History of the PC

The fourth and current generation of the modern computer includes those that incorporate microprocessors in their designs. Of course, part of this fourth generation of computers is the personal computer, which itself was made possible by the advent of low-cost microprocessors and memory.

In 1973, some of the first microcomputer kits based on the 8008 chip were developed. These kits were little more than demonstration tools and didn't do much except blink lights. In late 1973, Intel introduced the 8080 microprocessor, which was 10 times faster than the earlier 8008 chip and addressed 64KB of memory.

This was the breakthrough the personal computer industry had been waiting for. A company called MITS introduced the Altair kit in a cover story in the January 1975 issue of Popular Electronics.

The Altair kit, considered the first personal computer, included an 8080 processor, a power supply, a front panel with a large number of lights, and 256 bytes (not kilobytes) of memory.

The kit sold for $395 and had to be assembled. Assembly back then meant you got out your soldering iron to actually finish the circuit boards, not like today where you can assemble a system of premade components with nothing more than a screwdriver.

The Altair included an open architecture system bus called the S-100 bus because it had 100 pins per slot. The open architecture meant that anybody could develop boards to fit in these slots and interface to the system. This prompted various add-ons and peripherals from numerous aftermarket companies.

The new processor inspired software companies to write programs, including the CP/M (control program for microprocessors) operating system and the first version of the Microsoft BASIC (beginners all-purpose symbolic instruction code) programming language.

IBM introduced what can be called its first personal computer in 1975. The Model 5100 had 16KB of memory, a built-in 16-line–by–64-character display, a built-in BASIC language interpreter, and a built-in DC-300 cartridge tape drive for storage.

The system's $9,000 price placed it out of the mainstream personal computer marketplace, which was dominated by experimenters (affectionately referred to as hackers) who built low-cost kits ($500 or so) as a hobby.

Obviously, the IBM system was not in competition for this low-cost market and did not sell as well by comparison. The Model 5100 was succeeded by the 5110 and 5120 before IBM introduced what we know as the IBM Personal Computer (Model 5150).

Although the 5100 series preceded the IBM PC, the older systems and the 5150 IBM PC had nothing in common. The PC that IBM turned out was more closely related to the IBM System/23 DataMaster, an office computer system introduced in 1980.

In fact, many of the engineers who developed the IBM PC had originally worked on the DataMaster. In 1976, a new company called Apple Computer introduced the Apple I, which originally sold for $666.66.

The selling price was an arbitrary number selected by one of Apple's cofounders, Steve Jobs. This system consisted of a main circuit board screwed to a piece of plywood; a case and power supply were not included.

Only a few of these computers were made, and they reportedly have sold to collectors for more than $20,000. The Apple II, introduced in 1977, helped set the standard for nearly all the important microcomputers to follow, including the IBM PC.

The microcomputer world was dominated in 1980 by two types of computer systems. One type, the Apple II, claimed a large following of loyal users and a gigantic software base that was growing at a fantastic rate.

The other type, CP/M systems, consisted not of a single system but of all the many systems that evolved from the original MITS Altair. These systems were compatible with one another and were distinguished by their use of the CP/M operating system and expansion slots, which followed the S-100 standard.

All these systems were built by a variety of companies and sold under various names. For the most part, however, these systems used the same software and plug-in hardware. It is interesting to note that none of these systems was PC compatible or Macintosh compatible, the two primary standards in place today.

A new competitor looming on the horizon was able to see that to be successful, a personal computer needed to have an open architecture, slots for expansion, a modular design, and healthy support from both hardware and software companies other than the original manufacturer of the system.

This competitor turned out to be IBM, which was quite surprising at the time because IBM was not known for systems with these open-architecture attributes! IBM, in essence, became more like the early Apple, and Apple itself became like everybody expected IBM to be.

The open architecture of the forthcoming IBM PC and the closed architecture of the forthcoming Macintosh caused a complete turnaround in the industry.


At the end of 1980, IBM decided to truly compete in the rapidly growing low-cost personal computer market. The company established the Entry Systems Division, located in Boca Raton, Florida, to develop the new system.

The division was located intentionally far away from IBM's main headquarters in New York, or any other IBM facilities, so that this new division would be able to operate independently as a separate unit.

This small group consisted of 12 engineers and designers under the direction of Don Estridge and was charged with developing IBM's first real PC. (IBM considered the previous 5100 system, developed in 1975, to be an intelligent programmable terminal rather than a genuine computer, even though it truly was a computer.)

Nearly all these engineers had come to the new division from the System/23 DataMaster project, which was a small office computer system introduced in 1980 and was the direct predecessor of the IBM PC. Much of the PC's design was influenced by the DataMaster design.

In the DataMaster's single-piece design, the display and keyboard were integrated into the unit. Because these features were limiting, they became external units on the PC, although the PC keyboard layout and electrical designs were copied from the DataMaster.

Several other parts of the IBM PC system also were copied from the DataMaster, including the expansion bus (or input/output slots), which included not only the same physical 62-pin connector, but also almost identical pin specifications.

This copying of the bus design was possible because the PC used the same interrupt controller as the DataMaster and a similar direct memory access (DMA) controller. Also, expansion cards already designed for the DataMaster could easily be redesigned to function in the PC.

The DataMaster used an Intel 8085 CPU, which had a 64KB address limit and an 8-bit internal and external data bus. This arrangement prompted the PC design team to use the Intel 8088 CPU, which offered a much larger (1MB) memory address limit and an internal 16-bit data bus, but only an 8-bit external data bus.

The 8-bit external data bus and similar instruction set enabled the 8088 to be easily interfaced into the earlier DataMaster designs. IBM brought its system from idea to delivery of functioning systems in one year by using existing designs and purchasing as many components as possible from outside vendors.

The Entry Systems Division was granted autonomy from IBM's other divisions and could tap resources outside the company, rather than go through the bureaucratic procedures that required exclusive use of IBM resources. IBM contracted out the PC's languages and operating system to a small company named Microsoft.

That decision was the major factor in establishing Microsoft as the dominant force in PC software today. On August 12, 1981, a new standard was established in the microcomputer industry with the debut of the IBM PC.

Since then, hundreds of millions of PC-compatible systems have been sold, as the original PC has grown into an enormous family of computers and peripherals. More software has been written for this computer family than for any other system on the market.