## Fixed-Point Designer |

This example shows the basics of how to use the fixed-point numeric object `fi`

.

**Notation**

The fixed-point numeric object is called ** fi** because J.H. Wilkinson used

`fi`

**Setup**

This example may use display settings or preferences that are different from what you are currently using. To ensure that your current display settings and preferences are not changed by running this example, the example automatically saves and restores them. The following code captures the current states for any display settings or properties that the example changes.

originalFormat = get(0, 'format'); format loose format long g % Capture the current state of and reset the fi display and logging % preferences to the factory settings. fiprefAtStartOfThisExample = get(fipref); reset(fipref);

**Default Fixed-Point Attributes**

To assign a fixed-point data type to a number or variable with the default fixed-point parameters, use the `fi`

constructor. The resulting fixed-point value is called a `fi`

object.

For example, the following creates `fi`

objects `a`

and `b`

with attributes shown in the display, all of which we can specify when the variables are constructed. Note that when the `FractionLength`

property is not specified, it is set automatically to "best precision" for the given word length, keeping the most-significant bits of the value. When the `WordLength`

property is not specified it defaults to 16 bits.

a = fi(pi)

a = 3.1416015625 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 13

b = fi(0.1)

b = 0.0999984741210938 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 18

**Specifying Signed and WordLength Properties**

The second and third numeric arguments specify `Signed`

(`true`

or 1 = `signed`

, `false`

or 0 = `unsigned`

), and `WordLength`

in bits, respectively.

```
% Signed 8-bit
a = fi(pi, 1, 8)
```

a = 3.15625 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 8 FractionLength: 5

The `sfi`

constructor may also be used to construct a signed `fi`

object

a1 = sfi(pi,8)

a1 = 3.15625 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 8 FractionLength: 5

```
% Unsigned 20-bit
b = fi(exp(1), 0, 20)
```

b = 2.71828079223633 DataTypeMode: Fixed-point: binary point scaling Signedness: Unsigned WordLength: 20 FractionLength: 18

The `ufi`

constructor may be used to construct an unsigned `fi`

object

b1 = ufi(exp(1), 20)

b1 = 2.71828079223633 DataTypeMode: Fixed-point: binary point scaling Signedness: Unsigned WordLength: 20 FractionLength: 18

**Precision**

The data is stored internally with as much precision as is specified. However, it is important to be aware that initializing high precision fixed-point variables with double-precision floating-point variables may not give you the resolution that you might expect at first glance. For example, let's initialize an unsigned 100-bit fixed-point variable with 0.1, and then examine its binary expansion:

a = ufi(0.1, 100);

bin(a)

ans = 1100110011001100110011001100110011001100110011001101000000000000000000000000000000000000000000000000

Note that the infinite repeating binary expansion of 0.1 gets cut off at the 52nd bit (in fact, the 53rd bit is significant and it is rounded up into the 52nd bit). This is because double-precision floating-point variables (the default MATLAB® data type), are stored in 64-bit floating-point format, with 1 bit for the sign, 11 bits for the exponent, and 52 bits for the mantissa plus one "hidden" bit for an effective 53 bits of precision. Even though double-precision floating-point has a very large range, its precision is limited to 53 bits. For more information on floating-point arithmetic, refer to Chapter 1 of Cleve Moler's book, Numerical Computing with MATLAB. The pdf version can be found here: http://www.mathworks.com/company/aboutus/founders/clevemoler.html

So, why have more precision than floating-point? Because most fixed-point processors have data stored in a smaller precision, and then compute with larger precisions. For example, let's initialize a 40-bit unsigned `fi`

and multiply using full-precision for products.

Note that the full-precision product of 40-bit operands is 80 bits, which is greater precision than standard double-precision floating-point.

a = fi(0.1, 0, 40); bin(a)

ans = 1100110011001100110011001100110011001101

b = a*a

b = 0.0100000000000045 DataTypeMode: Fixed-point: binary point scaling Signedness: Unsigned WordLength: 80 FractionLength: 86

bin(b)

ans = 10100011110101110000101000111101011100001111010111000010100011110101110000101001

**Access to Data**

The data can be accessed in a number of ways which map to built-in data types and binary strings. For example,

**DOUBLE(A)**

a = fi(pi); double(a)

ans = 3.1416015625

returns the double-precision floating-point "real-world" value of `a`

, quantized to the precision of `a`

.

**A.DOUBLE = ...**

We can also set the real-world value in a double.

a.double = exp(1)

a = 2.71826171875 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 13

sets the real-world value of `a`

to `e`

, quantized to `a`

's numeric type.

**STOREDINTEGER(A)**

storedInteger(a)

ans = 22268

returns the "stored integer" in the smallest built-in integer type available, up to 64 bits.

**Relationship Between Stored Integer Value and Real-World Value**

In `BinaryPoint`

scaling, the relationship between the stored integer value and the real-world value is

There is also `SlopeBias`

scaling, which has the relationship

where

and

The math operators of `fi`

work with `BinaryPoint`

scaling and real-valued `SlopeBias`

scaled `fi`

objects.

