# Multiply-Add

Multiply-add combined operation

## Library

HDL Coder / HDL Operations

## Description

The Multiply-Add block computes the product of the first two inputs, a and b, and adds the result to the third input, c. The inputs can be vectors or scalars.

The multiplication operation is full precision, regardless of the output type. The
**Integer rounding mode**, **Output data type**,
and **Saturate on integer overflow** settings apply only to the
addition operation.

Use the Multiply-Add block to map a combined multiply-add or a
multiply-subtract operation to a DSP unit in your target hardware. You can select the
**Function** setting in the Block Parameters dialog box for
the Multiply-Add block.

To map to a DSP unit, specify the `SynthesisTool`

property for your
model. When you generate HDL code for your model, HDL Coder™ configures the multiply-add operation so that your synthesis tool can map
to a DSP unit.

**Note**

Some DSP units do not have the multiply-add capability. To see if your hardware has the multiply-add capability, refer to the documentation for the hardware.

## Data Type Support

The Multiply-Add block accepts and outputs signals of numeric data type
that Simulink^{®} supports, including fixed-point data types.

You can use matrix data types with the Multiply-Add block. When you use
these types, the port dimensions of the inputs `a`

and
`b`

must match. For example, in MATLAB^{®}, you can perform these matrix
operations:

a = [1 2; 3 4]; b = [5; 6]; c = 7; c + (a.*b)

ans = 12 17 25 31

```
Error in port widths or
dimensions
```

.## Parameters

### Function

Specify the function to perform a combined multiply and add or a multiply and subtract operation.

**Settings**

**Default:**
`c+(a.*b)`

You can set the function to:

`c+(a.*b)`

`c-(a.*b)`

`(a.*b)-c`

### Output data type

Specify the output data type.

**Settings**

**Default:**
`Inherit: Inherit via internal rule`

Set the output data type to:

A rule that inherits a data type, for example,

`Inherit: Same as input`

An expression that evaluates to a valid data type, for example,

`fixdt([],16,0)`

Click the **Show data type assistant** button
to display the Data Type Assistant dialog
box, which helps you to set the **Output data type**
parameter.

For more information, see Control Data Types of Signals .

### Integer rounding mode

Specify the rounding mode for fixed-point operations.

**Settings**

**Default:**
`Floor`

`Ceiling`

Rounds positive and negative numbers toward positive infinity. Equivalent to the MATLAB

`ceil`

function.`Convergent`

Rounds number to the nearest representable value. If a tie occurs, rounds to the nearest even integer. Equivalent to the Fixed-Point Designer™

`convergent`

function.`Floor`

Rounds positive and negative numbers toward negative infinity. Equivalent to the MATLAB

`floor`

function.`Nearest`

Rounds number to the nearest representable value. If a tie occurs, rounds toward positive infinity. Equivalent to the Fixed-Point Designer

`nearest`

function.`Round`

Rounds number to the nearest representable value. If a tie occurs, rounds positive numbers toward positive infinity and rounds negative numbers toward negative infinity. Equivalent to the Fixed-Point Designer

`round`

function.`Simplest`

Chooses between rounding toward floor and rounding toward zero to generate rounding code that is as efficient as possible.

`Zero`

Rounds number toward zero. Equivalent to the MATLAB

`fix`

function.

**Command-Line Information**

See Block-Specific Parameters for the command-line information.

**See Also**

For more information, see Rounding.

### Saturate on integer overflow

Specify whether overflows saturate.

**Settings**

**Default:** Off

- On
Overflows saturate to either the minimum or maximum value that the data type can represent.

For example, an overflow associated with a signed 8-bit integer can saturate to -128 or 127.

- Off
Overflows wrap to the appropriate value that the data type can represent.

For example, the number 130 does not fit in a signed 8-bit integer and wraps to -126.

**Tips**

Consider selecting this check box when your model has a possible overflow and you want explicit saturation protection in the generated code.

Consider clearing this check box when you want to optimize efficiency of your generated code.

Clearing this check box also helps you to avoid overspecifying how a block handles out-of-range signals. For more information, see Troubleshoot Signal Range Errors.

When you select this check box, saturation applies to every internal operation on the block, not just the output or result.

In general, the code generation process can detect when overflow is not possible. In this case, the code generator does not produce saturation code.

**Command-Line Information**

Parameter:
`SaturateOnIntegerOverflow` |

Type: character vector |

Value:
`'off'` | `'on'` |

Default:
`'off'` |

## Algorithms

## Extended Capabilities

**Introduced in R2015b**