An illustrated Guide to CPU's from 8086 to Pentium-II
Click & Learn. Module 3a. WWW.MKDATA.DK
About CPU's
To understand the data processing methodology, an understanding of the design and function of the CPU is essential. The following subjects will be covered on these pages. They are divided in three sub modules, which ought to be read as a unit.
●Brief CPU review from 1st to 6th generation
●What is a CPU?
●8086 compatibility
●The CPU improvements
●About clock doubling
●Cache RAM
●The CPU – areas of development
●The CPU – speed measurements
●CPU changes - historic review of 286, 386, 486
●Pentium
●MMX
●Cyrix 6X86
●Pentium Pro
●Pentium II
●CPU sockets and chip set
●Miscellaneous notes about CPU's
●Clocking and over clocking
You ought to read modules 2a, 2b, 2c, 2d, and 2e first.
The CPU is certainly the most important PC component. CPU stands for Central Processing Unit. Let us briefly study that name:
●It is a processor, because it processes data.
●It is central, because it is the center of PC data processing.
●It is a unit, because it is a chip, which contains millions of transistors.
Without the CPU, there would be no PC. Like all other hardware components, the CPU's are continually undergoing further development. You can see the explosive technological development in data processing most clearly in the development of newer and faster CPU's. The CPU's have for years doubled their performance about every 18 months, and there are no
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An illustrated Guide to CPU's from 8086 to Pentium-II
indications that this trend will stop.
When we now look at all the CPU's from a broader perspective, we can see that:
●The CPU history is closely tied to the companies IBM and especially Intel.
●The CPU's have their roots back to Intel's chip 4004 from 1971.
●You can identify six CPU generations up till today.
●The compatibility concept has been important throughout the development.
CPU's - brief review
CPU history starts in 1971, when a small unknown company, Intel, for the first time combined multiple transistors to form a central processing unit - a chip called Intel 4004. However, it was 8 years before the first PC was constructed.
PC's are designed around different CPU generations. Intel is not the only company manufacturing CPU's, but by far the leader one. The following table shows the different CPU generations. They are predominantly Intel chips, but in the 5th generation we see alternatives:
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An illustrated Guide to CPU's from 8086 to Pentium-II
We will start by looking at what the CPU really does:
What is a CPU?
The CPU is centrally located on the system board. Since the CPU carries out a large share of the work in the computer, data pass continually through it. The CPU continually receives instructions to be executed. Each instruction is a data processing order. The work itself consists mostly of calculations and data transport:
Data have a path to the CPU. It is kind of a data expressway called the system bus. You will hear much more about this later.
Two types of data
The CPU is fed long streams of data via the system bus. The CPU receives at least two types of data:
●Instructions on how to handle the other data.
●Data, which must be handled according to the instructions.
What we call instructions is program code. That includes those messages, which you continuously send to the PC from the mouse and keyboard. Messages to print, save, open, etc. Data are typically user data. Think about the letter, which are writing to Aunt Karen. The contents, letters, images, etc., are user data. but then you say "print," you are sending program code (instructions):
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8086 compatible instructions
The biggest job for the CPU consists of decoding the instructions and localizing data. The calculations themselves are not heavy work.
The decoding consists of understanding the instructions, which the user program sends to the CPU. All PC CPU's, are "8086 compatible." This means that the programs communicate with the CPU in a specific family of instructions.
These instructions were originally written for the Intel 8086 processor, which founded the concept "the IBM compatible PC." The 8086 from 1978 received its instructions in a certain format. Since there was a desire that subsequent CPU generation should be able to handle the same instructions which the 8086 could, it was necessary to make the instruction sets compatible. The new CPU's should understand the same instructions. This backwards compatibility has been an industry standard ever since. All new processors, regardless of how advanced, must be able to handle the 8086 instruction format.
Thus, the new CPU's must use much effort to translate the 8086 instruction format to internal instruction codes:
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CISC and RISC instructions and their handling
The first CPU's had a so called Complex Instruction Set Computer (CISC). This means that the computer can understand many and complex instructions. The X86 instruction set, with its varying length from 8 to 120 bit, was originally developed for the 8086 with its mere 29000 transistors.
Reduced Instruction Set Computer (RISC): The RISC instructions are brief and the same length (for example 32 bit long, as in Pentium Pro), and they process much faster than CISC instructions. Therefore, RISC is used in all newer CPU's. However, the problem is that the instructions arrive at the CPU in 8086 format. Thus, they must be decoded
For every new CPU generation, the instruction set has been expanded. The 386 came with 26 new instructions, the 486 with 6 new instructions, and Pentium with 8 new instructions. These changes mean that some programs require at least a 386 or a Pentium processor to work.
There is also a continuous optimizing of the instruction handling process. One is that the clock frequency increases, as we will see later - the faster, the better. But what can the CPU do in one clock tick. That is critical to its performance. For example, a 386 needed 6 clock ticks to add a number to a sub total. A job which the 486 manages in only two clock ticks, because of more effective instruction decoding, 5th and 6th generation CPU's can execute more than one of those operations in one clock tick, since they contain more processing lines (pipelines),
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which work parallel.
Read also the section about MMX instructions.
Floating-point unit - FPU
The first CPU's could only work with whole numbers. Therefore, it was necessary to add a mathematical co-processor (FPU), when better math power was needed. Later, this FPU was built into the CPU:
It is said that Intel's CPU's have by far the best FPU units. Processors from AMD and Cyrix definitely have a reputation for providing sub standard performance in this area. But, you may not utilize the FPU. That depends on the applications (user programs) you are using. Common office programs do not use the floating point operations, which the FPU can handle. However, 3D graphics programs like AutoCad do. Therefore, if you use your PC in advanced design applications, the FPU performance becomes significant. For most users, it is only of limited importance.
Many brand names
As mentioned, there are CPUs of many brand names (IBM, Texas, Cyrix, AMD), and often they make models, which overlap two generations. This can make it difficult to keep of track of CPU's. Here is an attempt to identify the various CPU's according to generation:
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Now, let us see how CPU speed has been improved through generations. Click for Module 3b.
To overview
Last revised: 20 May 1998. Copyright (c) 1996, 1997, 1998 by Michael B. Karbo. WWW.MKDATA.DK.
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