Operating System Layer

The Operating System layer is the view of a systems programmer (for example someone who writes compilers) or a system administrator's view.

Virtual Multi-Tasking

Virtual multi-tasking means creating the appearance that there are multiple virtual machines, each doing something different but at the same time. For example, word processing in one window, web surfing in another.

In reality, the instructions being executed by these parallel virtual machines are taking turns being executed by a single machine.

Virtual Memory

The size of main memory for a virtual machine can actually be larger than the size of memory for the implementing machine at the next lower level.

This is made possible by automatically swapping infrequently used data to secondary memory until it is needed. The user is never aware of this swapping.

Files and File Systems

Secondary memory at the operating system layer is the file system.

The file system is a tree. Parent nodes are directories or folders, leaf nodes are files such as programs and documents.

User Interfaces

MS Windows Explorer (GUI):

DOS Command Console (CUI):