History of CPU`s

Where it all began

The "brain" of a computer is the central processor chip - this does all the mathematical manipulations, and controls the way data is fed into and out of the computer.

The central processor chip, commonly known as the CPU, is supported within the computer by a number of other specialised chips, which help in the data processing and data flow.

The original Intel 8080 was the starting point for the development of the modern PC. The 8080 was an 8 bit device, which required an external clock generator, and external chips to control its bus system.

This was followed by the 8085, which had internal clock generation.

The 8085 was an 8 bit device, with 16 bit address capability. The first 8 address lines were also the 8 data lines, and during the execution of each instruction, some of the time these lines would carry address information, and some of the time they would carry data.

The next generation of CPU was the 8086. This had a 16 bit architecture, so had 16 data lines, and 20 address lines.

As with the 8085, some pins had dual functions, with the first 16 address lines also functioning as data lines, and the top 4 address lines also functioning as status indicator lines, again on a time shared multiplexing basis.

The 8086 was used in the XT range of computers.

286

The next development was the 80286, known as a 286.

This was still a 16 bit processor, but had 24 address lines, so could address 16Mbytes of memory.

Two new types of package were developed, with separate pins for each function - no multiplexing was neccessary. The packages were either Plastic Leaded Chip Carrier, or Pin Grid Array. Both had 68 pins.

It was the first CPU in the line to enable virtual memory, and multitasking, which allowed the use of a Windows environment or OS/2.

Although there were a few computers produced known as XT/286, mostly the 286 was used in the AT range of computers.

Within the AT there was a 16 bit data bus.

The 286 could operate in two different modes :-

Real mode - where the 286 behaved like a 8086, and used the 8086 instruction set.

The CPU would only use the bottom 640K of RAM, and had an address limit of 1Mb.

DOS typically utilised real mode.
Protected mode - where the CPU would recognise the full 16Mb of memory, and used all of its features.

Protected mode was the mode used by the higher level operating systems such as Windows or OS/2.

386

The next development in CPU`s was the 80386, or 386.

This is a 32 bit processor, made in two versions :-

386DX - which is a 32 bit processor with a 32 bit data bus
386SX - which is a 32 bit processor, but only has a 16 bit data bus

Both versions have a 32 bit address bus.

In order to accommodate all the connections to the CPU, the processor used another new package in which all pins had unique functions, and multiplexing was not required - this package is a Pin Grid Array with 132 pins.

With the 386, the previously used ISA busses were not sufficient in capacity, and the computer industry split - one manufacturer produced the microchannel bus, and a consortium of other manufactures produced the EISA bus system.

Although the microchannel system had a number of advantages, it was new, and all previous expansion cards were not useable with it.

The EISA system retained compatibility with all the existing 8 bit and 16 bit ISA expansion cards.

The 386 CPU also has the same two modes of operation as the 286, ie, real mode and protected mode. However it has a larger instruction set than the 286, and runs faster - up to 25MHz or 40 MHz.

486

As technology advanced, the 386 was superseded by the 486 - this is again 32 bit architecture, and has a similar instruction set to the 386.

However it is faster - up to 100MHz.

It also concentrated on increasing performance - one method is to incorporate a short term memory cache of 8K inside the CPU - this saves time because the CPU does not have to access external memory for short term storage.

The 486 uses another new package - a Pin Grid Array with 168 pins.

In order to achieve the lowest possible impedance on the power supply lines, of these 168 pins, 24 are used for the +ve supply, and 28 are used for the ground supply.

The 486 is made in two versions :-

486DX - this contains a maths coprocessor within the chip
486SX - this does not - when this version is used the motherboard nearly always has an additional socket fitted for a separate maths coprocessor.

486 based computers often feature one of a further two types of internal bus structure :-

VESA or VLB - this is an additional connector placed in line with ISA slots, so that an extender card can plug into the two ISA connectors plus the VESA connector.

Backward compatibility is retained, in that 8 bit and 16 bit ISA cards can still be used.

The motherboard is designed so that the VESA connectors are physically adjacent to the CPU, and the VESA signals are accessed directly to the CPU, rather than through a seperate BUS controller, as with the ISA signals. VESA signals will match the CPU clock speed up to 33 MHz.

A typical motherboard will have 4 or 5 ISA expansion slots, and 2 or 3 ISA expansion slots with in-line VESA slots.
PCI - this is a separate connector ( often coloured white ), which is again located physically close to the CPU.

A PCI bus controller is used to interface with the CPU busses.

