IEEE 754-1985

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The first IEEE Standard for Binary Floating-Point Arithmetic (IEEE 754-1985) set the standard for floating-point computation for 23 years. It became the most widely-used standard for floating-point computation, and is followed by many CPU and FPU implementations. Its binary floating-point formats and arithmetic are preserved in the new IEEE 754-2008 standard which replaced it.

The 754-1985 standard defines formats for representing floating-point numbers (including negative zero and denormal numbers) and special values (infinities and NaNs) together with a set of floating-point operations that operate on these values. It also specifies four rounding modes and five exceptions (including when the exceptions occur, and what happens when they do occur).



IEEE 754-1985 specifies four formats for representing floating-point values: single-precision (32-bit), double-precision (64-bit), single-extended precision (≥ 43-bit, not commonly used) and double-extended precision (≥ 79-bit, usually implemented with 80 bits). Only 32-bit values are required by the standard; the others are optional. Many languages specify that IEEE formats and arithmetic be implemented, although sometimes it is optional. For example, the C programming language, which pre-dated IEEE 754, now allows but does not require IEEE arithmetic (the C float typically is used for IEEE single-precision and double uses IEEE double-precision).

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