Download Desktop PC Operating Systems

The Functions of
Operating Systems
Desktop PC Operating Systems
Learning Objectives
Describe the main components of a typical
desktop PC operating system.
Define the terms boot file and a file
allocation table (FAT).
Main components of a typical
desktop PC operating system
HCI

Type / allow communication.
Utility programs

Routines that the OS makes available to the user.
Hardware control/Input and Output

Software routines to control the hardware/device
drivers
Multi tasking capability

Allows different Windows/user can carry on more
than one task at a time.
Spooling

Queue jobs for input / printing / …
Main components of a typical
desktop PC operating system
Security

Ensure that different users can keep files
confidential.
Host software

Scheduler to schedule instructions.
Memory management

To allocate memory to data/software.
Interrupt handling

Schedule jobs through the system.
Translators

Produce object code.
Boot Program
A file containing commands to automatically
configure a personal computer on start up.

Stored on the ROM as it is unchangeable.
Power-up process:

POST (power on self-test)
http://en.wikipedia.org/wiki/Power-on_self-test

The boot program then retrieves user-defined /
configurable / changeable parameters stored in the boot
file / Basic Input Output System (BIOS) and uses them to
configure the system.
Note the BIOS in a PC is also known as autoexec.bat or config.sys.

Finally it loads the first instruction of the operating system
into memory.
Format
A routine that is part of
the OS which enables the
OS to store and search
for files in sectors on a
disk.
Disk Structure




(A) Track
(B) Geometrical or
mathematical Sector
(C) Track Sector /
Computing Sector / Disk
Sector
(D) Cluster
2 or more track sectors.
Sector / Disk sector
The word ‘sector’ in computing is used to mean
a ‘track sector’.

Mathematically a sector is a ‘slice of the pie’ but in
computing a sector means a ‘track sector’.
A sector is the smallest physical / accessible
amount of data on a medium.

It is possible to think of a sector as a block but the
word block in computing means data chunks of
varying sizes in data streams so it is not used here.
Clusters
A unit of disk space allocated for files and
directories.
The smallest logical amount of disk space that
can be allocated to hold a file.



Large clusters means small files will waste disk space
called ‘slack space’.
However, large cluster sizes reduces bookkeeping
overhead and fragmentation, which may improve
reading and writing speed overall.
So a balance is required usually meaning that a
cluster is around the size of an average file.
File Allocation Table (FAT)
Enables the OS to find files on a disk and to be able to
store user's files.

i.e. Maintain and manage disk space used for file storage.
Uses linked lists to point to the blocks on the disk that
contain each file (each linked list contains the addresses of each
memory block that contains the file in question, if followed the file
“units” will be found in the same order as the original file).
However, to find the first file “unit” a “Root Directory”
table is used, this lists the files and their starting clusters.

See next few slides for diagrams and more detailed
explanations.
A typical FAT Root Directory
table
Filename
Starting Cluster
File1
2
File2
4
File Allocation Table (FAT)
(map to each cluster on the volume)
The first column gives the
cluster number and the
second column is a pointer to
the next cluster used to store
a file.
The last cluster used has a
null pointer (usually FFFFH)
to indicate the end of the
linking.
The directory entry for a file
has a pointer to the first
cluster in the FAT table.
The diagram shows details of
two files stored on a disk.
Note the actual contents of
each cluster (the ‘parts’ of
each file) is not shown here.
Only the pointer to the next
‘part’ of the file.
Storage Clusters on a disk
In order to find a file,
the OS looks in the
directory for the
filename and, if it
finds it, the OS gets
the cluster number
for the start of the
file.
The OS can then
follow the pointers in
the FAT to find the
rest of the file.
Opening a file
1. Find the filename in the FAT Root Directory
table.
2. Look up the Starting Cluster.
3. Go to that cluster and retrieve the data.
4. Go to the FAT table and find the starting cluster
and then look up the Pointer Cluster.
5. Go to that cluster and retrieve the data
(combining it with previous data for the file).
6. Repeat steps 3 – 5 until a null pointer is found
(which indicates the end of the file).
File Allocation Table (FAT)
Unused clusters have a zero entry pointer.
When a file is deleted, the clusters that were
used to save the file are set to zero pointer.
In order to store a new file, the OS finds the first
cluster with a zero pointer entry and enters the
cluster number in the directory table.
OS searches linearly for clusters with zero
pointer entries to set up the linked list (entering the
pointer to the next cluster in the previously used cluster
entry in the FAT table).
Speed
Using linear searches will take a long time.
However, the FAT table is normally loaded
into RAM so that continual disk accesses
can be avoided.

Speeds up the search of the FAT.
Plenary
What is a boot file and a file allocation
table (FAT)?
Plenary
A boot file:

A file containing commands to automatically
configure a personal computer on start up.
A file allocation table (FAT):

A list held on disk by an operating system to
maintain and manage disk space used for file
storage.
Plenary
How does a PC operating system use a
file allocation table to store and find files
when necessary?
Plenary
The disk surface must be divided up into small
areas.
Files are stored in these small areas.
Each file will normally use more than one area
The table of files has an entry pointing to the first
area on the disk surface used by that file and a
pointer to the next area.
Each subsequent area has a pointer to the next
area, as in a linked list with a null value to signify
the end of the file.