Operating Systems I An Introduction to Operating System

Operating Systems I An Introduction to Operating System Concepts CMSC 104 1

OS Introduction Computer applications today require a single machine to perform many operations and the applications may compete for the resources of the machine. l This demands a high degree of coordination l This coordination is handled by system software known as the operating system l CMSC 104 2

Evolution of Operating System l OS for batch jobs o Program execution required significant preparation of equipment o Program execution (job) o OS was a system to simplify program setup and simplify transition between jobs o Physical separation of users and equipment led to computer operators CMSC 104 3

OS Evolution (cont’d) o User’s left jobs with the operator and came back the next day (batch jobs) o Users had no interaction with computer during program execution. Maybe okay for some applications, but not for all. CMSC 104 4

OS Evolution l OS for Interactive Processing o Allowed programs to carry on dialogue with user via remote terminals (workstations) o Real-time processing o Users demand timely response o Machines too expensive to serve only one user o Common for several users to want interactive services at the same time CMSC 104 5

OS Evolution (cont’d) l OS for time-sharing o To accommodate multiple real-time users, the OS rotates its various jobs in and out of execution via time-sharing o Each job gets a predetermined “time slice” o At end of time slice current job is set aside and a new one starts o By rapidly shuffling jobs, illusion of several jobs executing simultaneously is created CMSC 104 6

OS Evolution (cont’d) o without time slicing, a computer spends most of its time waiting for peripheral devices or users o A collection of tasks can be completed in less time with time-sharing than when completed sequentially CMSC 104 7

Different Operating Systems on the Same Machine ? It is possible to have more than one operating system available to be used on a machine. l Only one operating system is run at a time, though. l Examples: l o VAX - VMS or Ultrix o IBM PCs - DOS or Linux CMSC 104 8

Types of software l Applications software Performs tasks specific to the machine’s utilization. l Generally transportable l l System Software Performs tasks common to computer systems in general l Operating systems vary based on the hardware they’re used on l CMSC 104 9

Types of software (cont’d) l Utility software o providing fundamental activities, yet not included with OS o “extend” the OS o Distinction between applications and utilities is often vague o Distinction between OS and utilities is also vague CMSC 104 10

The OS Shell l Defines interface between OS and users o Windows GUI o UNIX command line o UNIX users can choose among a variety of shells �csh is the “C shell” �tcsh is an enhanced “C shell” o Shell programming CMSC 104 11

OS Shell interface Users O/S Users shell CMSC 104 12

The OS Kernel The internal part of the OS is often called the Kernel l Kernel Components l o File Manager o Device Drivers o Memory Manager o Scheduler o Dispatcher CMSC 104 13

OS File Manager Maintains information about the files that are available on the system l Where files are located in mass storage, their size and type and their protections, what part of mass storage is available l Files usually allowed to be grouped in directories or folders. Allows hierarchical organization. l CMSC 104 14

OS Device Drivers Software to communicate with peripheral devices or controllers l Each driver is unique l Translates general requests into specific steps for that device l CMSC 104 15

OS Memory Manager This unit is responsible for coordinating the use of the machine’s main memory. l It decides what area of memory is to be allocated for a program and it’s data l It allocates and deallocates memory for different programs and always knows what areas are free. l CMSC 104 16

OS Scheduler l Maintains a record of processes that are present, adds new processes, removes completed processes o memory area(s) assigned o priority o state of readiness to execute (ready/wait) CMSC 104 17

OS Dispatcher Ensures that processes that are ready to run are actually executed l Time is divided into small (50 ms) segments called a time slice. l When the time slice is over, the dispatcher allows scheduler to update process state for each process, then selects the next process to run l CMSC 104 18

OS Summary Shell -- interface to user l File Manager -- manages mass memory l Device Drivers -- communicate with peripherals l Memory Manager -- manages main memory l Scheduler & Dispatcher -- manage processes l CMSC 104 19

Utilities Operating Systems usually come with some associated utility programs l UNIX usually has the text editors emacs and vi (and sometimes pico) l UNIX has it’s own sort utility l UNIX has it’s own mail utility l CMSC 104 20