Why the IBM PC Had an Open Architecture

In some ways, the most far-reaching decision made by the team that built the IBM PC was to use an open architecture rather than one that was proprietary to IBM. That decision led to the market for add-in boards, for large numbers of third-party applications, and eventually to a large number of competitors all creating "IBM-compatible" machines.

It was a complete break from what IBM had ever done before, but at the time, the decision wasn't particularly controversial. According to Blue Magic: The People, Power and Politics Behind the IBM Personal Computer, by James Chposky and Ted Leonsis, when Entry Level Systems Lab Director Bill Lowe went to IBM's Corporate Management Committee in July 1980 to propose the project, he told them, "The only way we can get into the personal computer business is to go out and buy part of a computer company, or buy both the CPU (central processing unit) and software from people like Apple or Atari—because we can't do this within the culture of IBM."

Chposky and Leonsis note that no one person on the team ever took credit for the open-architecture idea, saying that having an open architecture allowed the hardware and software development to be independent and thus made the process faster. The team only had a year between the project's approval and the announcement of the PC.

In addition, IBM had tried a number of personal computer projects before, including the DataMaster. That experience convinced the engineers that an open approach would work better.

One of those engineers was Dave Bradley, a member of the PC development team who had previously worked on device control code for the DataMaster and would go on to do similar work on the PC (including being credited for coming up with the Control-Alt-Delete reset key combination).

Recounting the process for Byte in September 1990, he said, "We had learned from the DataMaster development and from the experiences of others that even a company the size of IBM couldn't develop all the hardware and software to make a personal computer a success."

Only the system unit itself and the keyboard (which had controversial key positions) were actually new designs from IBM, with the system designed to use an existing IBM Japan monitor and a dot-matrix printer made by Epson.

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IBM published a technical reference of the system's circuit designs and software source codes. Because this was public, and because most of the components were well-known within the industry, it was much easier for other companies to develop software and build peripherals. For example, the 8-bit data bus opened the door for add-on board manufacturers, which almost immediately started offering boards that had serial or parallel ports, graphics adapters, or extra memory up to 256K per board. These boards could be combined so the machine could use the full 640K of the processor's 1MB of space allocated to physical memory addresses. Different vendors offered a wide variety of add-on features. Major early players included Tecmar, Quadram, and AST, which originally gained fame with its Sixpack add-on card.

Indeed, a variant of that architecture designed for the IBM AT (running the Intel 286) became known as the Industry Standard Architecture (ISA) bus and would dominate PCs for many years. Hardware makers pushed hard for their machines to be known as "IBM-compatible," making sure the IBM versions of programs such as Lotus 1-2-3 and Microsoft Flight Simulator would run on their machines.

For more, check out PCMag's full coverage of the 40th anniversary of the IBM PC:

• The Golden Age of IBM PCs

• The IBM PC at 40: Lessons Learned

• Why the IBM PC Used an Intel 8088

• The IBM PC's Killer Apps: Where Are They Now?

• Teardown! Inside PC Labs' IBM PC Model 5150

• Project Chess: The Story Behind the Original IBM PC

• The Rise of DOS: How Microsoft Got the IBM PC OS Contract

• Here's What Bill Gates Told PCMag About the IBM PC in 1982

• 40 Years Later: Read PCMag's First Impressions of the IBM PC

• Spec Showdown: The Original IBM PC vs. a Lenovo ThinkCentre AIO