Within the 8 bits of the exponent the

Within the 8 bits of the exponent the total number of values possible
are as follows:

28 = 256

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If unsigned numbers were to be stored in the exponent they would range
from (as shown in the right)

0, 1, 2…                 127, 128…         255

But, the exponent can be negative or positive. In that case the signed
range that can be contained within the exponent would be as shown on the right.

-127, -126, -125… -0, +0, 1, 2… 128

 

         
         negative numbers                    positive numbers

As it can been seen from above there are two 0s, one negative, one positive. So
to solve this problem of the extra, redundant 0, the numbers from the signed range
below (negative and positive) are mapped to unsigned range above as shown by
the red arrows (as displayed in the two ranges below)
0, 1, 2… 127 128, 129… 255

The left range maps to all values from -127 to -1 and 0 (128 numbers),
the right range maps to all values from 1 to 128 (127 numbers). However, in IEEE
754, the minimum exponent of
-127 (where every bit is equal to 0) and +128 (all 1s) are reserved for special
values such as
+- ? (Infinity), denormalized numbers and NaNs (Not a
Number). Hence, the actual exponent range is -126… 127 where -126 is stored
as 1 and 0 is stored as 127. It allows every number from -127 to 127 to be
uniquely mapped from 0-254 and avoids mapping the 0 twice. This implies that
the exponent of 0 is stored with a bias of 127 (0 + 127), the exponent of -126
is stored as 1 (-126 + 127), the exponent of 127 is stored as 254 (127 + 127),
and so on.

Biasing is essential since exponents have to be signed values in order
to represent values ranging from very small to very large. However, it might
become difficult to compare two or more values due to two’s complement (how signed
values are usually represented). This is why in order to store the exponent in
8 bits, a bias of +127 is added to the base 10 exponent and then it is converted
into a base 2 representation (8-bits) (e.g., an exponent of 4 is adjusted with
a bias of 127 to become 131(10) which is then converted into an
8-bit IEEE 754 single precision representation as 1000 0011(2)).