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## SMLSLD, SMLSLDX

Signed Multiply Subtract Long Dual performs two signed 16 x 16-bit multiplications. It adds the difference of the products to a 64-bit accumulate operand.

Optionally, the instruction can exchange the halfwords of the second operand before performing the arithmetic. This produces top x bottom and bottom x top multiplication.

Overflow is possible during this instruction, but only as a result of the 64-bit addition. This overflow is not detected if it occurs. Instead, the result wraps around modulo 264.

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

### A1

 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 != 1111 0 1 1 1 0 1 0 0 RdHi RdLo Rm 0 1 M 1 Rn cond

#### SMLSLD (M == 0)

SMLSLD{<c>}{<q>} <RdLo>, <RdHi>, <Rn>, <Rm>

#### SMLSLDX (M == 1)

SMLSLDX{<c>}{<q>} <RdLo>, <RdHi>, <Rn>, <Rm>

```dLo = UInt(RdLo);  dHi = UInt(RdHi);  n = UInt(Rn);  m = UInt(Rm);  m_swap = (M == '1');
if dLo == 15 || dHi == 15 || n == 15 || m == 15 then UNPREDICTABLE;
if dHi == dLo then UNPREDICTABLE;```

### CONSTRAINED UNPREDICTABLE behavior

If dHi == dLo, then one of the following behaviors must occur:

• The instruction is undefined.
• The instruction executes as NOP.
• The value in the destination register is unknown.

### T1

 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 1 1 1 0 1 1 1 1 0 1 Rn RdLo RdHi 1 1 0 M Rm

#### SMLSLD (M == 0)

SMLSLD{<c>}{<q>} <RdLo>, <RdHi>, <Rn>, <Rm>

#### SMLSLDX (M == 1)

SMLSLDX{<c>}{<q>} <RdLo>, <RdHi>, <Rn>, <Rm>

```dLo = UInt(RdLo);  dHi = UInt(RdHi);  n = UInt(Rn);  m = UInt(Rm);  m_swap = (M == '1');
if dLo == 15 || dHi == 15 || n == 15 || m == 15 then UNPREDICTABLE;
// Armv8-A removes UPREDICTABLE for R13
if dHi == dLo then UNPREDICTABLE;```

### CONSTRAINED UNPREDICTABLE behavior

If dHi == dLo, then one of the following behaviors must occur:

• The instruction is undefined.
• The instruction executes as NOP.
• The value in the destination register is unknown.

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

### Assembler Symbols



 Is the general-purpose source register holding the lower 32 bits of the addend, and the destination register for the lower 32 bits of the result, encoded in the "RdLo" field.
 Is the general-purpose source register holding the upper 32 bits of the addend, and the destination register for the upper 32 bits of the result, encoded in the "RdHi" field.
 Is the first general-purpose source register, encoded in the "Rn" field.
 Is the second general-purpose source register, encoded in the "Rm" field.

### Operation

```if ConditionPassed() then
EncodingSpecificOperations();
operand2 = if m_swap then ROR(R[m],16) else R[m];
product1 = SInt(R[n]<15:0>) * SInt(operand2<15:0>);
product2 = SInt(R[n]<31:16>) * SInt(operand2<31:16>);
result = product1 - product2 + SInt(R[dHi]:R[dLo]);
R[dHi] = result<63:32>;
R[dLo] = result<31:0>;```