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COMPUTING SCIENCE

Qwerks of History

Brian Hayes

Eighty-Sixed

The chip that founded the Intel dynasty was designed for a Japanese desktop calculator. The processor, called the 4004, had about 2,300 transistors and operated on four bits of data at a time. The 4004 was followed by the 8008 and the 8080, which were eight-bit processors, and later by the 16-bit 8086. (A transitional morph designated the 8088 was chosen for the first IBM PC in 1981.) Later came the 80186 and the 80286; then, expanding the data path to 32 bits, the 80386 and the 80486. Just when we thought we understood the naming scheme, the next chip in the series was called the Pentium. There have been several further iterations; the current model is the Pentium 4, with a transistor count of 42 million.

The processors in the "x86" series have much more in common than just the corporate logo stamped on the package. (Indeed, chips made by several other companies belong to the same family.) All of the processors since the 8086 are machine-code compatible; that is, a program written for the 8086 in 1978 should run without change on the latest Pentium. This impressive feat of backward compatibility did not come without cost and effort. Through all the generations of processors, the designers have had to preserve the same basic architecture—features such as a set of eight general-purpose registers, a partitioning of the computer's memory space into fixed-size segments, and an instruction set that by modern standards might best be described as funky. The instructions vary in length from eight bits to 108, and they include a baffling variety of modes for addressing data in memory.

Fashions in computer engineering have changed since 1969, and the x86 instruction set now looks as retro as a pair of wide-wale bell-bottoms. Stylish microprocessors these days are mostly RISC designs—an acronym for reduced-instruction-set computer. The instructions are kept simple and few so that they can be executed very fast. This strategy frees up space on the chip for other resources, such as a large set of registers. Uniformity and orthogonality are also prized virtues of RISC designs: Usually, all the instructions have the same length and the same format, and all the registers are interchangeable.

Figure 1. Although the world of computing is ever-changingClick to Enlarge Image

By now, RISC is everywhere in the world of microprocessors—except the x86 line. Even Intel has introduced RISC chips, as well as another adventurous new technology called a very-long-instruction-word processor, but these products are not being marketed for mainstream personal computers. One reading of this situation is that Intel has been trapped by its own success. The designers might well prefer to leave behind the qwerks of the past, but the marketplace will not allow them to abandon 25 years' worth of "legacy code" written for the x86 chips.

It's interesting to compare Intel's predicament with the experience of Apple Computer, which converted the Macintosh to a RISC architecture in 1994, yet maintained compatibility with older software. The changeover was rather like jacking up a house and moving it to a new foundation while the family inside sat down for dinner. The key to backward compatibility was an emulator, a program running on the new hardware that impersonates the old.

Why couldn't Intel do something similar to liberate their customers from the x86 architecture? In fact they did, although the impersonation is done in hardware rather than software. Inside the latest Pentium processors is a core that looks nothing like the earlier x86 chips; it is a RISC-like machine with a large bank of registers and a repertoire of simple instructions called R-ops (for RISC operations). But this core is entirely hidden from the programmer, who continues to deal with the chip as if it had the usual set of eight general-purpose registers and other qwerks. A decoder on the chip reads incoming x86 instructions and translates them into R-ops. Most of the translations are straightforward, but a few of the x86 instructions give rise to streams of more than 100 R-ops.

One might try to argue that the Pentium 4 would be faster without the overhead of translation. The chip real estate dedicated to the decoder could be put to some other use that would improve overall performance. This may be true. On the other hand, users of Pentium 4 systems are not complaining about traveling in the slow lane while the world passes them by; on the contrary, makers of other chips have had to struggle to keep pace. The fastest Pentium has a clock rate greater than 3 gigahertz. It is the gasoline engine of computing.




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