An Operating Systems Vade Mecum

ion. Operating systems, like other software components, construct higher-level (virtual) resources out of lower-level (physical) ones. Details of the lower-level structures are hidden, and higher-level structures are introduced. From the point of view of a process, the physical machine is enhanced by the operating system into a virtual machine. Enhancement includes both simplicity (hiding details) and function (introducing new structures). Neither time (ability to execute) nor space (main store) appears to be shared with other processes. The virtual machine is thus simpler than the physical machine. The process interface provides extra instructions that improve on the basic hardware instruction set, particularly with respect to transput. The virtual machine is thus more functional than the physical machine. From the point of view of the user, the operating system provides services that are not present in the underlying machine. These services include loading and running programs, provi..