Phil Storrs PC Hardware book

The history of the Personal Computer system

The term Personal Computer was first used by IBM as a name for the computer that marked IBMs entry into the market for desk top computers. The IBM PC, released in late 1981, used an Intel 8088 microprocessor chip and used PCDOS as its operating system.

This was not the first Personal Computer, Apple, Commodore, and Tandy to mention only the most successful manufacturers, had been marketing personal computers based on various eight bit processor chips since about 1975. The PCDOS operating system was made available to any computer manufacturer that wished to use it under the name MSDOS and soon many manufacturers were producing DOS computers that were more or less compatible with the IBM PC.

Where do personal computers fit into the computer scene ?

Main Frame Mini Computer Personal Computers Definition: A personal computer is a stand-alone computer, used by one person at a time.

The first readily available personal computers were produced in 1975 to 1977 and were the:

Personal Computers before the IBM PC

These first generation Personal Computers used an 8 bit microprocessor chip and so were limited to a maximum of 64K Byte of memory. This limit was imposed by the 16 bit Address Buss available to 8 bit processors (216 = 65356 or 64K). The opening user interface was BASIC, a high level computer language, and applications were generally written in this language and loaded and run from the Basic Interpreter built into the computers firmware.

Many of these early Personal Computers used different microprocessor chips and different Microprocessors use quite different "machine codes" and programming techniques. Programs were not transportable from one brand of computer to another. Two of the most popular computers used the Z80 or the 6502 processors. Tandy TRS 80 = Z80 and the Apple II = 6502. Different brands of computer used different data storage techniques on audio cassette tape or floppy disks and so not even data was transportable between computers. Memory and disk storage was very expensive so few early PCs had more than 16k Bytes of RAM and even fewer had Floppy Disk Drives !.

Although the TRS 80 and Apple II were the most popular Personal Computers in the late 70s, several other manufacturers produced 8 bit computers.

CPM The first universal Operating System ?

An attempt was made at producing a common operating system. CPM (Control Program for Microcomputer) was meant to be a universal operating system that was loaded from a Floppy Disk and provided a simple user interface that would allow applications written or compiled into machine code to be loaded and run from the opening prompt. CPM would only work on computers using the 8080 family of processors, these were the 8080, 8085 and the Z80. As Apple and Commodore computers were not based on the 8080 family of processors they required add-on hardware to provide CPM functionality. CPM was the starting point for DOS. Not only did CPM require an 8080 family processor, manufacturers soon used different floppy disk formats and this meant an end to transportability between many brands of computer.

DOS, the successful universal Operating System

DOS provided a simple user interface that enabled the user to load and run "applications" (programs) written or compiled into the processors machine code.

1980 saw the availability of 16 bit processors with 16 bit data processing and a 20 bit address bus that gave the processor access to 1 Meg Byte of memory space (220 = 1,1024,000). The first two 16 bit processors available from Intel were the 8086 and the 8088. The 8088 processor is the most important one of these because it was the chip used in the IBM PC and PC/XT computers. The 8086 processor was only used in a few clone PC type computers.

IBM had started developing an 8 bit CPM PC but when Apple displayed a 16 bit computer in late 1979, IBM called in INTEL and MICROSOFT to help it with the design of the PC and its operating system. MICROSOFT insisted on the operating system being available for all who wanted to use it, and so MSDOS was born.

PCDOS was a version for IBM and MSDOS was the version for the rest of the computer industry. For all practical purposes, the early MSDOS and PCDOS were the same.

The INTEL 16 bit Microprocessors

The IBM PC and PC/XT used an INTEL 8088 processor. The 8088 processor is a 16 bit processor that has a 16 bit internal data bus and an 8 bit external data bus. The 8 bit external data bus meant that the PC could use the cheap, readily available old 8 bit I/O chips, used in the 8 bit computers. This processor has a 20 bit address bus and this gives it access to a maximum of 1 Meg Byte of addressable memory. The Bus Slots in computers using this processor are called Eight bit Slots because of the eight bit external data bus. The Bus Connector is a double sided edge connector and has 62 pins.

Some other early DOS Computer manufacturers used the 8086 processor instead of the 8088. The 8086 is a true 16 bit processor with a 16 bit internal data bus and a 16 bit external data bus. These computers accessed the memory 16 bits at a time, instead of eight bits at a time like the real IBM but they still used the same eight bit external data slot. Another processor that was sometimes used in PC/XT type computers, from other manufacturers, was the V20 from NEC. This chip is a cloned 8088 and performs faster for the same clock speed than the original Intel 8088 processor. V20 was the industry name for the chip but it's actual type number was D70108C-x where x is a number that indicated the maximum clock speed the chip could operate at. NEC also built a V30 chip, a cloned 8086 processor.

The IBM PC and PC/XT

 

The IBM PC is on the left and the IBM PC/XT is on the right.

