) is odd, see "FSTDBMX, FSTMIAX".
if n == 15 && (wback || CurrentInstrSet() != InstrSet_A32) then UnpredictableProcedure(); end;
if regs == 0 || regs > 16 || (d+regs) > 32 then UnpredictableProcedure(); end;
if imm8[0] == '1' && (d+regs) > 16 then UnpredictableProcedure(); end;
regs == 0
The instruction operates as a VSTM with the same addressing mode but stores no registers.
regs > 16 || (d+regs) > 16
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.
<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.
<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.
!
Specifies base register writeback,
W
!
0
[absent]
1
[present]
if ConditionPassed() then
EncodingSpecificOperations();
CheckVFPEnabled(TRUE);
var address : bits(32) = if add then R(n) else R(n)-imm32;
for r = 0 to regs-1 do
if single_regs then
MemA{32}(address) = S(d+r);
address = address+4;
else
// Store as two word-aligned words in the correct order for current endianness.
if BigEndian(AccessType_ASIMD) then
MemA{32}(address) = D(d+r)[63:32];
MemA{32}(address+4) = D(d+r)[31:0];
else
MemA{32}(address) = D(d+r)[31:0];
MemA{32}(address+4) = D(d+r)[63:32];
end;
address = address+8;
end;
end;
if wback then R(n) = if add then R(n)+imm32 else R(n)-imm32; end;
end;
2026-03-12 12:23:09
2025-09_rel_asl1