The Intel processor line, colloquially referred to as x86, has followed a long, evolutionary development. It started with one of the first single-chip, 16-bit microprocessors, where many compromises had to be made due to the limited capabilities of integrated circuit technology at the time. Since then, it has grown to take advantage of technology improvements as well as to satisfy the demands for higher performance and for supporting more advanced operating systems.

The list that follows shows some models of Intel processors and some of their key features, especially those affecting machine-level programming. We use the number of transistors required to implement the processors as an indication of how they have evolved in complexity (K denotes 1000, and M denotes 1,000,000).

• 8086: (1978, 29 K transistors). One of the first single-chip, 16-bit microprocessors. The 8088, a variant of the 8086 with an 8-bit external bus, formed the heart of the original IBM personal computers. IBM contracted with then-tiny Microsoft to develop the MS-DOS operating system. The original models came with 32,768 bytes of memory and two floppy drives (no hard drive). Architecturally, the machines were limited to a 655,360-byte address space—addresses were only 20 bits long (1,048,576 bytes addressable), and the operating system reserved 393,216 bytes for its own use. In 1980, Intel introduced the 8087 floating-point coprocessor (45 K transistors) to operate alongside an 8086 or 8088 processor, executing the floating-point instructions. The 8087 established the floating-point model for the x86 line, often referred to as “x87.”
• 80286: (1982, 134 K transistors). Added more (and now obsolete) addressing modes. Formed the basis of the IBM PC-AT personal computer, the original platform for MS Windows.
• i386: (1985, 275 K transistors). Expanded the architecture to 32 bits. Added the flat addressing model used by Linux and recent versions of the Windows family of operating system. This was the first machine in the series that could support a Unix operating system.
• i486: (1989, 1.2 M transistors). Improved performance and integrated the floating-point unit onto the processor chip but did not significantly change the instruction set.
• Pentium: (1993, 3.1 M transistors). Improved performance, but only added minor extensions to the instruction set.
• PentiumPro: (1995, 5.5 M transistors). Introduced a radically new processor design, internally known as the P6 microarchitecture. Added a class of “conditional move” instructions to the instruction set.
• Pentium II: (1997, 7 M transistors). Continuation of the P6 microarchitecture.
• Pentium III: (1999, 8.2 M transistors). Introduced SSE, a class of instructions for manipulating vectors of integer or floating-point data. Each datum can be 1, 2, or 4 bytes, packed into vectors of 128 bits. Later versions of this chip went up to 24 M transistors, due to the incorporation of the level-2 cache on chip.
• Pentium 4: (2000, 42 M transistors). Extended SSE to SSE2, adding new data types (including double-precision floating point), along with 144 new instructions for these formats. With these extensions, compilers can use SSE instructions, rather than x87 instructions, to compile floating-point code. Introduced the NetBurst microarchitecture, which could operate at very high clock speeds, but at the cost of high power consumption.
• Pentium 4E: (2004, 125 M transistors). Added hyperthreading, a method to run two programs simultaneously on a single processor, as well as EM64T, Intel’s implementation of a 64-bit extension to IA32 developed by Advanced Micro Devices (AMD), which we refer to as x86-64.
• Core 2: (2006, 291 M transistors). Returned back to a microarchitecture similar to P6. First multi-core Intel microprocessor, where multiple processors are implemented on a single chip. Did not support hyperthreading.
• Core i7: (2008, 781 M transistors). Incorporated both hyperthreading and multi-core, with the initial version supporting two executing programs on each core and up to four cores on each chip.

Each successive processor has been designed to be backward compatible—able to run code compiled for any earlier version. There are many strange artifacts in the instruction set due to this evolutionary heritage. Intel has had several names for their processor line, including IA32, for “Intel Architecture 32-bit,” and most recently Intel64, the 64-bit extension to IA32, which we will refer to as x86-64. We will refer to the overall line by the commonly used colloquial name “x86,” reflecting the processor naming conventions up through the i486.