This document discusses disk structures and file handling. It covers floppy disks, hard disks, file allocation, how DOS reads and writes files using file handles, and basic file operations like create, write, read, delete and close. Disk structures include tracks, sectors, cylinders and partitions. File allocation uses a file allocation table to track file fragments across the disk.

1. DISK
STRUCTURES
&
FILE HANDLING

2. FLOPPY
DISK
3 ½ INCH
5 ¼ INCH
HARD DISK
FILE ALLOCATION
DISK STRUCTURES:
Tracks, sectors, and
cylinders.
Disk partitions.
Disk capacity.
DISK STRUCTURES

4. FILE
FILE HANDLING
FILE HANDLE
FILE ERRORS
FILE POINTER
OPERATIONS OF
FILE
• 3CH
• 3DH
• 3EH
• 3FH
• 40H
• 41H
• 42H
FILE HANDLING

5. DISK STRUCTURES
The actual physical details of disk

6. FLOPPY DISK

7. 1)Internal parts of a 3½-inch floppy disk.
2)A hole that indicates a high-capacity disk.
3)The hub that engages with the drive motor.
4)A shutter that protects the surface when
removed from the drive.
5)The plastic housing.
6)A polyester sheet reducing friction against the
disk media as it rotates within the housing.
7)The magnetic coated plastic disk.
8)A schematic representation of one sector of
data on the disk; the tracks and sectors are
not visible on actual disks.
9)The write protection tab (unlabeled) is upper
left.

8.  3 ½ INCH
 5 ¼ INCH

9. HARD DISK

10. 1) Actuator that moves the
read-write arm.
2) Read-write arm swings
read-write head back and
forth across platter.
3) Central spindle allows
platter to rotate at high
speed.
4) Magnetic platter stores
information in binary form.
5) Plug connections link hard
drive to circuit board in
personal computer.
6) Read-write head is a tiny
magnet on the end of the
read-write arm.
7) Circuit board on underside
controls the flow of data to
and from the platter.
8) Flexible connector carries
data from circuit board to
read-write head and platter.
9) Small spindle allows read-
write arm to swing across
platter.

11. WORKING OF HARD DISK

12. STRUCTURE OF
DISK

14. CAPACITY OF THE 5 ¼ INCH DOUBLE
DENSITY FLOPPY DISK

15. CAPACITY OF HARD DISK

16. FILE ALLOCATION
To keep the track of data stored on a disk, DOS uses
a directory structure. The first tracks and sectors of a
disk contain information about the disk’s file
structure.

18. FILE DIRECTORY:

19. ATTRIBUTE BYTE

20. Cluster
“A fixed number of sector (depends on the kind of
disk) is called cluster”

21. (File Allocation Table):
“A table that the operating system uses to
locate files on a disk. Due to fragmentation, a file
may be divided into many sections that are
scattered around the disk. The FAT keeps track of all
these pieces.”
F
A
T

22. DOS
HOW DOS READ A FILE?
• DOS gets the starting
cluster number from
directory.
• DOS read that particular
cluster number from disk
and stored in DTA (Data
Transfer Area). The
program that initiated the
read retrieves data from
HOW DOS STORES A FILE?
• DOS locates an unused
directory entry and stores
the filename, attribute,
creation time and date.
• DOS searches the FAT for
the first entry indicating
unused cluster (000 means
unused) and stores the
starting cluster number in
the directory. Let’s suppose

23. FILE HANDLING

24. FILE
FILE HANDLING
FILE HANDLE
FILE ERRORS
FILE POINTER
OPERATIONS OF
FILE
• 3CH
• 3DH
• 3EH
• 3FH
• 40H
• 41H
• 42H
FILE HANDLING

25. File ?
File Handling ?
A file is an object on a computer that stores data, information, settings, or commands
that are used with a computer program.
Through file handling, one can perform operations like create, modify , delete etc
on File.
File Create Write Rename Delete

26. FILE HANDLES
When file is created or open in a
program, DOS assign it a unique
number called file handle.

28. FILE ERRORS

30. OPERATIONS OF FLE

31. 3CH
3DH
3EH
3FH40H
41H
42H

32. 3CH
42H
3EH3DH
40H 41H3FH
OPEN A NEW FILE /
REWRITE A NEW FILE
MOVE A FILE POINTER
DELETE A FILEWRITE A FILEREAD A FILE
CLOSE A FILE
OPEN AN
EXISTING A FILE

33. Creating File & Writing
FILE DB "c:example_newhello.txt",0
TEXT DB "HELLO I AM TEXT",0
TEXT_SIZE = $ - OFFSET TEXT
HANDLE DW ?
System Call AH BX CX DX
File Open 3CH 0 Offset Filename
File Write 40H Handle Text Size Offset Text
File Close 3EH Handle
Handle ?
Handle is some resources/memory to store temporary data , used in file operation

34. Create File Operation
MOV CX,0
MOV DX, OFFSET FILE ; FILENAME
MOV AH, 3CH ; 3CH FOR CREATING FILE
INT 21H
Write into File
MOV HANDLE, AX ; MAKE RESOURCES HANDLE
MOV AH, 40H ; 40H FOR WRITING INTO FILE
MOV BX, HANDLE ; COPY RESOURCES HANDLE
MOV DX, OFFSET TEXT ; TEXT TO WRITE
MOV CX, TEXT_SIZE ; TEXT SIZE
INT 21H;

35. Close File
MOV AH, 3EH ; 3EH FOR CLOSE FILE
MOV BX, HANDLE ; COPY RESOURCES HANDLE
INT 21H

36. Delete File
.DATA
FILE DB "C:EXAMPLE_NEWWORLD.TXT",0
.CODE
MOV DX, OFFSET FILE ; OLD FILENAME
MOV AH, 41H ; 41H FOR DELETE FILE
INT 21H
System Call AH BX CX DX
File Delete 41H Offset OldFilename

37. File pointer
 The file pointer is used to locate a position in the file.
 File is opened – file pointer is at the beginning of file.
 After read operation – it the next byte to be read.
 After writing – file pointer at end of file.

38. File pointer
Move File pointer
INT 21H, FUNCTION 42H
INPUT AH = 42H
AL = MOVEMENT CODE:
• 0 MOVE RELATIVE TO BEGINNIG
• 1 MOVE RELATIVE TO CUUERENT POINTER LOCATION
• 2 MOVE RELATIVE TO THE EOF
BX = FILE HANDLE
CX:DX = NUMBER OF BYTES TO MOVE
OUTPUT DX:AX = NEW POINTER LOCATION IN BYTES FROM THE BEGNNING OF FILE.
IF CF = 1 ERROR CODE IN AX (1,6).
System Call AH BX CX:DX
File pointer 42H File
handle
Number of
byte to
move

39. Caption