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FLDM*X (FLDMDBX, FLDMIAX)

FLDMDBX is the Decrement Before variant of this instruction, and FLDMIAX is the Increment After variant. FLDM*X loads multiple SIMD&FP registers from consecutive locations in the Advanced SIMD and floating-point register file using an address from a general-purpose register.

Arm deprecates use of FLDMDBX and FLDMIAX, except for disassembly purposes, and reassembly of disassembled code.

Depending on settings in the CPACR, NSACR, and HCPTR 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
!= 1111110PUDW1RnVd1011imm8<7:1>1
condimm8<0>

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

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

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

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

if P == '0' && U == '0' && W == '0' then SEE "Related encodings";
if P == '1' && W == '0' then SEE "VLDR";
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 "FLDM*X".
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 VLDM with the same addressing mode but loads 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.
  • One or more of the SIMD and floating-point registers are unknown. If the instruction specifies writeback, the base register becomes unknown. This behavior does not affect any general-purpose registers.

T1

15141312111098765432101514131211109876543210
1110110PUDW1RnVd1011imm8<7:1>1
imm8<0>

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

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

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

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

if P == '0' && U == '0' && W == '0' then SEE "Related encodings";
if P == '1' && W == '0' then SEE "VLDR";
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 "FLDM*X".
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 VLDM with the same addressing mode but loads 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.
  • One or more of the SIMD and floating-point registers are unknown. If the instruction specifies writeback, the base register becomes unknown. This behavior does not affect any general-purpose registers.

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.

!

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
            S[d+r] = MemA[address,4];  address = address+4;
        else
            word1 = MemA[address,4];  word2 = MemA[address+4,4];  address = address+8;
            // Combine the word-aligned words in the correct order for current endianness.
            D[d+r] = if BigEndian(AccType_ATOMIC) then word1:word2 else word2:word1;
    if wback then R[n] = if add then R[n]+imm32 else R[n]-imm32;