2. IEEE-754 format

IEEE-754 format

IEEE-754 format

The IEEE-754 format is a standard specified by the **IEEE** for representing floating-point binary numbers. The two versions of the format that are most common are **single precision** (32-bits) and **double precision** (64-bits).

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IEEE-754 formats specify:

• Number of bits for the significand and exponent.
• A bias for the exponent.

  Exponent bias

  The bias is an offset which is always applied to the exponent of a floating point number, $127$ for single precision and $1023$ for double precision.

  We use a bias instead of two’s complement as it allows for the numeric ordering to be the same as the lexicographic ordering which allows fast size comparisons of numbers.

  Example: convert single-precision exponent $0111010_2$:

  $$0111010_2=122_{10}$$

  Subtract the bias to find $122_{10}-127_{10}=-5_{10}$.

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• A normalised format.

  Normalisation

  When the significand of a floating point number is normalised, there is an implied leading $1$ before the stored significand.

  $$01011000[..]\to 1.01011000[..]$$

  By normalising floating point values, we: ?

  • Gain an extra bit of precision because of the implied leading $1$.
  • Ensure that there is a unique representation for values. In non-normalised form, $2.5$ can be represented as $10.1\times 2^0$, $1.01\times 2^1$ and so on.

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Bit lengths for IEEE-754 format

Each IEEE-754 format uses a different number of bits for each part of the number:

Format	Sign	Exponent	Significand	Total
single	1	8	23	32
double	1	11	52	64

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The sign bit behaves the same way as in signed-magntiude. The exponent and significand are not just unsigned.

title: Example 1
Take $11000001001011000000000000000000$.
We first break it down to individual components: $\color{green}1 \color{blue}1000010\color{red}01011$.
- $\color{green}1$ is the $\color{green}\text{sign bit}$ which tells us it is negative.
- $\color{blue}1000010$ is the $\color{blue}\text{exponent}$.
  This value is $130_{10} - 127_{10} (\text{bias}) = 3_{10}$.
- $\color{red}01011$ is the $\color{red}\text{significand}$.
  Significand is $1.01011$ with the implied leading $1$.
  
So, our final value is $- 1.01011 \times 2^3 = - 1010.11_2 \rightarrow - 10.75_{10}$.

title: Example 2
Take $-133.25_{10}$.
Convert to binary and normalise:
 
$$
	\begin{aligned}
		133.25 &\rightarrow 1000101.01 \times 2^0 \\
		&= 1.000010101 \times 2^7 \\
		&= {\color{red}00001010100000000000000_2} \\
		\\
		2^7 &\rightarrow 7 + 127 \\
		&= 137 \\
		&= {\color{blue}10000110_2}
	\end{aligned}
$$
 
Sign is negative so $\color{green}1$.
Hence final value is ${\color{green}1}{\color{blue}10000110}{\color{red}00001010100000000000000}$.

Special values

Using normalised form, it is not possible to represent $0$, so there is a special value specified which we use for $0$.

Decimal	IEEE-754 single precision
$\pm 0$	$0/10000000000000000000000000000000$
$\pm \infty$	$0/11111111100000000000000000000000$
$\text{NaN}$	$0/111111111\text{(any non-zero significand)}$

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