Engineering Math  

 

 

 

Floating Point Number

A floating-point number is a way to represent real numbers in computers, particularly those that are very large or very small. They're called "floating-point" because the decimal (or binary) point can "float" to different positions within the number. This allows for a wide range of values to be represented with a fixed number of digits.

How it works:

A floating-point number consists of three parts:

  • Sign: Indicates whether the number is positive or negative.
  • Significand (or Mantissa): Contains the significant digits of the number.
  • Exponent: Specifies where the decimal point is placed relative to the significand.

Why use floating-point numbers?

  • Dynamic range: They can represent both very small (close to zero) and very large numbers efficiently.
  • Efficiency: They use a fixed amount of memory, regardless of the magnitude of the number.

Limitations:

Approximation: Most real numbers cannot be represented exactly as floating-point numbers, leading to rounding errors. This is because computers have finite memory, and floating-point numbers are essentially approximations of real numbers.

Uneven distribution: The spacing between representable floating-point numbers is not uniform, meaning that the accuracy can vary depending on the magnitude of the number.

Important Note: When working with floating-point numbers in calculations, it's important to be aware of their limitations and potential for rounding errors.

Floating-Point Representation

In IEEE 754 single-precision floating-point representation, a number is represented as:

    (-1)s × 1.m × 2e

  • s: Sign bit (1 bit) : 1 -> Negative, 0 -> Positive
  • e: Exponent (8 bits, with a bias of 127)
  • m: Mantissa (23 bits, representing the fraction part)

How to calculate Exponent ?

The exponent in an IEEE 754 single-precision floating-point number is not directly stored. Instead, it's represented using a biased format. This means a fixed value, called the bias, is added to the actual exponent before storing it.  For single-precision, this bias is 127.

To determine the actual exponent, you'll need to extract the 8-bit exponent field from the 32-bit floating-point representation. Convert this binary value to decimal, and then subtract the bias (127) to obtain the true exponent.  This process effectively shifts the range of representable exponents to allow for both positive and negative values without needing an explicit sign bit for the exponent itself.

Remember that special cases exist for representing zero, infinity, and NaN (Not a Number), which have specific exponent field values. Understanding the bias and its role in exponent representation is crucial for interpreting and manipulating floating-point numbers accurately.

Steps to Calculate the Exponent

  1. Identify the Bias:
    • For single-precision (32-bit) floating-point numbers, the exponent field is 8 bits.
    • The bias for the exponent is 127 (i.e., 2(8-1) - 1).
  2. Calculate the Unbiased Exponent:
    • Determine the exponent e such that the number can be represented as (-1)s × 1.m × 2e, where m is the mantissa.
  3. Calculate the Biased Exponent:
    • The biased exponent is calculated by adding the bias to the actual (unbiased) exponent:
    • Biased Exponent = e + 127
  4. Convert the Biased Exponent to Binary:
    • Convert the biased exponent to an 8-bit binary number.

Example Calculation

Let's take an example to illustrate the calculation:

Example Number: 12.375

  1. Convert to Binary:
    • The binary representation of 12.375 is 1100.011.
  2. Normalize the Binary Representation:
    • Normalize the binary number so that it is in the form 1.m × 2e.
    • For 12.375, this becomes 1.100011 × 23.
  3. Identify the Components:
    • Sign bit s: 0 (positive number)
    • Mantissa m: 100011 (after the decimal point)
    • Unbiased Exponent e: 3
  4. Calculate the Biased Exponent:
    • Biased Exponent = Unbiased Exponent + Bias = 3 + 127 = 130.
  5. Convert the Biased Exponent to Binary:
    • 130 in binary is 10000010.

IEEE 754 Representation of 12.375

    Sign bit: 0

    Exponent: 10000010

    Mantissa: 10001100000000000000000 (padded with zeros to make 23 bits)

Putting it all together, the IEEE 754 single-precision representation of 12.375 is:

0 10000010 10001100000000000000000

Explanation of Each Component

  1. Sign Bit (1 bit):
    • 0 for positive numbers
    • 1 for negative numbers
  2. Exponent (8 bits):
    • Stored as a biased exponent.
    • For single-precision, the bias is 127.
  3. Mantissa (23 bits):
    • Represents the fractional part of the number after normalization.
    • The leading 1 is implicit and not stored, saving space for additional precision.

