What operating systems support preemptive multitasking?

Answer 1

Examples of preemptive operating systems include AmigaOS, the Windows NT family (including XP and Vista), Linux, *BSD, OS/2 2.X - OS/2 Warp 3 - 4.5, Mac OS X and Windows 95/98/ME (32-bit applications only)[1]. Unix and Unix-based systems, and VMS, as well as other systems used in the academic and medium-to-large business markets, have always supported preemptive multitasking, but for a long time were beyond the reach of most users either because of the costs of licensing or the expensive hardware required to support them.

Examples of older, non-preemptive (cooperative) operating systems include Windows 1.x, 2.x, 3.x, Windows for Workgroups, Windows 95/98 (when running 16-bit applications), NetWare, and Classic Mac OS versions (system 5.0 and up). Non-multitasking operating systems include older versions of Mac OS, MS DOS, and Commodore 64 OS which could only execute one program at a time.

Amiga OS, based on the preemptive multitasking TRIPOS system, was the first such system widely available to home users (1985). Running on Motorola 68000-based Amiga systems without memory management, the system used dynamic loading of relocatable code blocks ("hunks" in Amiga jargon) to preemptively multitask all processes in the same flat address space.

Early PC operating systems such as MS-DOS and DR-DOS, did not support multitasking at all. Novell NetWare, Microsoft Windows and OS/2 systems introduced cooperative multitasking to the PC, but did not support preemptive multitasking. In the case of the PC, the slow start was partly because of the need to support a large legacy code base of DOS software written to run in single-user mode on a 8086-based PC, whereas the Amiga system was designed to multitask from the beginning.

The earliest version of Windows to support a limited form of preemptive multitasking was Windows 2.1x, which used the Intel 80386's Virtual 8086 mode to run DOS applications in virtual 8086 machines--commonly known as "DOS boxes"--which were preemptible. In Windows 95 and its successors, Windows 98 and Windows Me, 32-bit applications were made preemptible by running each one in a separate address space, but 16 bit applications remained cooperative. [1] Windows NT, which ran processes in a 32-bit paged address space from the outset, has always supported preemptive multitasking, and this has been inherited by the NT line of Windows systems, including Windows 2000, Windows XP and Windows Vista.

Although there were plans to upgrade the cooperative multitasking Mac OS to a preemptive model (and a preemptive API did exist in Mac OS 9, although in a very limited sense[2] and rarely exploited), these were abandoned in favor of Mac OS X, a re-engineered system based on the BSD-derived Darwin kernel, which supports preemptive multitasking.

OS/2 Warp, IBM's rewrite of an earlier IBM/Microsoft collaboration, OS/2, targeted at 386 systems, supported preemptive multitasking of native applications, and also permitted several different Windows sessions to be multitasked preemptively.

Preemptive multitasking is a rare example of an advanced feature of the Amiga operating system which was not found on other desktops of a similar price range during the heyday of the Amiga. Unfortunately the company behind the Amiga, Commodore International, failed to recognise its potential in the business world and promoted it almost exclusively as a 'games machine' which hampered its acceptance in the business community, a grave tactical error. Very soon after the Commodore company crashed and the Amiga was left with no support. The Amiga OS was so good that various companies have bought the Amiga and to its credit, although it has never again become mainstream, it still has an active community15 years after the Commodore company went bankrupt.