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multitasking
multitasking refers to the running of multiple independent computer programs on the same computer; giving the appearance that it is performing the tasks at the same time. since most compueter can do at most one or two things at one time, this is generally done via time-sharing, which means that each program uses a share of the computer's time to execute.
An operating systemkernel contains a pieces of software called a scheduler which determines how much time each program will spend executing, and in which order execution control should be passed to programs. control is passed to a process by the kernel, which allows the program acess to the CPU and memory. later, control is returned to the kernel through some mechanism, so that another program may be allowed to use CPU.this so-called passing of control between the kernel and applications is called a context switch.
an early model which governed the allocation of time to programs was called cooperative multitasking. in this model, when control is passed to a program by the kernel. it may execute for as long as it wants before explicitly returning control to the kernel. this means that a malicious ormalfunctioning program may not only prevent any other programs from using the CPU, buy it can hang the entire system if it enters an infinite loop.
modern operating systems extend the concepts of application preemption to device drivers and kernel code, so that the operating system has preemptive control over internal run- times as well.
The philosophy governing preemptive miltitasking is that ofensuring that all programs are given regular time an the CPU. this implies that all programs must br limited in how much time they are allowed to spend on the CPU without being interrupted. To accomplish this, modern operating system kernels make use of a timed interrupt. A protected mode timer is by the kernel which triggers a return to supervisor mode after the specified time has elapsed. (see abovesections on interrupts and dual mode operation.)
On many single user operating systems cooperative multitasking is perfectly adeguate, as home computers generally run a small number of well tested programs. The amigaOS is an exception , having pre-emptive multitasking from its very first version. Windows NT was the first version of Microsoft windows which enforced preemptive multitasking, but itdidn’t reach the home user market until windows XP (since windows NT was targeted at professionals).
DISK ACCESS AND FILE SYSTEMS:
Access to data stored on disks is a central feature of all operating systems. Computers store data on disks using files which are structured in specific ways in order to allow for faster access, higher reliability, and to make better use out of the drive’s availablespace.the specific way in which files are stored on a disk is called a file system. And enables files to have names and attributes. Is also allows them to be stored in a hierarchy of diredtories or folders arrangedin a directory tree.
Early operating systems generally supported a single type of disk drive and only one kind of file system.
Early file system were limited in their capacity, speed,and in the kinds of file names and directory structures they could use. These limitations often reflected limitations in the operating systems they were designed for, making it very difficult for an operating system to support more than one file system.
While many simpler aperating systems support a limited range of options for accessing storage systems, operating systems like UNIX and Linuxsupport a technology known as a virtual file systemor VFS. An operating system such as UNIX supports a wide array of storage devices, regardless of their desing or file systems, allowing them to be accessed through a common application programminginterface (API). This makes it unnecessary for programs to have any knowledge about the device they are accessing. A VFS allows the operating system to...