I/0 SYSTEM AND CASE STUDY

Submitted ToMs. R. Madhubala, MCA.,

By G.Ramya Bharathi(14BIT046)

Contents

1) Disk Structure2) Disk Scheduling3) Disk Management4) Swap-space Management5) Swap-space use and Location6) Windows 2000

Disk Structure

Disks provide the bulk of secondary storage for modern computer systems.Magnetic tape was used an early secondary storage medium but the acces time is much slower than 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 the logical block is usually 512 bytes,although some disks can be low-level formatted to choose a different logical block size,such as 1024 bytes.

Disk Scheduling The disk drives meeting this responsibility entails having a fast

access time and disk bandwidth. 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 additional time waiting for the disk to

rotate the desired sector to the disk head. The disk bandwidth is the total number of bytes transferd divided

by the total time between the first request for service and the completion of the last transfer.

We can improve the both the access time and the bandwodth 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 is What the memory address for the transfer

is What the number of bytes to be

transferred is If the desired disk drive and controller are

available,the requests can be serviced immediately.

Disk Management

The operating system is responsible for severalother aspects of disk management too. Here we discuss disk initialization,booting from

disk,and bad-block recovery. Disk Formatting Boot Block Bad Blocks

Disk Formating

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 each sector. The header & trailer contain information used by the

disk controller such as sector number and an Error-correcting code(ECC).

. The operating system still needs to record its own

data structures on the disk. It does so in two steps. The first step is to partition the disk into one or

more groups of cylinders. The operating system can treat each partition as

though it were a separate disk. After partitioning,the second step is logical

formatting. In this step the operating system stores the initial

file-system data structure onto the disk.

Boot Block

This initial bootstrap program tends to be simple.It initializes all aspects of the system,from CPU registers to device controllers and the contents of main memory and then starts the operating system.

For most computers,the bootstrap is stored in Read-only memory(ROM).

A disk that has a boot partition is called a Boot disk or system disk.

The code in the boot ROM instructs the disk controller to read the boot blocks into memory and then starts executing that code.

Bad Blocks

More frequently,one or more sectors become defective. Most disks even come from the factory with bad blocks. On simple disks,such as some disks with IDE

controllers,bad blocks are handled manually. For instance,the MS-DOS format command does a

logical format and as a part of the process scans the disk to the find bad blocks.

As an alternative to sector sparing some controllers can be instructed to replace a bad block by sector sliping.

. sector 0 sector 1

MS-DOS disk layout More sophisticated disks,such as the SCSI disks

used in high-end pcs and more workstations an servers are smarter about bad-black recovery.

boot block

FAT

root directory

data blocks(subdirectries)

Swap-space Management:

Swap-space management is another low-level task of the operating system .

Virtual memeory uses disk space as an extension of main memory.

In this section,we discuss how swap space is used, where swap space is located on disk,and how swap

space is managed.

.Swap-Space Use swap space is used in various ways by different

operating systems,depending on the implementated memory-management algorithm.

For instance,systems that implement swapping may use swap space to hold the entire process image,including the code and data segments.Some operating systems.

such as UNIX,allow the use of multiple swap spaces. These swap spaces are usually put on separate disks ,so

the load placed on the I/O system by paging and swapping can be spread over the system’s I/O devices.

.Swap-space Location: A swap 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.

This approach creates a fixed amount of swap space during disk partitioning.

.Swap-space Management:An Example Swap space is allocated to a process when the

process is started.Enough space is set aside to hold the program,known as the text pages or the text segment,and the data segment of the process.Two process swap maps are used by the kernal to track swap space use. map

m

BSD text-segment swap map

...

512k 512k 512k 71k

Windows 2000

Microsoft windows 2000 operating system is a 32-bit preemptive multitask operating system for Intel pentium and later microprocessors.T

he success windows NT operating system,it was previously named windows version 5.0.

History In 1980’s Microsoft and IBM cooperated to develop the

os/2 operating system. Thus portability now refers to portability Intel

architecture systems.

Design principles Extensibility refers to the capacity of an operating

system to keep advances in computing technology.

Among them environmental subsystems that can different operating systems.

An operating system is portable if it can be moved from one hardware to another with relatively few changes.

All processor –dependent code is isolated in a link library called the Hardware-abstraction layer(HAL).

It provides source level compatibility to application that a IEEE 1003.1

System Components

The user-mode subsystem are in two categories.The environmental subsystem emulates different operating systems that was subsystems provide security function.

Hardware –Abstraction Layer HAL is the layer of software that hides hardware

differences from of the operating system,to help make windows 2000 portable.

For performance reasons,I/O drivers can access the hardware directly.

Kernel

The 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 object type in windows 2000 is a subsystem data type that has a set of attributes and a set of operations.

The thread object is the entity that is run kernel and is associated with a process object.

Timer objects are used to keep track of the time and to signal timeouts when operations take and need to be interrupted.

Virtual-Memory Manager

The virtual memory operation of the windows 2000 executive is the Virtual memory manager.

The VM manager windows 2000 uses a page-based management scheme with a page size of the data that are assigned to a process but are not in physical memory stored in the paging file on disk.

Windows provides an alternative ,called a section object,to present a block of memory.

I/O Manager

I/O manager is responsible for file systems,cache management,device and network drivers.

The I/O Manager converts the requests it receives into a standard called an I/O request packet(IRP).

Eack cache block is described a Virtual-address control block(VACB)that stores the virtual address and offset for that view,as well as the number of processes that are using that manager.

..

Disk driver

File system

Process

Cache manager

VM Manager

Cached I/O

data copy

Page fault

I/O I/O manager

noncached I/O

File I/O

MS-DOS Environment

The MS-DOS environment does not have the complexity of the other 2000 environmental subsystems.

It is provide by a win32 application the virtual DOS machine(VDM).

File system Historically,MS-DOS systems have used the file

allocation table. The 16-bit FAT file system has several

shortcomings,including fragmentation,a size limitation of 2 GB,and a access protection.

Volume management&Tolerance:

In windows 2000 volume called a volume set,which can consist of up to 32 physical parts.

LCNS 0-128000

LCNS 128001-783361

Disk C:(FAT)

logical drive D

disk 1(2.5GB)

Disk 2(2.5 GB)

Protocols

The Several message-block(SMB) protocol was first introduced .

The system uses the protocol to send I/O request over the network SMB protocol has four message types.

Windows 2000 uses the Data-link control(DLC) protocol to access IBM frames and HP printers that are connected directly to the network.

The AppleTalk protocol was designed as a low-cost connectivity .