Virtual Machines
The concept of virtual machines is closely related to layering.
In a typical multi-user system, users are expected to know that the machine is shared by other users, and that resources such as devices are shared between all the users.
In virtual machine operating systems an addition layer of abstraction is placed between the users and the system so that it appears to the user that they are using a machine dedicated to them.
Usually it is the case that a more powerful machine is used to host several virtual machines. For example, the 80386 and later Intel CPUs supported virtual 8086 machines. Thus, an operating system designed for the 80386 could actually run several copies of MS-DOS and appear to the user to be several different PCs at the same time.
Another example of a virtual machine system is the IBM 370 running the VM operating system. This allowed users to work as if they had a dedicated (but smaller, less powerful) 370 completely at their disposal.
In a typical multi-user system, users are expected to know that the machine is shared by other users, and that resources such as devices are shared between all the users.
In virtual machine operating systems an addition layer of abstraction is placed between the users and the system so that it appears to the user that they are using a machine dedicated to them.
Usually it is the case that a more powerful machine is used to host several virtual machines. For example, the 80386 and later Intel CPUs supported virtual 8086 machines. Thus, an operating system designed for the 80386 could actually run several copies of MS-DOS and appear to the user to be several different PCs at the same time.
Another example of a virtual machine system is the IBM 370 running the VM operating system. This allowed users to work as if they had a dedicated (but smaller, less powerful) 370 completely at their disposal.
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