Steps in Code (Python Example)

import struct

def float_to_ieee_754(num):
    # Pack the float into 4 bytes using IEEE 754 format
    packed = struct.pack('!f', num)
    # Unpack the bytes into a single integer
    unpacked = struct.unpack('!I', packed)[0]
    # Convert the integer to a 32-bit binary string
    binary_str = f'{unpacked:032b}'
    
    # Extract the sign, exponent, and mantissa
    sign = binary_str[0]
    exponent = binary_str[1:9]
    mantissa = binary_str[9:]
    
    return sign, exponent, mantissa

num = 12.375
sign, exponent, mantissa = float_to_ieee_754(num)
print(f'Number: {num}')
print(f'Sign: {sign}')
print(f'Exponent: {exponent}')
print(f'Mantissa: {mantissa}')

Output

Number: 12.375
Sign: 0
Exponent: 10000010
Mantissa: 10001100000000000000000

Summary

  • Exponent Calculation: Add the bias (127) to the actual exponent to get the biased exponent.
  • Binary Conversion: Convert the biased exponent to an 8-bit binary number.
  • IEEE 754 Representation: Combine the sign bit, the 8-bit biased exponent, and the 23-bit mantissa to form the 32-bit representation.

 

How to Calculate Mantissa ?

The mantissa, also known as the significand, in an IEEE 754 single-precision floating-point number represents the fractional part of the number.  This 23-bit field, however, only stores the digits after the leading 1.

This leading 1 is implicit and not stored, which allows for an additional bit of precision.  To obtain the full mantissa, you'll combine this hidden bit with the stored bits from the 23-bit field.  Convert this combined binary value into decimal, and then divide it by 2 raised to the power of 23 to normalize it to a fraction between 1 and 2.

Remember, this process only applies to normal numbers, as special cases like zero, infinity, and NaN have specific mantissa values. Recognizing the role of the hidden bit and normalization in mantissa calculation is key to accurately interpreting the precision and magnitude of floating-point numbers.

Steps to Calculate the Mantissa

  1. Convert the Number to Binary:
    • Convert the integer and fractional parts of the number to binary separately.
    • Combine them to form the complete binary representation of the number.
  2. Normalize the Binary Representation:
    • Normalize the binary number so that it is in the form 1.m × 2e, where m is the mantissa.
    • The leading 1 before the binary point is implicit and not stored.
  3. Extract the Mantissa:
    • Take the fractional part (after the binary point) of the normalized binary number.
    • Pad or truncate the fractional part to fit into 23 bits for single-precision.

Example Calculation

Let's take an example to illustrate the calculation:

Example Number: 12.375

  1. Convert to Binary:
    • The integer part 12 in binary is 1100.
    • The fractional part 0.375 in binary is .011.
    • Combine them to get 1100.011.
  2. Normalize the Binary Representation:
    • Normalize the binary number so that it is in the form 1.m × 2e.
    • For 12.375, this becomes 1.100011 × 23.
    • The exponent e is 3.
  3. Extract the Mantissa:
    • The normalized form is 1.100011.
    • Remove the leading 1 and take the remaining part: 100011.
    • Pad with zeros to make it 23 bits: 10001100000000000000000.

IEEE 754 Representation of 12.375

    Sign bit: 0

    Exponent: 10000010

    Mantissa: 10001100000000000000000

Putting it all together, the IEEE 754 single-precision representation of 12.375 is:

0 10000010 10001100000000000000000

Explanation of Each Component

  1. Sign Bit (1 bit):
    • 0 for positive numbers
    • 1 for negative numbers
  2. Exponent (8 bits):
    • Stored as a biased exponent.
    • For single-precision, the bias is 127.
  3. Mantissa (23 bits):
    • Represents the fractional part of the number after normalization.
    • The leading 1 is implicit and not stored, saving space for additional precision.

