OPERATION SYSTEM

K.Rohinipriya

DISK STRUCTURE Disks provide the bulk of secondary storage for modern

computer system. Magnetic tape was used as an early secondary storage medium

but the access time is much slower then for disks. Modern disk drives are addressed as large one dimensional array

of logical blocks where the logical block is the smallest unit of transfer.

The size of a logical block is usually 512 bytes although some disks can be low level formatted to choose a different logical block size such as 1,024 bytes.

The one dimensional array of digital blocks is mapped onto the sectors of the disk sequentially.

The mapping proceeds in order through that track then through the rest of the cylinder from outermost to innermost.

Send the number of sectors per track is not a constant on some drives.

On media that use constant linear velocity(CLV) the density of bits per track is uniform.

Tracks in the outermost zone typically hold 40 percent more sectors then do tracks in the innermost zone.

This method is used in CD-ROM and DVD-ROM drives. This methods is used in hard disks and is know as constant

angular velocity(CAV)

DISK SCHEDULING One of the responsibilities of the operating system is to use

the hardware efficiently. The disk drives meeting this responsibility entails having a

fast access time and disk bandwidth. The access time has two major components . The seek time is the time for the disk arm to move the heads

to the cylinder containing the desired sector. The rotational latency is the addition time waiting for the

disk to rotate the desired sector to the disk head. We can improve both the access time and the bandwidth by

scheduling the servicing of disk I/O requests in a good order.

The request specifies several pieces of information.Whether this operation is input or output

What the disk address for the transfer isWhat the memory address for the transfer isWhat the number of byte to be transferred is

If the desired disk drive and controller are available the request can be serviced immediately.

Thus when one request is completed the operation system chooses which pending request to service next.

FCFS SCHEDULING The simplest form of disk scheduling is of course the first

served(FCFS) algorithm. This algorithm is intrinsically fair but it generally does not

provide the fastest service If the disk head is iruitially at cylinder 53 it will first move

from 53 to 98 then to 183,37,122,14,124,65, and finally to 67, for a total head movement of 640 cylinders

SSTF SCHEDULING It seems reasonable to service all the requests close to the

current head position before moving the head far away to service other requests

This assumption is the basis for the shortest seek time first (SSTF)algorithm

Since seek time increase with the number of cylinders traversed by the head SSTF chooses the pending request closest to the current head position

SSTF scheduling is essentially a form of shortest job first (SJF) scheduling and like SJF scheduling it may cause starvation of some request

SCAN SCHEDULING In the SCAN algorithm the disk arm starts at one end of

the disk and moves toward the other end servicing request as it reaches each cylinder until it gets to the other end of the disk

The head continuously scan back and forth across the disk Before applying SCAN to schedule the requests on

cylinder98,183 ,we need to know the direction of head movement in addition to the head current position

These request have also waited the longest so why not go there first that is the idea of the next algorithm

C-SCAN SCHEDULING SCAN(C-SCAN) scheduling is variant of SCAN designed

to provide a more uniform wait time Like SCAN,C-SCAN moves the head from one end of the

disk to the other servicing request along the way The C-SCAN scheduling algorithm essentially treats the

cylinder s as a circular list that wraps around from the final cylinder to the first one

LOOK SCHEDULING As we described them both SCAN and C-SCAN move the

disk arm across the full width of the disk In practice neither algorithm is implemented this way Then it reverses direction immediately without going to all

the way to the end of the disk Because they look for a request before continuing to move

in a given direction

DISK MANAGEMENTDisk Formatting: A new magnetic disk is a blank slate .It is just platters of a

magnetic recording material. Before a disk can store data it must be divided into sectors

that the disk controller can read and write. This process is called low level formatting Low level formatting fills the disk with a special data

structure for a sector typically consists of a header a data and a trailer .

Some operating systems can handle only a sector size of 512 bytes.

SWAP SPACE MANAGEMENTo Swap space management is another low level task of the

operating system.o Virtual memory uses disk space as an extension of main

memory.o Since disk access is much slower than memory access

using swap space significantly decreases system performs.

SWAP SPACE USESwap space is used in various ways by different operating systems depending on the implemented memory management algorithms.The amount of swap space needed on a system can therefore very depending on the amount of physical memory the amount of virtual memory it is backing and the in which the virtual memory is used.Overestimation wastes disk space that could otherwise be used for files but does no other harm.

