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Note for Operating Systems - OS By JNTU Heroes

  • Operating Systems - OS
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  • Jawaharlal nehru technological university anantapur college of engineering - JNTUACEP
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UNIT – I Introduction to Operating System 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 UsersPeople, 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

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Computer Startup     bootstrap program is loaded at power-up or reboot Typically stored in ROM or EPROM, generally known as firmware Initializes 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 interruptnA 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: vectored interrupt system Separate segments of code determine what action should be taken for each type of interrupt 

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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. 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

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