By the time personal computers based on microprocessors began to emerge in the mid-1970s, programmers had been writing operating systems – the software that manages the computer hardware and provides commonly used services for application programs – for about twenty years. Big mainframe computers had operating systems that were huge and complicated, created from hundreds of thousands of lines of code. But other operating systems, designed to fit in the small memory of minicomputers, were tiny. That was the kind that the PCs could use.
Computer Scientist Gary Kildall created just such an operating system in 1974 for a small computer called the “Intellec-8” that Intel had designed to showcase their new microprocessors. Called “CP/M”, it was unlike most other operating systems for small computers because it was written in PL/M, a portable higher-level language that he had designed earlier, rather than in the assembly-language of a particular computer. That meant that CP/M could be ported to run on many different personal computers. And if the applications were written in PL/M, they could be ported as well.
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