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Fixed-point numbers are limited in that they cannot simultaneously represent very large or very small numbers using a reasonable word size. This limitation can be overcome by using scientific notation. With scientific notation, you can dynamically place the binary point at a convenient location and use powers of the binary to keep track of that location. Thus, you can represent a range of very large and very small numbers with only a few digits.
You can represent any binary floating-point number in scientific notation form as ±f × 2±e where f is the fraction (or mantissa), 2 is the radix or base (binary in this case), and e is the exponent of the radix. The radix is always a positive number, while f and e can be positive or negative.
When performing arithmetic operations, floating-point hardware must take into account that the sign, exponent, and fraction are all encoded within the same binary word. This results in complex logic circuits when compared with the circuits for binary fixed-point operations.
The Simulink Fixed Point software supports single-precision and double-precision floating-point numbers as defined by the IEEE Standard 754. Additionally, a nonstandard IEEE-style number is supported.
A direct analogy exists between scientific notation and radix point notation. For example, scientific notation using five decimal digits for the fraction would take the form
where p is an integer of unrestricted range. Radix point notation using five bits for the fraction is the same except for the number base
where q is an integer of unrestricted range. The previous equation is valid for both fixed- and floating-point numbers. For both these data types, the fraction can be changed at any time by the processor. However, for fixed- point numbers the exponent never changes, while for floating-point numbers the exponent can be changed any time by the processor.
For fixed-point numbers, the exponent is fixed but there is no reason why the binary point must be contiguous with the fraction. For example, a word consisting of three unsigned bits is usually represented in scientific notation in one of these four ways.
If the exponent were greater than 0 or less than -3, then the representation would involve lots of zeros.
These extra zeros never change to ones, however, so they don't show up in the hardware. Furthermore, unlike floating-point exponents, a fixed-point exponent never shows up in the hardware, so fixed-point exponents are not limited by a finite number of bits.
Note Restricting the binary point to being contiguous with the fraction is unnecessary; the Simulink Fixed Point software allows you to extend the binary point to any arbitrary location. |
The IEEE Standard 754 has been widely adopted, and is used with virtually all floating-point processors and arithmetic coprocessors—with the notable exception of many DSP floating-point processors.
Among other things, this standard specifies four floating-point number formats, of which singles and doubles are the most widely used. Each format contains three components: a sign bit, a fraction field, and an exponent field. These components, as well as the specific formats for singles and doubles, are discussed in the sections that follow.
While two's complement is the preferred representation for signed fixed-point numbers, IEEE floating-point numbers use a sign/magnitude representation, where the sign bit is explicitly included in the word. Using this representation, a sign bit of 0 represents a positive number and a sign bit of 1 represents a negative number.
In general, floating-point numbers can be represented in many different ways by shifting the number to the left or right of the binary point and decreasing or increasing the exponent of the binary by a corresponding amount.
To simplify operations on these numbers, they are normalized in the IEEE format. A normalized binary number has a fraction of the form 1.f where f has a fixed size for a given data type. Since the leftmost fraction bit is always a 1, it is unnecessary to store this bit and is therefore implicit (or hidden). Thus, an n-bit fraction stores an n+1-bit number. The IEEE format also supports denormalized numbers, which have a fraction of the form 0.f. Normalized and denormalized formats are discussed in more detail in the next section.
In the IEEE format, exponent representations are biased. This means a fixed value (the bias) is subtracted from the field to get the true exponent value. For example, if the exponent field is 8 bits, then the numbers 0 through 255 are represented, and there is a bias of 127. Note that some values of the exponent are reserved for flagging Inf (infinity), NaN (not-a-number), and denormalized numbers, so the true exponent values range from -126 to 127. See the sections Inf and NaN.
The IEEE single-precision floating-point format is a 32-bit word divided into a 1-bit sign indicator s, an 8-bit biased exponent e, and a 23-bit fraction f. A representation of this format is given below.
The relationship between this format and the representation of real numbers is given by
Exceptional Arithmetic discusses denormalized values.
The IEEE double-precision floating-point format is a 64-bit word divided into a 1-bit sign indicator s, an 11-bit biased exponent e, and a 52-bit fraction f. A representation of this format is shown in the following figure.
The relationship between this format and the representation of real numbers is given by
Exceptional Arithmetic discusses denormalized values.
The Simulink Fixed Point software supports a nonstandard IEEE-style floating-point data type. This data type adheres to the definitions and formulas previously given for IEEE singles and doubles. You create nonstandard floating-point numbers with the float function:
float(TotalBits,ExpBits)
TotalBits is the total word size and ExpBits is the size of the exponent field. The size of the fraction field and the bias are calculated from these input arguments. You can specify any number of exponent bits up to 11, and any number of total bits such that the fraction field is no more than 53 bits.