Steps in Code (Python Example)

import struct

def float_to_ieee_754(num):
    # Pack the float into 4 bytes using IEEE 754 format
    packed = struct.pack('!f', num)
    # Unpack the bytes into a single integer
    unpacked = struct.unpack('!I', packed)[0]
    # Convert the integer to a 32-bit binary string
    binary_str = f'{unpacked:032b}'
    
    # Extract the sign, exponent, and mantissa
    sign = binary_str[0]
    exponent = binary_str[1:9]
    mantissa = binary_str[9:]
    
    return sign, exponent, mantissa

num = 12.375
sign, exponent, mantissa = float_to_ieee_754(num)
print(f'Number: {num}')
print(f'Sign: {sign}')
print(f'Exponent: {exponent}')
print(f'Mantissa: {mantissa}')

Output

Number: 12.375
Sign: 0
Exponent: 10000010
Mantissa: 10001100000000000000000

Summary

  • Mantissa Calculation: Convert the number to binary, normalize it, and extract the fractional part after the binary point. Pad or truncate it to fit 23 bits.
  • Binary Conversion: Convert the biased exponent to an 8-bit binary number.
  • IEEE 754 Representation: Combine the sign bit, the 8-bit biased exponent, and the 23-bit mantissa to form the 32-bit representation.

Floating-Point Compression - BF1(Block Floating Point Compression)

Block Floating Point (BFP) compression, specifically BF1, is a technique used to reduce the storage space required for a set of IEEE 754 single-precision floating-point numbers. The core idea is to find a common exponent that can represent the entire block of numbers with reasonable accuracy. This common exponent is stored once, and then only the mantissas of the individual numbers are stored, effectively saving the space that would have been used to store individual exponents for each number.

To compress a block of floating-point numbers using BF1, you first find the largest absolute value within the block. The exponent of this largest value becomes the shared exponent for the entire block. Next, each number's mantissa is adjusted by shifting it to match the shared exponent. Finally, only these adjusted mantissas and the shared exponent are stored.

Decompression involves reversing this process. The shared exponent is applied to each stored mantissa by shifting it back to its original position. This restores the original floating-point representation of each number within the block.

While BFP compression can significantly reduce storage requirements, it does introduce some loss of precision due to the shared exponent approximation. The effectiveness of BFP depends on the characteristics of the data being compressed, such as the range of values and the desired level of accuracy.

Following diagram depicts the overall process of BFP based on how I understand the process. In BFP, compression (i.e, reduced number of stored bits) happens at two level

  • Level 1 : store only one e (exponent, common exponent in this case) and does not store the exponent for each individual number
  • Level 2 : reduce the bit length of matissa by quantization and truncation

Introduction to BF1 Compression

Block Floating Point (BF1) compression is a technique used to compress a block of floating-point numbers by leveraging a common exponent for all the numbers in the block. This method is particularly effective in reducing the data size while maintaining a reasonable level of precision. The key steps in BF1 compression involve determining a common exponent, normalizing the numbers, quantizing the mantissas, and then storing the compressed data.

Steps in BF1 Compression

  1. Determine the Common Exponent:
    • Calculate the exponents of all the floating-point numbers in the block.
    • Select a common exponent that minimizes the total error when the numbers are normalized using this exponent.
    • The common exponent is typically the minimum or average exponent from the block.
  2. Normalize the Numbers:
    • Divide each number by 2common_exponent to normalize them.
    • This normalization step shifts the exponent so that all numbers have a similar scale.
  3. Quantize the Mantissas:
    • Extract the mantissa (fractional part) of each normalized number.
    • Quantize the mantissa to reduce its precision, typically by truncating or rounding to a fixed number of bits.
    • This step reduces the number of bits needed to store each mantissa.
  4. Store the Compressed Data:
    • Store the common exponent separately.
    • Store the quantized mantissas for each number in the block.

NOTE : Why we don't store the exponent part of every number at step 4 ?