The original IBM PC, released in 1981, used an 8088 processor, had 16k of RAM as standard (expandable to 64k on the system board), 5 Bus Slots, a Cassette Tape I/O Port for program and data storage, a Basic Interpreter in four 8 KByte ROMs, and an 8 KByte BIOS ROM. The Power Supply provided 63 Watt of power as +/- 5 volt and +/- 12 volts. The Internal Data Bus was 16 bit wide but the External Data Bus was only 8 bit wide, and the Address Bus was 20 bit wide. The Firmware that started up the computer and provided Basic Input Output Services was contained in the 8KByte BIOS ROM.

A Floppy Disk Interface and Floppy Disk Drives were available as an option, as were plug in cards that provided Parallel, Serial and Games Ports, and more RAM.

Two optional Video Interfaces were available, a Colour Graphics Adaptor (CGA) and a Mono Display Adaptor (MDA). The MDA card provided a character based only display with no graphics capability, and also had a built in Parallel Port for connection of a printer.

The IBM PC/XT, released in 1982, was a more advanced PC with one Floppy Drive, a 10 MEG Hard Disk Drive and 256k of RAM as standard. The System Board had 8 bus slots and no cassette tape interface, and the power supply could supply 130 Watts. This computer also had a Basic Interpreter, but did not have the Cassette Tape I/O Port. The PC/XT still used the 8088 processor so it had a 16 bit internal data bus and an 8 bit external data bus and a 20 bit address bus.

Note:- The 8 bit bus slots first used in the IBM PC are still in use today in our most modern hardware - they form part of the ISA Bus slots. The ISA Bus was introduced with the PC/AT and was created by adding an extra 36 pin connector to provide the extra data and address lines. The eight bit data bus slots used in the PC and PC/XT type computers use a 62 pin double sided edge connector and are referred to as the Eight bit Bus slots.

Other DOS Computer Manufacturers and CLONES

As DOS was available to all computer manufacturers many manufacturers, some of who had produced CPM computers, soon produced DOS computers. Some of these computers were less than successful and it soon became evident that to be successful, DOS computers had to be IBM compatible. Most used different Video Display standards and some used different Floppy Disk formats. NECs Power Mate range started with eight inch Floppy Disk Drives and later went to 80 track 5.25 inch Floppy Disk Drives, rather than IBM's 40 track Floppy Disk Drives.

It was not long before manufacturers in Asia cloned the IBM PC and PC/XT and sold copies at half to one third of the price of an IBM computer. Some of these were exact copies of the IBM product. The other DOS computer manufacturers had to avoid legal action by IBM over copyright so they built DOS computers that obeyed DOS rules, but were not always IBM compatible.

To be IBM compatible, a computer had to use the same I/O addresses, the same Video Display standards and have a very similar BIOS (Basic Input Output Services) to the real IBM. Some software even looked for the word IBM inside the computers BIOS ROM to make sure it only ran on the "real thing".

The BIOS is the code in a ROM, the firmware, that starts the computer at the time of switch-on and provides many of the Basic Input Output Services required by DOS and applications.

IBM compatibility was solved by the eventual availability of compatible BIOS code. The compatible BIOS even had the word IBM, written as this is not IBM, in the appropriate spots.

The TURBO computer

The other manufacturers soon developed the PC/XT and way past the IBM original specification. IBM did not try to catch up with these manufacturers, it continued to make the same hardware in spite of the competition. IBM did make one or two other models of computer but these were far from successful in the market place and soon disappeared. The model JX is one such example.

Most eight bit bus (XT compatible) computers soon had the clock speed increased from IBMs 4.7 MHz to 7, 8, 10, 12 and then 16 MHz. Most processor chips have a number or letter after the type number that indicates the maximum clock speed the device can operate at.

The "other brands" hardware came with Real Time Clocks, Serial and Parallel Ports and Games Ports as standard. These were often still options with IBM hardware.

Towards the end of the time these first generation PCs were manufactured, some manufacturers even used 80286 processors but still used the eight bit bus slots. This provided 16 bit access to the memory and allowed the use of processors with clock speeds of up to 20MHz. These computers were still PC/XT type computers, limited to a maximum of one Meg of memory space.

The PC/AT (80286 based) Computer

IBMs next development of the PC was the PC/AT, released late in 1983. The AT used a 80286 processor, a true 16 bit processor with a 16 bit External Data Bus and a 24 bit Address Bus. The 24 bit Address Bus gave the AT access to a maximum of 16 Meg Byte of addressable memory space (224 = 16,384,000). Other manufacturers built similar computers based on an 80286 processor and the alternative title, a 286 Computer, was often used to describe them. The 16 bit bus introduced in the IBM PC/AT became known as the ISA Bus, (Industry Standard Architecture) and was based on the original eight bit Bus introduced in the original IBM PC. A 36 pin connector was added to the end of the 62 pin connector to provide the extra eight data lines and four address lines. The ISA Bus also provides more "IRQ" and "DMA" lines.

Other manufacturers soon pushed the ATs clock speed from IBMs 6 MHz to 8, 10, 12, 16 and then 20 MHz. IBM stuck to its 6 MHz and did not develop the AT computer any further.

