Introduction to OS [3]

• operating system provides environment for program execution

• primary goal: make the computer convenient to use

• secondary goal: use the hardware in an efficient manner.

• OS is intermediary between user (and applications) and H/W

• OS is a manager and allocator of resources

• must allocate resources efficiently and fairly

• OS is a control program

• controls the execution of user programs

• prevents errors and improper use of the computer

• OS is the one program running at all times on the computer

Intro: Abstract View of OS as Intermediary [4]

Intro: Early Systems [5]

• large and expensive machines run from a console

• programmer was also the operator

• many separate steps:

• loading/unloading of magnetic and paper tapes, punch cards

• while tapes were being mounted:

• CPU sat idle - no work was being done

Intro: Evolutionary Steps [6]

• professional operator was hired

• jobs with similar needs were batched together and run as a group

• resident monitors transferred control from job to job

• overlapped CPU and I/O operations were introduced

• replaced card readers, line printers with magnetic-type units

• readers and printers were operated off-line

• replaced tapes (sequential-access) with disks (random-access)

• introduced simultaneous peripheral operation on-line (spooling)

Intro: Spooling [7]

Intro: Multiprogramming [8]

• increases CPU utilization

• organizes jobs so that CPU always has something to execute

• OS keeps several jobs in memory

• OS picks and begins to execute one of the jobs

• OS switches to/executes another job when current job waits

• when a program does I/O, the CPU works on another program

• all the jobs are kept in the job pool

• on disk awaiting allocation of main memory

• if not enough memory, OS must choose (job scheduling)

• OS allocates memory (memory management)

• OS allocates CPU to one process (CPU scheduling)

Intro: Memory Layout for Multiprogramming [9]

Intro: Batch Systems [10]

• multiprogrammed but do not permit interaction with the user

• appropriate for executing large jobs that need little interaction

• But

• user provides commands to handle job steps (may be dependent)

• programs must be debugged statically

Intro: Time-Sharing Systems [10]

• multiprogrammed and do allow interaction with the user

• switch rapidly from one user to the next

• each user has impression of their own computer

• programs can be larger than physical memory (virtual memory)

• programs interact with user (short time before it needs to do I/O)

Intro: Process Management [11]

• OS manages processes, memory, files, networking

• a process is program in execution

• OS is responsible for process management:

• creation/deletion of both user and system processes

• suspension/resumption of processes

• process synchronization, communication and deadlock handling

Intro: Memory Management [11]

• CPU directly addresses main memory (and no other storage device)

• OS is responsible for memory management:

• which parts of memory are being used and by whom

• which processes are to be loaded into memory

• allocate/deallocate memory space as needed

Intro: Secondary-Storage Management [12]

• main memory is too small for all data and programs

• info lost when power is lost

• secondary storage (disk) is used to back up main memory

• OS is responsible for disk management:

• item free-space management

• storage allocation

• disk scheduling

Intro: File Management [13]

• info stored on several different types of physical media

• OS provides a uniform logical view of info (unit of storage: file)

• OS is responsible for file management:

• creation/deletion of files and directories

• support of primitives for manipulating files and directories

• mapping of files onto secondary storage

• backup of files on stable (nonvolatile) storage media

Intro: Network Management [13]

• a distributed system collects

• physically separate, possibly heterogeneous systems

• into a single coherent system

• processors in system are connected by communication network

• LANs (local-area networks) and WANs (wide-area networks)

• OS generalizes network access as a form of file access

Intro: OS Services [14]

• OS provides services to programs and users:

• program execution and I/O operations

• file-system manipulation

• communications

• error detection

• resource allocation

• accounting and protection

Intro: System Calls [14]

• system calls: interface between running program and OS

• process control

• file manipulation

• device manipulation

• information maintenance

• communications

Intro: System Programs [15]

• convenient environment for program development/execution:

• file manipulation and modification

• status information

• programming-language support

• program loading and execution

• communications

• application programs

Intro: UNIX Limited Structure [16]

Intro: VENUS Layered Structure [17]

• OS are complex and require a layered solution

• increases modularity

• a function only uses functions of lower-level layers