Tài liệu CPU Architecture

4.1 Chapter Overview
This chapter discusses history of the 80x86 CPU family and the major improvements occuring along the line.
The historical background will help you better understand the design compromises they made as well as understand
the legacy issues surrounding the CPU s design. This chapter also discusses the major advances in computer
architecture that Intel employed while improving the x861.
4.2 The History of the 80x86 CPU Family
Intel developed and delivered the first commercially viable microprocessor way back in the early 1970 s: the
4004 and 4040 devices. These four-bit microprocessors, intended for use in calculators, had very little power.
Nevertheless, they demonstrated the future potential of the microprocessor — an entire CPU on a single piece of
silicon2. Intel rapidly followed their four-bit offerings with their 8008 and 8080 eight-bit CPUs. A small outfit
in Santa Fe, New Mexico, incorporated the 8080 CPU into a box they called the Altair 8800. Although this was
not the world s first personal computer (there were some limited distribution machines built around the 8008
prior to this), the Altair was the device that sparked the imaginations of hobbyists the world over and the personal
computer revolution was born.
Intel soon had competition from Motorola, MOS Technology, and an upstart company formed by disgrunteled
Intel employees, Zilog. To compete, Intel produced the 8085 microprocessor. To the software engineer, the
8085 was essentially the same as the 8080. However, the 8085 had lots of hardware improvements that made it
easier to design into a circuit. Unfortunately, from a software perspective the other manufacturer s offerings
were better. Motorola s 6800 series was easier to program, MOS Technologies 65xx family was easier to program
and very inexpensive, and Zilog s Z80 chip was upwards compatible with the 8080 with lots of additional
instructions and other features. By 1978 most personal computers were using the 6502 or Z80 chips, not the Intel
offerings.
Sometime between 1976 and 1978 Intel decided that they needed to leap-frog the competition and produce a
16-bit microprocessor that offered substantially more power than their competitor s eight-bit offerings. This initiative
led to the design of the 8086 microprocessor. The 8086 microprocessor was not the world s first 16-bit
microprocessor (there were some oddball 16-bit microprocessors prior to this point) but it was certainly the highest
performance single-chip 16-bit microprocessor when it was first introduced.
During the design timeframe of the 8086 memory was very expensive. Sixteen Trạng Quỳnhbytes of RAM was selling
above $200 at the time. One problem with a 16-bit CPU is that programs tend to consume more memory
than their counterparts on an eight-bit CPU. Intel, ever cogniscent of the fact that designers would reject their
CPU if the total system cost was too high, made a special effort to design an instruction set that had a high memory
density (that is, packed as many instructions into as little RAM as possible). Intel achieved their design goal
and programs written for the 8086 were comparable in size to code running on eight-bit microprocessors. However,
those design decisions still haunt us today as you ll soon see.