IBM PC clone

An IBM PC clone, as its name implies, is a clone of the IBM PC, albeit often extended beyond the original IBM PC's abilities.

If you had a personal computer, it was almost definitely an IBM PC clone, unless you were poor enough to use a Soviet ZX Spectrum clone (unheard of outside of the Soviet Union), or rich enough to use an Apple Mac (almost unheard of outside of the US).

As the IBM PC used off-the-shelf components, with the only original part being the BIOS, all a competitor needed to do to clone it was to make or buy a compatible BIOS that functioned exactly the same, but technically was made by someone who hadn't knowingly copied any of the source code.

Companies like Phoenix Technologies and American Megatrends provided just such a BIOS, by having some engineers document what IBM's BIOS did, which they then passed along to an engineer who hadn't seen IBM's code, who then replicated its functionality without using the original code as a reference. That way, even if part of it had the exact same code because it was the logically sensible way to accomplish the needed result, they could plausibly claim in court that they hadn't copied the code itself, only the outcome produced by the code.^[1]

This enabled any manufacturer with sufficient capital to create their own IBM PC clone, by combining a Phoenix or American Megatrends BIOS chip with regular parts. (Preinstalling DOS helped, but IBM didn't own that either. Microsoft did, and they were happy to sell it.) IBM was unable to sue the companies who cloned the BIOS, or the companies who slapped together whole machines, and so their particular personal computer became the de facto standard, with almost everyone buying a compatible machine, but almost no-one buying an official one.

References

"Phoenix Says Its BIOS May Foil IBM's Lawsuits" James Langdell, PC Mag, Jul 1984, p. 56

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