Editing X86 (section)

===Floating-point unit===
{{Main|x87}}
{{Further|Floating-point unit}}
[[File:Intel chips 386 387.jpg|thumb|An Intel 386 with the 387 co-processor]]
Early x86 processors could be extended with [[floating-point]] hardware in the form of a series of floating-point [[numerical analysis|numerical]] [[co-processor]]s with names like [[Intel 8087|8087]], 80287 and 80387, abbreviated x87. This was also known as the NPX (''Numeric Processor eXtension''), an apt name since the coprocessors, while used mainly for floating-point calculations, also performed integer operations on both binary and decimal formats. With very few exceptions, the 80486 and subsequent x86 processors then integrated this x87 functionality on chip which made the x87 instructions a [[de facto]] integral part of the x86 instruction set.

Each x87 register, known as ST(0) through ST(7), is 80&nbsp;bits wide and stores numbers in the [[IEEE floating-point standard]] double extended precision format. These registers are organized as a stack with ST(0) as the top. This was done in order to conserve opcode space, and the registers are therefore randomly accessible only for either operand in a register-to-register instruction; ST0 must always be one of the two operands, either the source or the destination, regardless of whether the other operand is ST(x) or a memory operand. However, random access to the stack registers can be obtained through an instruction which exchanges any specified ST(x) with ST(0).

The operations include arithmetic and transcendental functions, including trigonometric and exponential functions, and instructions that load common constants (such as 0; 1; e, the base of the natural logarithm; log2(10); and log10(2)) into one of the stack registers. While the integer ability is often overlooked, the x87 can operate on larger integers with a single instruction than the 8086, 80286, 80386, or any x86 CPU without to 64-bit extensions can, and repeated integer calculations even on small values (e.g., 16-bit) can be accelerated by executing integer instructions on the x86 CPU and the x87 in parallel. (The x86 CPU keeps running while the x87 coprocessor calculates, and the x87 sets a signal to the x86 when it is finished or interrupts the x86 if it needs attention because of an error.)