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21CS208 – Operating Systems Unit-II21CS208 – OPERATING SYSTEMS
UNIT II - OVERVIEW OF OPERATING SYSTEMS
Introduction: Computer System Organization, Computer System Architecture, Operating System
Operations. Operating System Structure: OS Services, System calls, Types of System Calls,
Operating – System Structure, OS Generation and System Boot.
----------------------------------------------************-----------------------------------------------------
Text Book: Abraham Silberschatz, Peter Baer Galvin and Greg Gagne, “Operating System
Concepts”, 9thEdition,John Wiley and Sons Inc., 2018.
Operating System:
➢ An operating system is a program that manages the computer hardware. It also provides a
basis for application programs and acts as an intermediary between a user of a computer
and the computer hardware.
The purpose of an operating system is to provide an environment in which a user can execute
programs.
• Exploits the hardware resources of one or more processors (cores)
• Provides a set of services (system calls) to system users
• Manages main/secondary memory and I/O devices
Goals of an Operating System:
➢ The primary goal of an operating system is thus to make the computer system convenient
to use.
➢ The secondary goal is to use the computer hardware in an efficient manner.
Components of a Computer System:
A computer system can be divided roughly into four components.
i. Hardware
ii. Operating system
iii. The application programs
iv. Users
2.1 Computer System Organisation:
Computer-system operation
1
,21CS208 – Operating Systems Unit-II
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
A trap or exception 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
2
, 21CS208 – Operating Systems Unit-II
➢ 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 OS 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
✓ OS indexes into I/O device table to determine device status and to modify table
entry to include interrupt
Storage Structure
➢ Main memory – only large storage media that the CPU can access directly
✓ Random access
✓ Typically volatile
➢ Secondary storage – extension of main memory that provides large nonvolatile storage
capacity
➢ Hard disks – rigid metal or glass platters covered with magnetic recording material
• Disk surface is logically divided into tracks, which are subdivided into
sectors
3
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