Pentium (original)

The Pentium (also referred to as the i586 or P5 Pentium) is a microprocessor introduced by Intel on March 22, 1993. It is the first CPU using the Pentium brand. Considered the fifth generation in the x86 (8086) compatible line of processors, succeeding the i486, its implementation and microarchitecture was internally called P5.

Like the Intel i486, the Pentium is instruction set compatible with the 32-bit i386. It uses a very similar microarchitecture to the i486, but was extended enough to implement a dual integer pipeline design, as well as a more advanced floating-point unit (FPU) that was noted to be ten times faster than its predecessor.

The Pentium was succeeded by the Pentium Pro in November 1995. In October 1996, the Pentium MMX was introduced, complementing the same basic microarchitecture of the original Pentium with the MMX instruction set, larger caches, and some other enhancements. Intel discontinued the original Pentium (P5) processors, which were sold as a lower-cost option after the Pentium II's release in 1997, on December 31, 2001. This coincided with Microsoft ending support for classic versions of Windows such as Windows 95. The Pentium line was gradually replaced by the Celeron processor, which also took over the role of the 80486 brand.

The P5 Pentium is the first superscalar x86 processor, meaning it was often able to execute two instructions at the same time. Some techniques used to implement this were based on the earlier superscalar Intel i960 CA (1989), while other details were invented exclusively for the P5 design. Large parts were also copied from the i386 or i486, especially the strategies used to cope with the complicated x86 encodings in a pipelined fashion. Just like the i486, the Pentium used both an optimized microcode system and RISC-like techniques, depending on the particular instruction, or part of instruction. The dual integer pipeline design is something that had been argued being impossible to implement for a CISC instruction set, by certain academics and RISC competitors.^[who?]

Other central features include a redesigned and significantly faster floating-point unit, a wide 64-bit burst-mode data bus (external as well as internal), separate code and data caches, and many other techniques and features to enhance performance. It contains 256-bit internal data buses and write-back caches. It does contain System Management Mode that has been implemented since the Intel's SL architecture.

The 66-MHz Pentium processor operates at 112 V1.1 Dhrystone MIPS and has SPECint92 rating of 64.5, a SPECfp92 rating of 56.9 and an iCOMP index rating of 567. The performance difference between 60- and 66-MHz version is about 10%.

The P5 also has better support for multiprocessing compared to the i486, and is the first x86 CPU with hardware support for it similar to IBM mainframe computers. Intel worked with IBM to define this ability and also designed it into the P5 microarchitecture. This ability was absent in prior x86 generations and x86 processors from competitors.

In order to employ the dual pipelines at their full potential, certain compilers were optimized to better exploit instruction level parallelism, although not all applications would substantially gain from being recompiled. The faster FPU always enhanced floating point performance significantly though, compared to the i486 or i387. Intel spent resources working with development tool vendors, ISVs and operating system (OS) companies to optimize their products.