SWAP SPACE LOCATIONA swap space can reside in two places swap space can be carved out of the normal file system or it can be in a separate disk partition.Navigating the directory structure and the disk allocation data structures takes time and extra disk accesses. Alternatively swap space can be created in a separate disk partition.No file system or directory structure is placed on this space.

WINDOWS 2000Microsoft Windows 2000 operating system is a 32-bit preemptive multitask system for Intel Pentium and later microprocessors.

HistoryMicrosoft and IBM cooperated to develop the OS/2 operating which was written in assembly language for single processor Intel 80286 In the 1998.Server windows 4.0 adopted the windows 95 user interface and incorporated internet which and web-browser software. There are four versions of windows 2000.

DESIGN PRINCIPALExtensibility refers to the capacity of an operating system to keep advances in computing technology.So that changes are facilitated the developers implemented windows 2000.Among them are environmental subsystems that late different operating systems.As is true of the UNIX operating system the majority of the is written in C and C++.All process-dependent code is isolated in a link library (DLL)called the hardware-abstraction layer(HAL).String comparisons are account for varying character sets.

SYSTEM COMPONENTSThe architecture of windows 2000 is a layered of modelThe main layer are the HAL the kernel and all of which run in protected mode and a large collection of subsystem run is use mode.The user –mode subsystems are in two environmental subsystems emulate different operating systems the subsystems provide security function.

KERNELThe kernel of windows 2000 provides the foundation for the executive and subsystems.The kernel is never paged out of memory and its execution preempted.An objects type in widows 2000 is a system data type that has a set of attributes and a set of function.Kernel uses two sets of objects.The first set comprises the Dispatcher objects control dispatching and synchronization in the example.The thread objects is the entity that is run kernel and is associated with a process.

THREADS AND SCHEDULING

AS do many operating systems windows 2000 uses the model processes and thread for executable code.Each process has one or more threads which are the of execution dispatched by the kernel.Read means waiting to run.A thread is running when it is executing on a process run until it is preempted by a higher priority thread.A new thread is in the transition state while it is waiting resources necessary for execution.The never is lowered below the base priority however.

ENVIRONMENTAL SUBSYSTEMSWindows 2000 uses the win32 subsystem as the main operating environment and thus to start all process.The environmental subsystem uses the windows 2000 LPC facility to get services for the process.Windows 2000 prohibits applications mixing API routines from different environments.For instance a win32 application cannot call a POSIX routine.This transformation when be MS-DOS application as well as for POSIX command-line applications.

MS-DOS ENVIRONMENTThe MS-DOS environment does not have the complexity of the other window 2000 environmental subsystem.It is provide by a win32 application which the virtual DOS machine(VDM).The VDM is based on the source code it gives the application at least 620KB of memory.The windows 2000 is running on an x86 processor MS-DOS graphical applications run in full screen mode and character applications can run full in a windows.

WIN32 ENVIRONMENTMain subsystem in windows 2000 is theWin32 subsystem.It runs Win32 and management all keyboard mouse and screen I/O.Since it is the environment it is designed to be extremely robust.Several features Win32 contribute to has its own input queue.The reference counts kept by object manager prevent objects from being deleted while they are still being and prevent their use after they have been deleted.

FILE SYSTEMHistorically MS-DOS system have used the file allocation table(FAT) the item.The 16-bit FAT file system has several shortcomings including fragmentation a size limitation of 2GB and a lack access protection for the system.It was designed to include feature including data recovery security fault tolerance large files of systems.The windows 2000 provides support for the FAT and OS/2 HPP systems.

REPARSE POINTSReparse point are a new feature in the file system that in return an error when accessed.The reparse data then tell the I/O manager what to do.Mount points would allow you to create a volume on another drive move the old data to the new volume.The remote storage services facility also uses reparse points.For more information about hierarchical storage see section.

NETWORKING Windows 2000 supports both peer-to-peer and client-server

networking also has facilities for network management. The network device interface specification and the

transport driver interface(TDI). In terms of the OSI model the TDI the model between the

transport layer and the session layer the mechanism. TDI supports both connection based and connectionless

transports and the function to send any type of data.

PROTOCOLSWindows 2000 implements transports protocols as drives.Window 2000 comes will several networking protocols.The system uses the protocol to send I/O requests over the network the SMB protocol has four message types.Among the limitations of net BEUI are that it uses the actual of a computer as the address and that it does not support routing.The Apple Talk protocol was designed as a low cost connection by Apple low Macintosh computers to share files.