In BF1 compression, we don't need to store the exponent part of each number separately because we use a common exponent for the entire block of numbers. It means the every number of the compressed data uses the same exponent(common exponent). So saving only one exponent(common exponent) and quantized mantissa for each number is enough to recover every individual value.

NOTE : So in BF1, the compression happens in two steps ? by using a common exponent and quantized mantissa (shorter bit length) ?

Yes, that's correct! The BF1 (Block Floating Point) compression technique achieves data compression primarily through two key steps:

  • Using a Common Exponent: By selecting a common exponent for all the floating-point numbers in a block, we eliminate the need to store individual exponents for each number. This significantly reduces the data size, especially when dealing with large blocks of numbers.
  • Quantizing the Mantissa: After normalizing the numbers using the common exponent, the mantissas are extracted and quantized. Quantization involves reducing the precision of the mantissas, typically by truncating or rounding them to a fixed number of bits. This further reduces the amount of data that needs to be stored.

Example of BF1 Compression

Let's consider a block of floating-point numbers and compress them using BF1 compression.

Example Block: [0.015, 0.020, 0.018, 0.022, 0.016, 0.021, 0.017, 0.019]

  1. Calculate Exponents:
    • Convert each number to its binary representation and determine the exponent.
    • Example exponents: -7, -7, -7, -7, -7, -7, -7, -7
  2. Determine Common Exponent:
    • Select the common exponent, which in this case is -7.
  3. Normalize the Numbers:
    • Normalize each number by dividing by 2-7.
    • Normalized numbers: [1.92, 2.56, 2.3, 2.82, 2.05, 2.69, 2.18, 2.43]
  4. Quantize the Mantissas:
    • Extract and quantize the mantissa of each normalized number.
    • Example quantized mantissas: [1.92, 2.56, 2.3, 2.82, 2.05, 2.69, 2.18, 2.43]
  5. Store the Compressed Data:
    • Store the common exponent (-7) separately.
    • Store the sign bit for each number.
    • Store the quantized mantissas for each number.

BF1 Decompression

To decompress the data, the stored quantized mantissas are multiplied by 2common_exponent to restore the original scale of the numbers.

Example of Decompression

  1. Retrieve the Common Exponent:
    • Retrieve the stored common exponent, which is -7.
  2. Decompress the Mantissas:
    • Multiply each quantized mantissa by 2common_exponent. (decompressed data = quantized_mantissa * 2common_exponent)
    • Example decompressed numbers: [0.015, 0.020, 0.018, 0.022, 0.016, 0.021, 0.017, 0.019]+/- error

Python Code Example

Download from here (NOTE : This code is written by chatGPT and I did a lot of back-and-forth with chatGPT to produce the result in the sequence and format that I liked. I haven't verified the result by hand, but you may get the general idea on how/where compression happens and how it gets uncompressed)

The output of the python code are shown below.

###### Original Block ######

Original Block (Decimal): [0.015, 0.02, 0.018, 0.022, 0.016, 0.021, 0.017, 0.019]

Original Block (Binary Components):

Number: 0.015

s: 0

e: 01111000

mantissa: 11101011100001010001111

 

Number: 0.02

s: 0

e: 01111001

mantissa: 01000111101011100001010

 

Number: 0.018

s: 0

e: 01111001

mantissa: 00100110111010010111100

 

Number: 0.022

s: 0

e: 01111001

mantissa: 01101000011100101011000

 

Number: 0.016

s: 0

e: 01111001

mantissa: 00000110001001001101111

 

Number: 0.021

s: 0

e: 01111001

mantissa: 01011000000100000110001

 

Number: 0.017

s: 0

e: 01111001

mantissa: 00010110100001110010110

 

Number: 0.019

s: 0

e: 01111001

mantissa: 00110111010010111100011

 

###### Compressed Block ######

Common Exponent (Decimal): -7

Common Exponent (Binary): 01111000

Compressed Block (Decimal): [1.919921875, 2.5595703125, 2.3037109375, 2.8154296875, 2.0478515625, 2.6875, 2.17578125, 2.431640625]

Compression Percentage: 43.75%

Compressed Block (Binary Components):

Quantized Mantissa: 1.919921875

s: 0

e: 01111000 <== this individual exponent is not saved because this value for all elements in the block is same.

quantized mantissa: 11110101110 <== bit length of this mantissa is shorter than the mantissa of the original block.