PCI runs at a fixed speed of 33 MHz.

A typical motherboard will have 4 or 5 ISA expansion slots, and 2 PCI expansion slots.

Pentium

After a while, the 486 was superseded by the Pentium - again this performed all the functions that the 486 performed, but added a few more.

The Pentium chip contains two CPU`s inside the one chip, and a much faster maths coprocessor.

There is a 256 bit internal bus, and connection with the outside world is via a 64 bit bus.

The internal cache is increased to 16K, with 8k allocated to data, and 8k allocated to code.

The Pentium is available with speeds up to 233 Mhz.

The early Pentiums, running at 60 and 66 Mhz, used another new package with 273 pins.

The later Pentiums, running at 75 Mhz and above, come in yet another new package, a Staggered Pin Grid Array, with 296 pins. Of these, 53 pins are used for the +ve supply, and another 53 are used for the ground connection.

The Pentium chip contains a section that tries to guess what the next instruction is going to be, and downloads the required information from memory - if it is correct, then the instruction can be executed immediately, if it is wrong, the information is just dumped, and the required instruction is fetched from memory, and then performed.

The Pentium also contains two pipelines - one for each CPU.

A pipeline is used as a method of storing the next few instructions inside the chip - when a CPU is ready for the next instruction, it is just pulled straight out of the end of the pipeline. Along with the carrying out of the instruction, the CPU also fills up the queue of instructions waiting in the pipeline, by extracting instructions from external memory, and feeding them into the other end of the pipline.

Pipelining was used to a certain extent on the 386 and the 486, but it was never really pushed as a feature of these processors.

Pentium based PC`s nearly all use the PCI bus system.

The Pentium processor recognises the same instruction set as the 486.

The Pentium processor can operate in the same two modes as the 386 and the 486, ie, Real mode and Protected mode.

Pentium Pro and Pentium MMX

The Pentium was superseded by two different CPU`S - the Pentium MMX, and the Pentium Pro.

The Pentium PRO is designed for high-end professional computer systems such as servers.

It has 6 pipelines, more internal registers, and can use the 16k of internal cache either as two blocks of 8k, allocated respectively to data and to code, or as one block of 16k, allocated dynamically.

In addition, there is a second level cache of 256k inside the chip. It comes in an even bigger package, with 387 pins.

The Pentium MMX is designed to allow faster manipulation of data - such as for video decompression, synthesis of sound, and graphics.

It has 32k of internal cache, allocated 16k each to data and to code.

It has 57 additional instructions, each one being capable of manipulating eight bytes of data at one time, instead of the normal one byte.

Pentium II

The two types of Pentium have come together again with the creation of the Pentium II, which is essentially an improved Pentium Pro with MMX.

The Pentium II requires a completely redesigned mother board from the other Pentium versions.

The second level cache is now outside the CPU chip, very close to the CPU chip.

Currently Pentium II runs at up to 450 Mhz.

Effect of speed

Along with the development of CPU`s from the 8080 up to the Pentium II, has been the increase in the speed at which the CPU operates.

So whereas the 286 ran at a typical speed of between 16 Mhz and 25 Mhz, Pentium II`s run at up to 450 Mhz, and who knows what is on the horizon.

A side effect of faster operation is higher heat generation within each transistor within the chips.

Also, as the number of transistors in side the chips has increased, it is increasingly difficult for the transistors to loose heat.

The combined effects of these two conditions meant that steps had to be taken to try and reduce the amount of heat generated - this has been done by reducing the supply voltage from 5 volts to 3.3 volts in some 486`s and Pentium`s, and to 2.9 volts in Pentium II`s.

Compatibility

One of the key elements of the Intel CPU developments has been backward compatibility - a programme designed to run on a 386 will run on a modern Pentium based computer.

However sometimes a newer operating system can make this difficult.

Other CPU`s

There are now companies producing CPU`s that are equivalent to the above CPU`s.

Some of these equivalents are plug-in replacements for the above CPU`s, and some are different.

However provided the PC is an IBM compatible, the user will not be aware of the difference.

Performance

The substantial growth in both functional ability and in speed of the CPU`s has resulted in the ability of the desk top computer to process very sophisticated applications such as video, graphics, and 3-D CAD.

However the applications that many users require, ie, word processing, database management, and accountancy systems, are much less demanding in terms of raw processing power, and it is not neccessary to acquire computers at the leading edge of technology.

In many cases, therefore, a basic entry level PC is more than adequate for the needs of many users.

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