Disks provide the bulk of secondary storage for modern
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
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
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
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 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.
The simplest form of disk scheduling is of course the first
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
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
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
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
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
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
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
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 Management
o Swap space management is another low level task of the
o Virtual memory uses disk space as an extension of main
o Since disk access is much slower than memory access
using swap space significantly decreases system performs.
Swap Space Use
Swap space is used in various ways by different operating
systems depending on the implemented memory
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 Location
A 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
No file system or directory structure is placed on this
Microsoft Windows 2000 operating system is a 32-bit
preemptive multitask system for Intel Pentium and later
Microsoft 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-
There are four versions of windows 2000.
Extensibility refers to the capacity of an operating system
to keep advances in computing technology.
So that changes are facilitated the developers implemented
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.
The architecture of windows 2000 is a layered of model
The 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.
The kernel of windows 2000 provides the foundation for
the executive and subsystems.
The kernel is never paged out of memory and its execution
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.
Windows 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
This transformation when be MS-DOS application as well
as for POSIX command-line applications.
The 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
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.
Main subsystem in windows 2000 is theWin32 subsystem.
It runs Win32 and management all keyboard mouse and
Since it is the environment it is designed to be extremely
Several features Win32 contribute to has its own input
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.
Historically 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 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
For more information about hierarchical storage see
Windows 2000 supports both peer-to-peer and client-
server networking also has facilities for network
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.
Windows 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
The Apple Talk protocol was designed as a low cost
connection by Apple low Macintosh computers to share