DOS. 5/1/2006 Computer System Software CS 012 BE 7th Semester 2

Transcription

1 DOS File System

2 DOS The moment we turn the computer on, the computer loads a special program called the operating system into the computer s memory which provides an environment for us to run other programs. The OS provides an interface between the application programs and the hardware. DOS (Disk Operating System) is the operating system for IBM PCs and PC compatibles. There are 2 flavors of DOS viz., PC-DOS and MS-DOS. PC- DOS is mainly for IBM PCs where as MS-DOS is licensed by Microsoft for PC-compatibles. From the user perspective, there is no difference. 5/1/2006 Computer System Software CS 012 BE 7th Semester 2

3 MS DOS-File system MS-DOS has a tree structure i.e. files are organized on each logical drive within a hierarchical directory structure. Root directory is the top directory which holds numerous files and subdirectories. 5/1/2006 Computer System Software CS 012 BE 7th Semester 3

4 MS DOS-File system DOS uses the backslash character (\) to represent directory or sub-directory. The root directory, also known as home directory is represented by backslash (\) alone. This is the default directory if no name is specified after the backslash. DOS selects one disk drive as the current drive or current directory unless specified otherwise. The current directory can also be changed with the CHDIR command. 5/1/2006 Computer System Software CS 012 BE 7th Semester 4

5 MS DOS-File system Naming convention in DOS consists of Logical drive Path Filename Example: C:\CSIII\SS\Lecture.DOC 5/1/2006 Computer System Software CS 012 BE 7th Semester 5

6 MS DOS Data access Basic allocation units of each drive- CLUSTERS chkdsk is used to find the cluster size in a system Each drive has a file-allocation table (FAT) FAT is a roadmap that DOS follows to locate the clusters belonging to a particular file. 5/1/2006 Computer System Software CS 012 BE 7th Semester 6

7 Data Access The operating system allocates disk space on demand by user programs. A disk is divided into sectors and tracks. Each sector is usually 512 bytes. Normally, the space is allocated in units of fixed size known as clusters. A cluster is a multiple of sector size i.e. multiples of 512 bytes say 512, 1024, 2048 bytes etc. 5/1/2006 Computer System Software CS 012 BE 7th Semester 7

8 Cluster allocation When a user creates a directory, DOS allocates a cluster to hold the directory entries. Each directory entry requires 32 bytes. If a disk uses 2048-byte clusters (assuming), a single cluster can hold 64 directory entries(2048/32). Now, if the directory contains more than 64 entries, DOS must allocate a second cluster. 5/1/2006 Computer System Software CS 012 BE 7th Semester 8

9 Sectors & Clusters DOS stores files on disks in groups of sectors called clusters or allocation units. The file may fit in a single cluster or a group of clusters depending on the file s size. DOS tries to store the clusters belonging to a particular file in consecutive disk locations as far as possible. However, in some cases, the clusters of another file may prevent DOS from using consecutive locations. 5/1/2006 Computer System Software CS 012 BE 7th Semester 9

10 FAT The entry in the FAT corresponding to each cluster contains a value that indicates: which cluster is unused which cluster is reserved which cluster considered as a bad cluster the number of next cluster in the file the last cluster of the file 5/1/2006 Computer System Software CS 012 BE 7th Semester 10

11 How to access 5/1/2006 Computer System Software CS 012 BE 7th Semester 11

12 Accessing the contents of file The directory entry of a file mydoc.txt of 5456 bytes which begins with entry no. 3 which is the cluster no. of the first cluster of the file. Location 3 above stores the value Location 6 stores the value Location 7 stores the value FFFF which implies that it is the end of the file. Thus by accessing the FAT table, we can say that the contents of mydoc. txt file are there in cluster nos. 3, 6 and 7. 5/1/2006 Computer System Software CS 012 BE 7th Semester 12

13 Volume structure in MS-DOS Boot-sector : Contains a bootstrap loader and disk characteristics FAT : Main file allocation table Additional FAT : used for recovery during the failure of main FAT Root directory : It is in a fixed position of fixed size File space : Rest of disk space which is available for files and sub-directories. 5/1/2006 Computer System Software CS 012 BE 7th Semester 13

14 Booting The boot sector contains details about the characteristics of a disk and a bootstrap loader. When the computer is switched on first, the boot sector is automatically read from the disk by the ROM start up program and executed. This causes an immediate jump to the bootstrap loader code, which in turn finds the operating system files, MS-DOS.SYS and IO.SYS in the root directory. These are loaded into the memory and executed which in turn makes the operating system operational. 5/1/2006 Computer System Software CS 012 BE 7th Semester 14

15 File permissions in MS-DOS read-only : a file cannot be written or deleted hidden : such files ignored by many commands system : such files sre system files ( OS related files) archive : used for back-up 5/1/2006 Computer System Software CS 012 BE 7th Semester 15

16 Next Session Introduction to Unix 5/1/2006 Computer System Software CS 012 BE 7th Semester 16