**BIN(A), OCT(A), DEC(A), HEX(A)**

return the stored integer in binary, octal, unsigned decimal, and hexadecimal strings, respectively.

bin(a)

ans = 0101011011111100

oct(a)

ans = 053374

dec(a)

ans = 22268

hex(a)

ans = 56fc

**A.BIN = ..., A.OCT = ..., A.DEC = ..., A.HEX = ...**

set the stored integer from binary, octal, unsigned decimal, and hexadecimal strings, respectively.

```
a.bin = '0110010010001000'
```

a = 3.1416015625 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 13

```
a.oct = '031707'
```

a = 1.6180419921875 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 13

```
a.dec = '22268'
```

a = 2.71826171875 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 13

```
a.hex = '0333'
```

a = 0.0999755859375 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 13

**Specifying FractionLength**

When the `FractionLength`

property is not specified, it is computed to be the best precision for the magnitude of the value and given word length. You may also specify the fraction length directly as the fourth numeric argument in the `fi`

constructor or the third numeric argument in the `sfi`

or `ufi`

constructor. In the following, compare the fraction length of `a`

, which was explicitly set to 0, to the fraction length of `b`

, which was set to best precision for the magnitude of the value.

a = sfi(10,16,0)

a = 10 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 0

b = sfi(10,16)

b = 10 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 11

Note that the stored integer values of `a`

and `b`

are different, even though their real-world values are the same. This is because the real-world value of `a`

is the stored integer scaled by 2^0 = 1, while the real-world value of `b`

is the stored integer scaled by 2^-11 = 0.00048828125.

storedInteger(a)

ans = 10

storedInteger(b)

ans = 20480

**Specifying Properties with Parameter/Value Pairs**

Thus far, we have been specifying the numeric type properties by passing numeric arguments to the `fi`

constructor. We can also specify properties by giving the name of the property as a string followed by the value of the property:

```
a = fi(pi,'WordLength',20)
```

a = 3.14159393310547 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 20 FractionLength: 17

For more information on `fi`

properties, type

```
help fi
```

or

```
doc fi
```

at the MATLAB command line.

**Numeric Type Properties**

All of the numeric type properties of `fi`

are encapsulated in an object named `numerictype`

:

T = numerictype

T = DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 16 FractionLength: 15

The numeric type properties can be modified either when the object is created by passing in parameter/value arguments

T = numerictype('WordLength',40,'FractionLength',37)

T = DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 40 FractionLength: 37

or they may be assigned by using the dot notation

T.Signed = false

T = DataTypeMode: Fixed-point: binary point scaling Signedness: Unsigned WordLength: 40 FractionLength: 37

All of the numeric type properties of a `fi`

may be set at once by passing in the `numerictype`

object. This is handy, for example, when creating more than one `fi`

object that share the same numeric type.

```
a = fi(pi,'numerictype',T)
```

a = 3.14159265359194 DataTypeMode: Fixed-point: binary point scaling Signedness: Unsigned WordLength: 40 FractionLength: 37

```
b = fi(exp(1),'numerictype',T)
```

b = 2.71828182845638 DataTypeMode: Fixed-point: binary point scaling Signedness: Unsigned WordLength: 40 FractionLength: 37

The `numerictype`

object may also be passed directly to the `fi`

constructor

a1 = fi(pi,T)

a1 = 3.14159265359194 DataTypeMode: Fixed-point: binary point scaling Signedness: Unsigned WordLength: 40 FractionLength: 37

For more information on `numerictype`

properties, type

```
help numerictype
```

or

```
doc numerictype
```

at the MATLAB command line.

**Display Preferences**

The display preferences for `fi`

can be set with the `fipref`

object. They can be saved between MATLAB sessions with the `savefipref`

command.

**Display of Real-World Values**

When displaying real-world values, the closest double-precision floating-point value is displayed. As we have seen, double-precision floating-point may not always be able to represent the exact value of high-precision fixed-point number. For example, an 8-bit fractional number can be represented exactly in doubles

a = sfi(1,8,7)

a = 0.9921875 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 8 FractionLength: 7

bin(a)

ans = 01111111

while a 100-bit fractional number cannot (1 is displayed, when the exact value is 1 - 2^-99):

b = sfi(1,100,99)

b = 1 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 100 FractionLength: 99

Note, however, that the full precision is preserved in the internal representation of `fi`

bin(b)

ans = 0111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111

The display of the `fi`

object is also affected by MATLAB's `format`

command. In particular, when displaying real-world values, it is handy to use

format long g

so that as much precision as is possible will be displayed.

There are also other display options to make a more shorthand display of the numeric type properties, and options to control the display of the value (as real-world value, binary, octal, decimal integer, or hex).

For more information on display preferences, type

help fipref help savefipref help format

or

doc fipref doc savefipref doc format

at the MATLAB command line.

**Cleanup**

The following code sets any display settings or preferences that the example changed back to their original states.

% Reset the fi display and logging preferences fipref(fiprefAtStartOfThisExample); set(0, 'format', originalFormat);