In the beginning
Proprietary operating systems were made to sell the company's hardware. Without system software (compilers and operating systems), a budding hardware developer had great difficulty launching a computer; the availability of operating systems not tied to a single hardware supplier - such as Digital Research's CP/M for microcomputers, and Unix for larger computers - greatly transformed the computer industry; someone with an innovative idea could easily start producing hardware on which buyers could use standard software. In 1969-70, UNIX first appeared on the PDP-7 and later the PDP-11. It soon became capable of providing cross-platform time sharing using preemptive multitasking, advanced memory management, memory protection, and a host of other advanced features. UNIX soon gained popularity as an operating system for mainframes and minicomputers alike. Unix was inspired by Multics, as were several other operating systems, such as Data General's AOS-VS, and IBM's addition of such concepts as subdirectories to PC DOS in version 2.0.
Microsoft bought QDOS from Seattle Computer Products, a very simple diskette operating system somewhat similar to CP/M, to create an operating system, PC DOS, for the launch of the IBM PC, under a deal with IBM where Microsoft could still sell the operating system as MS DOS for non-IBM computers. Microsoft produced odd-numbered major version numbers while IBM was responsible for even revision numbers (2.0, 2.1, 4.0, etc) of the code base until version 6. There was very little difference between MS-DOS and PC-DOS, one example being the inclusion of GW-BASIC with MS-DOS (because some BASIC code in IBM PC ROMs was not legally allowed to be put into non-IBM computers). MS-DOS and PC-DOS soon became known simply as "DOS" (the term is now usually taken to also include other "DOSes" such as DR-DOS and FreeDOS, but it should not be confused with the command prompt program within some operating systems, COMMAND.COM). Although MS-DOS could be tailored to hardware significantly different to IBM's PC, it soon became common for hardware vendors to make their equipment as compatible as possible with the IBM PC and its immediate IBM successors (the PC-XT and later IBM PC-AT models), since many popular DOS programs bypassed the operating system to access hardware directly for speed, requiring other manufactures to closely copy the IBM design, including its limitations. The availability of MS-DOS had two major effects on the computer industry: the commercial acceptability of "sneaky tricks" (as documented, for example, in Ralf Brown's Interrupt List) to gain speed or functionality or copy-protection, and a market that demanded extreme compatibility (speed and cosmetic differences were the only acceptable innovations).
IBM PC compatibles could also run Microsoft Xenix, a UNIX-like operating system from the early 1980s. Xenix was heavily marketed by Microsoft as a multi-user alternative to its single user MS-DOS operating system. The CPUs of these personal computers could not facilitate kernel memory protection or provide dual mode operation, so Xenix relied on cooperative multitasking and had no protected memory.
The 80286-based IBM PC AT was the first IBM compatible personal computer capable of providing protected memory mode operation. However, this mode was hampered by software bugs in its implementation on the 286, and not widely accepted until the release of the Intel 80386. With the 386 porting BSD Unix to a PC became feasible, and various Unix-like systems (tagged "*nix" at the time), including Linux, arose, but IBM (and, initially, Microsoft) opted for OS/2 from the inception of the PS/2; Microsoft eventually going its own way with Microsoft Windows firstly as a GUI on top of DOS, then as a complete operating system.
Classic Mac OS, and Microsoft Windows 1.0-3.11 supported only cooperative multitasking (Windows 95, 98, & ME supported preemptive multitasking only when running 32-bit applications, but ran legacy 16-bit applications using cooperative multitasking), and were very limited in their abilities to take advantage of protected memory. Application programs running on these operating systems must yield CPU time to the scheduler when they are not using it, either by default, or by calling a function.
Windows NT's underlying operating system kernel which was designed by essentially the same team as Digital Equipment Corporation's VMS, a UNIX-like operating system which provided protected mode operation for all user programs, kernel memory protection, preemptive multi-tasking, virtual file system support, and a host of other features.
Classic AmigaOS and versions of Microsoft Windows from Windows 1.0 through Windows Me did not properly track resources allocated by processes at runtime.[citation needed] If a process had to be terminated, the resources might not be freed up for new programs until the machine was restarted.
The AmigaOS did have preemptive multitasking, as did operating systems for many larger ("supermini") computers that, despite being technically better, were struggling in sales when faced with the mass production of increasingly-faster "Personal" Computers and customers locked into non-portable software (legacy software and proprietary office documents).
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