Editing IBM hexadecimal floating-point (section)

== Extended-precision 128-bit ==
Called extended-precision by IBM, a [[quadruple-precision floating-point format|quadruple-precision]] HFP format 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 fraction field is wider, and the extended-precision number is stored as two double words (16 bytes):

:{|
|-
|colspan="8" |'''High-order part'''
|- style="text-align:center"
|style="width:20px"|1
|style="width:20px"|
|style="width:50px"|7
|style="width:20px"|
|style="width:20px"|
|style="width:320px"|56
|style="width:20px"|
|style="text-align:left"|''(width in bits)''
|- style="text-align:center"
|colspan="1" style="text-align:center;background-color:#FC9"|S
|colspan="3" style="text-align:center;background-color:#99F"|Exp
|colspan="3" style="text-align:center;background-color:#9F9"|Fraction (high-order 14 digits)
|colspan="1" style="text-align:center;background-color:#FFF"|&nbsp;
|- style="text-align:center"
|127
|126
|...
|120
|119
|...
|64
|align="left"|''(bit index)''*
|-
|colspan="8" |'''Low-order part'''
|- style="text-align:center"
|colspan="4"|8
|
|56
|
|style="text-align:left"|''(width in bits)''
|- style="text-align:center"
|colspan="4" style="text-align:center;background-color:#CCC"|Unused
|colspan="3" style="text-align:center;background-color:#9F9"|Fraction (low-order 14 digits)
|colspan="1" style="text-align:center;background-color:#FFF"|&nbsp;
|- style="text-align:center"
|63
|colspan="2"|...
|56
|55
|...
|0
|align="left"|''(bit index)''*
|-
|colspan="8"| ''* IBM documentation numbers the bits from left to right, so that the most significant bit is designated as bit number 0.''
|}

28 hexadecimal digits of precision is roughly equivalent to 32 decimal digits. A conversion of extended precision HFP to decimal string would require at least 35 significant digits in order to convert back to the same HFP value. The stored exponent in the low-order part is 14 less than the high-order part, unless this would be less than zero.