When specifying a nonstandard format, you should remember that the number of exponent bits largely determines the range of the result and the number of fraction bits largely determines the precision of the result.
Note These numbers are normalized with a hidden leading one for all exponents except the smallest possible exponent. However, the largest possible exponent might not be treated as a flag for Inf or NaN. |
The range of a number gives the limits of the representation while the precision gives the distance between successive numbers in the representation. The range and precision of an IEEE floating-point number depend on the specific format.
The range of representable numbers for an IEEE floating-point number with f bits allocated for the fraction, e bits allocated for the exponent, and the bias of e given by bias = 2(e - 1) - 1 is given below.
where
Normalized positive numbers are defined within the
range 2(1 - bias) to
(2 - 2-f)
· 2bias.
Normalized negative numbers are defined within the
range -2(1 - bias) to
-(2 - 2-f)
· 2bias.
Positive numbers greater than (2 - 2-f) · 2bias and negative numbers greater than -(2 - 2-f) · 2bias are overflows.
Positive numbers less than 2(1 - bias) and negative numbers less than -2(1 - bias) are either underflows or denormalized numbers.
Zero is given by a special bit pattern, where e = 0 and f = 0.
Overflows and underflows result from exceptional arithmetic conditions. Floating-point numbers outside the defined range are always mapped to ±Inf.
Note You can use the MATLAB commands realmin and realmax to determine the dynamic range of double-precision floating-point values for your computer. |
Because of a finite word size, a floating-point number is only an approximation of the "true" value. Therefore, it is important to have an understanding of the precision (or accuracy) of a floating-point result. In general, a value v with an accuracy q is specified by v ± q. For IEEE floating-point numbers,
and
Thus, the precision is associated with the number of bits in the fraction field.
The high and low limits, exponent bias, and precision for the supported floating-point data types are given in the following table.
Data Type | Low Limit | High Limit | Exponent Bias | Precision |
|---|---|---|---|---|
Single | 2-126 ≈ 10-38 | 2128 ≈ 3 · 1038 | 127 | 2-23 ≈ 10-7 |
Double | 2-1022 ≈ 2 · 10-308 | 21024 ≈ 2 · 10308 | 1023 | 2-52 ≈ 10-16 |
Nonstandard | 2(1 - bias) | (2 - 2-f) · 2bias | 2(e - 1) - 1 | 2-f |
Because of the sign/magnitude representation of floating-point numbers, there are two representations of zero, one positive and one negative. For both representations e = 0 and 0.f = 0.0.
In addition to specifying a floating-point format, the IEEE Standard 754 specifies practices and procedures so that predictable results are produced independently of the hardware platform. Specifically, denormalized numbers, Inf, and NaN are defined to deal with exceptional arithmetic (underflow and overflow).
If an underflow or overflow is handled as Inf or NaN, then significant processor overhead is required to deal with this exception. Although the IEEE Standard 754 specifies practices and procedures to deal with exceptional arithmetic conditions in a consistent manner, microprocessor manufacturers might handle these conditions in ways that depart from the standard. Some of the alternative approaches, such as saturation and wrapping, are discussed in Arithmetic Operations.
Denormalized numbers are used to handle cases of exponent underflow. When the exponent of the result is too small (i.e., a negative exponent with too large a magnitude), the result is denormalized by right-shifting the fraction and leaving the exponent at its minimum value. The use of denormalized numbers is also referred to as gradual underflow. Without denormalized numbers, the gap between the smallest representable nonzero number and zero is much wider than the gap between the smallest representable nonzero number and the next larger number. Gradual underflow fills that gap and reduces the impact of exponent underflow to a level comparable with roundoff among the normalized numbers. Thus, denormalized numbers provide extended range for small numbers at the expense of precision.
Arithmetic involving Inf (infinity) is treated
as the limiting case of real arithmetic, with infinite values defined
as those outside the range of representable numbers, or
. With the exception of the special
cases discussed below (NaN), any arithmetic operation
involving Inf yields Inf. Inf is
represented by the largest biased exponent allowed by the format and
a fraction of zero.
A NaN (not-a-number) is a symbolic entity
encoded in floating-point format. There are two types of NaN:
signaling and quiet. A signaling NaN signals an
invalid operation exception. A quiet NaN propagates
through almost every arithmetic operation without signaling an exception.
The following operations result in a NaN:
,
,
,
, and
.
Both types of NaN are represented by the largest biased exponent allowed by the format and a fraction that is nonzero. The bit pattern for a quiet NaN is given by 0.f where the most significant number in f must be a one, while the bit pattern for a signaling NaN is given by 0.f where the most significant number in f must be zero and at least one of the remaining numbers must be nonzero.
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