 

Quantized Mantissa: 2.5595703125

s: 0

e: 01111000

quantized mantissa: 101000111101

 

Quantized Mantissa: 2.3037109375

s: 0

e: 01111000

quantized mantissa: 100100110111

 

Quantized Mantissa: 2.8154296875

s: 0

e: 01111000

quantized mantissa: 101101000011

 

Quantized Mantissa: 2.0478515625

s: 0

e: 01111000

quantized mantissa: 100000110001

 

Quantized Mantissa: 2.6875

s: 0

e: 01111000

quantized mantissa: 101011000000

 

Quantized Mantissa: 2.17578125

s: 0

e: 01111000

quantized mantissa: 100010110100

 

Quantized Mantissa: 2.431640625

s: 0

e: 01111000

quantized mantissa: 100110111010

 

###### Decompressed Block ######

Decompressed Block (Decimal): [0.0149993896484375, 0.01999664306640625, 0.01799774169921875, 0.02199554443359375, 0.01599884033203125, 0.02099609375, 0.016998291015625, 0.0189971923828125]

Decompressed Block (Binary Components):

Decompressed Number: 0.0149993896484375 = 'quantized_mantissa * (2 ^ common_exponent)' = 1.919921875 * (2 ^-7)

s: 0

e: 01111000

mantissa: 11101011100000000000000

 

Decompressed Number: 0.01999664306640625 = 'quantized_mantissa * (2 ^ common_exponent)' = 2.5595703125 * (2 ^-7)

s: 0

e: 01111001

mantissa: 01000111101000000000000

 

Decompressed Number: 0.01799774169921875 = 'quantized_mantissa * (2 ^ common_exponent)' = 2.3037109375 * (2 ^-7)

s: 0

e: 01111001

mantissa: 00100110111000000000000

 

Decompressed Number: 0.02199554443359375 = 'quantized_mantissa * (2 ^ common_exponent)' = 2.8154296875 * (2 ^-7)

s: 0

e: 01111001

mantissa: 01101000011000000000000

 

Decompressed Number: 0.01599884033203125 = 'quantized_mantissa * (2 ^ common_exponent)' = 2.0478515625 * (2 ^-7)

s: 0

e: 01111001

mantissa: 00000110001000000000000

 

Decompressed Number: 0.02099609375 = 'quantized_mantissa * (2 ^ common_exponent)' = 2.6875 * (2 ^-7)

s: 0

e: 01111001

mantissa: 01011000000000000000000

 

Decompressed Number: 0.016998291015625 = 'quantized_mantissa * (2 ^ common_exponent)' = 2.17578125 * (2 ^-7)

s: 0

e: 01111001

mantissa: 00010110100000000000000

 

Decompressed Number: 0.0189971923828125 = 'quantized_mantissa * (2 ^ common_exponent)' = 2.431640625 * (2 ^-7)

s: 0

e: 01111001

mantissa: 00110111010000000000000

 

###### Verification ######

Original: 0.015000, Decompressed: 0.014999, Error: 0.000001

Original: 0.020000, Decompressed: 0.019997, Error: 0.000003

Original: 0.018000, Decompressed: 0.017998, Error: 0.000002

Original: 0.022000, Decompressed: 0.021996, Error: 0.000004

Original: 0.016000, Decompressed: 0.015999, Error: 0.000001

Original: 0.021000, Decompressed: 0.020996, Error: 0.000004

Original: 0.017000, Decompressed: 0.016998, Error: 0.000002

Original: 0.019000, Decompressed: 0.018997, Error: 0.000003

Summary

BF1 compression works by determining a common exponent for a block of floating-point numbers, normalizing the numbers using this common exponent, quantizing the mantissas, and storing the compressed data. During decompression, the stored quantized mantissas are multiplied by 2common_exponent to restore the original scale of the numbers.

Reference

YouTube