Real-Number Data in IEEE Format (30601)

SUMMARY

By default, the Microsoft Macro Assembler Versions 5.1 (MASM) and later assemble real-number data in IEEE format. The previous versions of the assemblers used the Microsoft binary format by default.

MORE INFORMATION

The following is the IEEE format for encoding 4- and 8-byte real numbers:

   Short Real Number
     31      23      15      7       0
    _________________________________
   |_|_______|_______|_______|_______|
   ^ ^       ^                       ^
   | |       |                       |
   | Exponent|                       |
   |         |Mantissa ------------->|
   |Sign

The parts of the real numbers are described below:

Sign (0 for positive or 1 for negative) is in the upper bit of the first byte.
Exponent is in the next bits in sequence (8 bits for a short real number or 11 bits for a long real number).
All except the first-set bit of Mantissa are in the remaining bits of the variable (13 bits for short real numbers, and 52 bits for long real numbers). Because the first significant bit is known to be set, it does not need to be stored.