COMPUTER PROGRAMME

A. Learning Objectives

1. Defining Operating System.
2. Identify the various types of OS and discuss the features of the each type

B. Learning Outcomes

1. Identify the various types of OS and discuss the features of the each type
2. To identify the key features of the each types of OS

Objectives
 To provide a grand tour of the major operating systems components
 To provide coverage of basic computer system organization

What is an Operating System?
 A program that acts as an intermediary between a user of a computer and the computer hardware.
 Operating system goals:
 Execute user programs and make solving user problems easier.
 Make the computer system convenient to use.
 Use the computer hardware in an efficient manner.

Computer System Structure
 Computer system can be divided into four components
 Hardware – provides basic computing resources
 CPU, memory, I/O devices
 Operating system
 Controls and coordinates use of hardware among various applications and users
 Application programs – define the ways in which the system resources are used to solve the computing problems of the users
 Word processors, compilers, web browsers, database systems, video games
 Users
 People, machines, other computers

Four Components of a Computer System


Operating System Definition

 OS is a resource allocator
 Manages all resources
 Decides between conflicting requests for efficient and fair resource use
 OS is a control program
 Controls execution of programs to prevent errors and improper use of the computer
 No universally accepted definition
 “Everything a vendor ships when you order an operating system” is good approximation
 But varies wildly
 “The one program running at all times on the computer” is the kernel. Everything else is either a system program (ships with the operating system) or an application program

Computer Startup
 bootstrap program is loaded at power-up or reboot
 Typically stored in ROM or EPROM, generally known as firmware
 Initializates all aspects of system
 Loads operating system kernel and starts execution

Computer System Organization
 Computer-system operation
 One or more CPUs, device controllers connect through common bus providing access to shared memory
 Concurrent execution of CPUs and devices competing for memory cycles



Computer-System Operation
 I/O devices and the CPU can execute concurrently.
 Each device controller is in charge of a particular device type.
 Each device controller has a local buffer.
 CPU moves data from/to main memory to/from local buffers
 I/O is from the device to local buffer of controller.
 Device controller informs CPU that it has finished its operation by causing an interrupt.

Common Functions of Interrupts
 Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines.
 Interrupt architecture must save the address of the interrupted instruction.
 Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.
 A trap is a software-generated interrupt caused either by an error or a user request.
 An operating system is interrupt driven.

Interrupt Handling
 The operating system preserves the state of the CPU by storing registers and the program counter.
 Determines which type of interrupt has occurred:
 polling
 vectored interrupt system
 Separate segments of code determine what action should be taken for each type of interrupt

Interrupt Timeline


I/O Structure
 After I/O starts, control returns to user program only upon I/O completion.
 Wait instruction idles the CPU until the next interrupt
 Wait loop (contention for memory access).
 At most one I/O request is outstanding at a time, no simultaneous I/O processing.
 After I/O starts, control returns to user program without waiting for I/O completion.
 System call – request to the operating system to allow user to wait for I/O completion.
 Device-status table contains entry for each I/O device indicating its type, address, and state.
 Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt.

Two I/O Methods (a) Synchronous (b) Asynchronous


Device-Status Table


Direct Memory Access Structure
 Used for high-speed I/O devices able to transmit information at close to memory speeds.
 Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention.
 Only one interrupt is generated per block, rather than the one interrupt per byte.

Storage Structure
 Main memory – only large storage media that the CPU can access directly.
 Secondary storage – extension of main memory that provides large nonvolatile storage capacity.
 Magnetic disks – rigid metal or glass platters covered with magnetic recording material
 Disk surface is logically divided into tracks, which are subdivided into sectors.
 The disk controller determines the logical interaction between the device and the computer.

Storage Hierarchy
 Storage systems organized in hierarchy.
 Speed
 Cost
 Volatility
 Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage.

Storage-Device Hierarchy


Caching
 Important principle, performed at many levels in a computer (in hardware, operating system, software)
 Information in use copied from slower to faster storage temporarily
 Faster storage (cache) checked first to determine if information is there
 If it is, information used directly from the cache (fast)
 If not, data copied to cache and used there
 Cache smaller than storage being cached
 Cache management important design problem
 Cache size and replacement policy

Performance of Various Levels of Storage
 Movement between levels of storage hierarchy can be explicit or implicit


Operating System Structure

 Multiprogramming needed for efficiency
 Single user cannot keep CPU and I/O devices busy at all times
 Multiprogramming organizes jobs (code and data) so CPU always has one to execute
 A subset of total jobs in system is kept in memory
 One job selected and run via job scheduling
 When it has to wait (for I/O for example), OS switches to another job
 Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing
 Response time should be < 1 second
 Each user has at least one program executing in memory process
 If several jobs ready to run at the same time  CPU scheduling
 If processes don’t fit in memory, swapping moves them in and out to run
 Virtual memory allows execution of processes not completely in memory