Operating System : Structure of OS

OPERATING SYSTEM STRUCTURE

     One of the most important thing in operating system is it provides the environment within which program are executed. Internally, operating system vary greatly in their operations, since they are organized along many different lines.

     One of the the most important aspect of operating system is the ability to multiprogram.

     A single program cannot, in general, keep either the CPU or I/O devices busy all the time.

     Single users frequently have multiple programs running

     Multiprogramming: It increases CPU utilisation by organising jobs (code and data) so that the CPU always has one to execute.

     The operating system keeps several job in memory simultaneously.

     Since in general main memory is too small to accommodate all jobs, the jobs are kept initially on the disk in the job pool. This pool consists of all processes residing on disk awaiting allocation of main memory. What is job pool?

     Where jobs are queued to be executed when resources are available. The job pool contains both jobs that are currently executing and jobs that have been scheduled but are not yet being executed. When a job is executing, it is fully present in memory.

     The set of jobs in memory can be a subset of the job kept in the job pool.

   Functions:

     The operating system picks and begins to execute one of the job in memory. Eventually, the job may have to wait for some task, such as an I/O operation to complete.

     In non multi-programmed system the CPU would stay idle.

     In multiprogramming system the operating system simply switches to, and execute another job. When that job need to wait the CPU switches to another job and so on.

     A system as large and complex as a modern operating system engineer carefully it is to function properly and be modified easily.

     Some of the components that are interconnected and melted into kernel.

     Simple structure layered approach microkernels modules hybrid system- Mac OS, iOS, Android

Simple Structure:

Application Program

Resident System Program

Memory Stores Device Drivers

ROM BIOS Device Drivers

      Many OS do not have well defined structure. MS DOS is an example offset system it is designed and implemented by few people who have no idea.

     It was written to provide the most functionality in the least space. So it was not carefully divided into modules.

      In MS DOS, the interface and the levels of functionality are not well separated. For instance, application programs are able to access basic I/O routines to write directly to the display and describe was such freedom leaves MS-DOS vulnerable to errant or malicious programs, causing entire system crash when user program fails. MS-DOS was also limited by the hardware of its era. Because the Intel 8088 for which it was written provides no dual mode and no hardware protection.

     Like MS DOS, Unix in initially was Limited buy hardware functionality.  

     It consists of two separable parts: the kernel & the system program.

     Kernel: It is further separated into a series of interfaces and device drivers. The kernel provides the file system, CPU scheduling, memory management and other operating system functions through system call.