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The IA32, x86-64, and Itanium processors support what is by far the most influential format on this standard, the Intel 80-bit (64 bit significand) "double extended" format, described in the next section.
This 80-bit format uses one bit for the sign of the significand, 15 bits for the exponent field (i.e. the same range as the 128-bit quadruple precision IEEE 754 format) and 64 bits for the significand. The exponent field is biased by 16383, meaning that 16383 has to be subtracted from the value in the exponent field to compute the actual power of 2. [20] An exponent field value of 32767 (all fifteen bits 1) is reserved so as to enable the representation of special states such as infinity and Not a Number. If the exponent field is zero, the value is a denormal number and the exponent of 2 is −16382. [21] The 8087 had 80-bit registers so that if the inputs to your computation had 64-bit accuracy, the outputs would also have 64-bit accuracy. Extended precision refers to floating-point number formats that provide greater precision than the basic floating-point formats. [1] 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 IEEE 754 floating-point standard recommends that implementations provide extended precision formats. The standard specifies the minimum requirements for an extended format but does not specify an encoding. [7] The encoding is the implementor's choice. [8] Calculations can be completed a little faster if all bits of the significand are present in the register.Extra digits make it easier for ordinary mortals to write floating-point calculations that won't go wrong for hard-to-analyze reasons. The more extra precision you have, the more you can imagine that your custom formulas (a + bx + cyz) will behave similarly to library functions (sin x) that were designed by experts to have a error of no more than 1 ulp over their whole domain. You don't get guaranteed accuracy, but you do get more reliable accuracy. The IBM 1130, sold in 1965, [2] offered two floating-point formats: A 32-bit "standard precision" format and a 40-bit "extended precision" format. Standard precision format contains a 24-bit two's complement significand while extended precision utilizes a 32-bit two's complement significand. The latter format makes full use of the CPU's 32-bit integer operations. The characteristic in both formats is an 8-bit field containing the power of two biased by 128. Floating-point arithmetic operations are performed by software, and double precision is not supported at all. The extended format occupies three 16-bit words, with the extra space simply ignored. [3]
The 80-bit floating-point format was widely available by 1984, [25] after the development of C, Fortran and similar computer languages, which initially offered only the common 32- and 64-bit floating-point sizes. On the x86 design most C compilers now support 80-bit extended precision via the long double type, and this was specified in the C99 / C11 standards (IEC 60559 floating-point arithmetic (Annex F)). Compilers on x86 for other languages often support extended precision as well, sometimes via nonstandard extensions: for example, Turbo Pascal offers an Extended type, and several Fortran compilers have a REAL*10 type (analogous to REAL*4 and REAL*8). Such compilers also typically include extended-precision mathematical subroutines, such as square root and trigonometric functions, in their standard libraries. Intel created a series of floating-point coprocessors for the x86 called the x87. In addition to supporting IEEE single and double precision numbers, it also supported an 80-bit extended precision number. Some C compilers (e.g. clang) mapped this to the long double type in C, but others (e.g. MSVC) didn't. Quiet Not a Number, the sign bit is meaningless. The 8087 and 80287 treat this as a Signaling Not a Number. Infinity. The sign bit gives the sign of the infinity. The 8087 and 80287 treat this as a Signaling Not a Number. The 8087 and 80287 coprocessors used the pseudo-infinity representation for infinities.double must have greater or equal precision as float. At no point it says one must be 64-bit and the other 32-bit precision. Unnormal. Only generated on the 8087 and 80287. The 80387 and later treat this as an invalid operand. The value is (−1) s × m × 2 e−16383 The x86 extended precision format is an 80-bit format first implemented in the Intel 8087 math coprocessor and is supported by all processors that are based on the x86 design that incorporate a floating-point unit (FPU).
Many languages have no built-in support for this type. The most recent example I know of that does is Swift, which has a Float80 type only available when compiling for Intel processors. (Swift also has CLongDouble which represents the exact type that the C compiler takes long double to mean, which is sometimes the same thing as Double.) The only time I've seen Float80 or long double used in practice is to use the increased precision to emulate a fused multiply-add instruction on older processors that don't support it, or very rarely to avoid loss of precision when converting from a 64-bit integer. And since it's a "legacy" instruction set, modern CPUs don't tend to optimize x87 instructions very well. If you don't need float80, then you have the option to do all your x86 floating point with SSE, which is a much more "normal" architecture with a random-access register file ( xmm), and ignore the x87 altogether. SSE is supported by all x86-64 CPUs, and by all 32-bit x86 CPUs from the last 20 years or so.The x87 and Motorola68881 80-bit formats meet the requirements of the IEEE 754 double extended format, [12] as does the IEEE754 128-bit format. long float: type to which float values are promoted for computations (and might be as small as float or as big as long double. This means that, for x64 applications, there is no guarantee the x87 instruction set will be present.
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