The third generation DOS computers

In 1987, when the 32 bit 80386 processor was released, IBM changed direction with a new naming convention, a new bus and a new operating system. IBM went to its PS/2 range of computers, many with a new bus called MCA (Micro Channel Architecture) and a new Operating System, called OS/2.

Two things prevented the MCA bus achieving much market penetration.

The rest of the personal computer industry went on to name computers after the processor they used, and many of these other manufacturers formed an Industry Committee to set standards for the personal computer industry.

Note: When IBM called its new bus, Micro Channel Architecture, the Industry Committee called the 16 bit bus, first used in the AT, the Industry Standard Architecture (ISA) Bus.

 

An IBM model PS/2 50

This same Industry Committee designed a new Bus, based on the ISA Bus, and called it the Extended Industry Standard Architecture (EISA) Bus. The EISA Bus has the advantage of being able to accept all the old plug-in cards that were made for the 8 bit (XT) Bus or the 16 bit ISA Bus.

The EISA Bus achieves compatibility by having a connector with two sets of contacts, one above the other. The ISA contacts are on the top and the extra EISA contacts are on the bottom. EISA cards have slots so that they can drop down and make contact with both row of contacts.

The 80386 processor was made in two variations, the 80386DX which had 32 bit external Data and Address busses, and the 80386SX with 16 bit external Data bus and 24 bit Address bus. The 80386 chip had a 32 bit internal Data bus. The 32 bit Address bus of the 80386DX processor gave this "chip" an Address range of 4.29Gig (232 = 4,294,967,296).

Fourth generation PC computers

The 80486 processor was the fourth generation processor and had 32 bit internal and external Data and 32 bit Address busses. This chip was also produced in several versions, starting with the 80486SX, then the 80486DX, 80486DX2 and the 80486DX4. The Cyrix / IBM 80586 chip is a Pentium OverDrive clone with a 64 bit internal data path and a 32 bit external data path and belongs to a "four and a half generation" !.

The fifth generation PC computer

Intel changed the naming of its processor chips with the introduction of the Pentium processor to stop other manufacturers using the name for their products. If Intel had called this chip the 80586 any other manufacturer could have called their clones of the chip, the 80586, because this is an industry standard naming convention. All of the Intel family of processor chips have been cloned by other manufacturers, Cyrix and IBM and AMD being the most prominent in recent times. Some manufacturers used the names 80586 and 80686 for clone Pentium chips.

The Pentium is a 64 bit processor and it works with 64 bit data. The Pentium has a 32 bit external Data bus and a 64 bit internal Data bus. The Address bus is still 32 bits.

The Pentium processor was soon enhanced with the release of the Pentium Pro processor in early 1996. This processor has the level 2 cache built into the processor rather than being located on the system board. In mid 1997 Intel released a Pentium II (Pentium two) processor and new versions of the original Pentium called the Pentium MMX. The other manufacturers, IBM and Cyrix and AMD also have more advanced processors on the market. You will learn about these in PC Servicing Two.

Alternative Personal Computer Platforms

The IBM personal computers were based on microprocessors from Intel. As other manufacturers started to make DOS computers the term PC tended to be used to describe any hardware that used the DOS operating system. It is possibly more correct to refer to these computers from "other" manufacturers as DOS computers or Intel processor based computers, rather than PC computers.

The term PC was registered by IBM as a trade name and strictly speaking, the other manufacturers of DOS computers could not use the name PC. In the early eighties the term PC referred to a particular model of IBM personal computer but as time has gone by, the term has been used to refer to any PC type computer, a computer using one of the Intel microprocessor chips or one of the clones of these processors.

You will see the term Platform used in describing the processor used in a personal computer, the computers we are studying in this course use the Intel Platform. Remember not all the processors we use in this platform are built by Intel, alternative devices are made by Cyrix and IBM and by AMD. The Apple Macintosh used a Motorola 68000 series processor and so this is referred to as being a Motorola Platform.

The PowerPC

The computer giants, Apple and IBM combined with Motorola and a number of other companies to develop an alternative PC platform and operating system. The aim of the consortium was to give computer users an alternative to products from Intel and Microsoft. In 1995 we had DOS, Windows, Windows 95, and Windows NT as alternative operating systems for the Intel platform, and AIX (a version of UNIX with a graphical user interface), Windows NT and Apples System 7.5 operating on PowerPC platforms. The PowerPC based systems from IBM have been far from successful in the market place and Apple has not captured a larger market share with it's offerings.

A traditional PC Computer system consists of:

*Note: In older traditional DOS computers the Floppy and Hard Drive Drive Interfaces, and the basic I/O functions were provided by plug in cards. Today the Floppy and Hard Drive Interfaces and the Parallel and Serial I/O Ports are provided on the System Board. The Games Port is usually provided on the Sound Card.

PC Computer case options

Modern PC computers are available in a wide range of cases and these include.

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Copyright © Phil. Storr, last updated 26th December 1998