Introduction of Floating Point Representation

1. To convert the floating point into decimal, we have 3 elements in a 32-bit floating point representation:
    i) Sign 
    ii) Exponent 
    iii) Mantissa 

• Sign bit is the first bit of the binary representation. ‘1’ implies negative number and ‘0’ implies positive number. 
  Example: 11000001110100000000000000000000 This is negative number.
• Exponent is decided by the next 8 bits of binary representation. 127 is the unique number for 32 bit floating point representation. It is known as bias. It is determined by 2^k-1 -1 where ‘k’ is the number of bits in exponent field. 

  There are 3 exponent bits in 8-bit representation and 8 exponent bits in 32-bit representation.

  Thus

  bias = 3 for 8 bit conversion (2^3-1 -1 = 4-1 = 3) 
  bias = 127 for 32 bit conversion. (2^8-1 -1 = 128-1 = 127) 

  Example: 01000001110100000000000000000000 
  10000011 = (131)₁₀
  131-127 = 4 

  Hence the exponent of 2 will be 4 i.e. 2⁴ = 16.

• Mantissa is calculated from the remaining 23 bits of the binary representation. It consists of ‘1’ and a fractional part which is determined by: 

  Example:

  01000001110100000000000000000000 

  The fractional part of mantissa is given by: 

  1*(1/2) + 0*(1/4) + 1*(1/8) + 0*(1/16) +……… = 0.625 

  Thus the mantissa will be 1 + 0.625 = 1.625 

  The decimal number hence given as: Sign*Exponent*Mantissa = (-1)⁰*(16)*(1.625) = 26

2. To convert the decimal into floating point, we have 3 elements in a 32-bit floating point representation:
    i) Sign (MSB) 
    ii) Exponent (8 bits after MSB) 
    iii) Mantissa (Remaining 23 bits) 

• Sign bit is the first bit of the binary representation. ‘1’ implies negative number and ‘0’ implies positive number. 
  Example: To convert -17 into 32-bit floating point representation Sign bit = 1
• Exponent is decided by the nearest smaller or equal to 2ⁿ number. For 17, 16 is the nearest 2ⁿ. Hence the exponent of 2 will be 4 since 2⁴ = 16. 127 is the unique number for 32 bit floating point representation. It is known as bias. It is determined by 2^k-1 -1 where ‘k’ is the number of bits in exponent field. 

  Thus bias = 127 for 32 bit. (2^8-1 -1 = 128-1 = 127) 

  Now, 127 + 4 = 131 i.e. 10000011 in binary representation.

• Mantissa: 17 in binary = 10001.

  Move the binary point so that there is only one bit from the left. Adjust the exponent of 2 so that the value does not change. This is normalizing the number. 1.0001 x 2⁴. Now, consider the fractional part and represented as 23 bits by adding zeros.

  00010000000000000000000

Related Link: 
https://www.youtube.com/watch?v=03fhijH6e2w

More questions on number representation: 
https://www.geeksforgeeks.org/number-representation-gq/

This article is contributed by Kriti Kushwaha

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 

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