IBM Floating Point Architecture - Extended-precision 128-bit

Extended-precision 128-bit

Extended-precision (quadruple-precision) was added to the System/370 series and was available on some S/360 models (S/360-85, -195, and others by special request or simulated by OS software). The extended-precision mantissa (fraction) field is wider, and the extended-precision number is stored as two double words (16 bytes):

High-Order Part: 1 7 56 bits +-+-----------+----------------------------------------------------+ |S| Exp | Fraction (left-most 14 of 28 digits) | +-+-----------+----------------------------------------------------+ 127 126 120 119 64 Low-Order Part: 8 56 bits +-------------+----------------------------------------------------+ | Unused | Fraction (right-most 14 of 28 digits) | +-------------+----------------------------------------------------+ 63 56 55 0

28 hexadecimal digits of precision is roughly equivalent to 32 decimal digits. A conversion of extended precision hexadecimal float to decimal string would require at least 35 significant digits in order to convert back to the same hexadecimal float value.