Floating point numbers are positive and negative fractions. They
always occupy a double word (32 bits). A floating point number is
represented as an exponential number. The mantissa is 24 bits long
and the exponent is 8 bits long.

Coding a floating point number:
31 30 ......... 24 23 22.................... 0
符号位 2**6 ... . ... 2**0 符号位 2**(-1) . . . . . . ... 2**(-23)
Exponent Mantissa