A Brief History of Intel CPUs, Part 1: The 4004 to the Pentium Pro

Intel 8008 The Intel 8008 traded some clock speed — it ran at 500KHz, as opposed to the 4004's 720KHz — for enhanced capabilities. While somewhat slower, it could operate on eight bits of data at a time, rather than the four-bit limitation of the 4004. Initially commissioned before the 4004, the chip was late to market and the company that initially ordered it, Computer Terminal Corporation, later decided not to use the design. The company agreed to allow Intel to market the chip independently, and the 8008 became a commercial success. It was used in the French-built Micral-N minicomputer in 1972 and the Canadian MCM/70 beginning in 1974. The chip used 3500 transistors, which made it significantly more complicated than the 4004 (back then, a few thousand transistors was a serious engineering effort.) Federico Faggin didn't lead the entire project, but he finalized the design and it was finished under his guidance. In 2008, Faggin called the 8008 "the ancestor of the Pentium Pro."

Intel 8080 The Intel 8080 was a 1974 design that followed the 8008 and drastically increased its clock speed, from 500KHz to 2MHz (later chips at up to 3.125MHz would be released). The 8080 was the first chip Faggin designed from scratch, and it used the n-channel MOS process (the 4004 and 8008 had used p-channel MOS), with fewer support chips required and 6000 transistors in the base design. Intel, back then, was known more for its memory than its CPUs, but the 8080 was a huge success in the budding microcomputer market, with prominent design wins and broad industry uptake. The 8080 was the base CPU model used for the fledgling operating system, CP/M. Intel would follow up the 8080 with the 8085 — a binary-compatible CPU with support for 5V operation and several new instructions to support new interrupts.

Intel 8086 The 8086 and 8088 were conceived of as short-term solutions that would generate revenue for Intel while it worked on its "real" next-generation chip, the i432APX. The 8086 and 8088 are nearly identical, but the 8086 implemented a 16-bit internal and external buses, while the 8088 connected to the rest of the system via an 8-bit bus. Ironically, Intel built the 8086 and 8088 partly to fend off competition from Federico Faggin, who had left the company and started his own competing business, Zilog. The "86" in the CPU name came from the fact that it had 8 general-purpose 16-bit registers. The final chip contained 20,000 transistors (29,000 if you counted the ROM and PLA), measured 33mm sq and was built on 3.2-micron technology. It's the grandfather of the modern x86 CPU industry.

Intel 80386 The 80386 was a substantial step forward for Intel's CPU designs. Built on 1.5-micron and 1-micron technology, it was Intel's first 32-bit x86 processor and the first Intel processor that could theoretically address up to 4GB of memory. It expanded and extended the 80286's Protected Mode and included a new virtual 8086 mode that allowed the 386 to emulate multiple 8086 chips simultaneously. This allowed the 386 to execute programs that could only run in Real Mode while using a Protected Mode operating system. The 80386 also supported a flat memory model while in Protected Mode that allowed programs to treat memory as a single, contiguous address space, even though the CPU itself actually used a segmented memory model. The 386SL supported off-chip cache memory mounted on the motherboard (16KB-64KB). As with the 286, other companies introduced 80386 clones — AMD released a 386 clocked up to 40MHz, on a 40MHz bus compared with the 25 and 33-MHz buses Intel supported.

Intel 80486 The 80486 launched in 1989, three years after the initial debut of the 80386. It was the first Intel CPU to contain over a million transistors, the first Intel x86 chip with an on-die L1 cache, and the first tightly-pipelined x86 core (a tight pipeline is one in which each stage performs its operations within the same time slot). The 80486 was the first Intel chip to implement a clock multiplier, in which the CPU runs at multiples of the base bus speed, and the first Intel chip that could regularly complete simple instructions within a single clock cycle. It was also the first Intel chip to implement an on-die FPU (the 486DX had this feature, while the 486SX and previous 386DX chips did not). Intel initially attempted to push bus speeds up to 50MHz but was forced to cut them back due to stability and heat problems.

Intel Pentium and Pentium MMX The Pentium debuted in 1993 as Intel's first non-numerical x86 processor brand. The new CPU included dual integer pipelines, meaning it could sometimes complete up to two instructions per clock cycle (this is referred to as a superscalar architecture). It used a 64-bit external bus instead of the 486's 32-bit bus and it offered separate instruction and data caches to boost performance. FPU performance was also dramatically improved. Intel would later introduce the Pentium MMX, with support for new multimedia instructions (MMX technology), larger caches, and higher overall performance. The Pentium and Pentium MMX have continued to be relevant to Intel's modern CPU business — its initial Atom core (Bonnell) drew inspiration from the 80486 and P5, as did the first few generations of the Xeon Phi (Knights Ferry, Knights Corner). When Intel wanted to demonstrate near-threshold voltage transistors, it again turned to the Pentium core to do so. While Intel's competitors again introduced their own chips, it was becoming increasingly difficult for the other x86 vendors to build products that could go toe-to-toe with Intel.

Intel Pentium Pro The Intel Pentium Pro is the great-great-grandfather of virtually every consumer CPU Intel has built over the past 21 years. It was the first Intel chip to translate x86 instructions into internal micro-ops and to reorder those translated operations for optimal execution within the CPU. Today, this is a common technique known as out-of-order execution, but it was radical at the time and cost Intel a significant amount of die space and power consumption compared with the Pentium. The rated TDP on a Pentium 200 was 15.5W, while a Pentium MMX had a 15.7W TDP. TDP for the Pentium Pro at 200MHz, in comparison, was 35W. This was partly due to the chip's large off-die L2 cache, which consumed significant amounts of power as well. The Pentium Pro's performance improvements were primarily linked to the use of 32-bit code; it could outperform an equivalent-clocked Pentium by 25-35 percent in these tests, but was only 5 percent faster when benchmarked in 16-bit code. This limited the chip's adoption, since MS-DOS, Windows 3.1, and Windows 95 were all entirely or significantly based on 16-bit code.