b is 8 bits

b0 b1 b2 ... b7

value of b = b0 * 2^0 + b1 * 2^1 + b2 * 2^2 + ... + b7 * 2^7

b = 0   => 00000000
b = 1   => 00000001
b = 2   => 00000010
b = 3   => 00000011
...
b = 9   => 00001001  1*2^0 + 1*2^3 = 1 + 8 = 9
...
b = 255 => 11111111

negative numbers are represented using a two's complement representation.