Extended precision

Extended precision refers to floating point number formats that provide greater precision than the basic floating point formats. Extended precision formats support a basic format by minimizing roundoff and overflow errors in intermediate values of expressions on the base format. In contrast to extended precision, arbitrary-precision arithmetic refers to implementations of much larger numeric types (with a storage count that usually is not a power of two) using special software (or, rarely, hardware).

The IBM 1130 offered two floating point formats: a 32-bit "standard precision" format and a 40-bit "extended precision" format. Standard precision format contained a 24-bit two's complement significand while extended precision utilized a 32-bit two's complement significand. The latter format could make full use of the cpu's 32-bit integer operations. The characteristic in both formats was an 8-bit field containing the power of two biased by 128. Floating-point arithmetic operations were performed by software, and double precision was not supported at all. The extended format occupied three 16-bit words, with the extra space simply ignored.

The IBM System/360 supports a 32-bit "short" floating point format and a 64-bit "long" floating point format. The 360/85 and follow-on System/370 added support for a 128-bit "extended" format. These formats are still supported in the current design, where they are now called the "hexadecimal floating point" (HFP) formats.

The Microsoft BASIC port for the 6502 CPU, such as in adaptations like Commodore BASIC, AppleSoft BASIC, KIM-1 BASIC or MicroTAN BASIC, supported an extended 40-bit variant of the floating point format Microsoft Binary Format (MBF) since 1977.

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