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).

Contents

Summary

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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