The document provides an overview of disk I/O in 3 sentences: It discusses the internal structure and working of floppy and hard disks, including the file directory, clusters, and file allocation table (FAT). The document also covers the various disk file functions in assembly language programming for opening, reading, writing, and closing files.

1. Presentation
(Assembly Language Programming)
Topic:
Disk I/O
Presented To:
Mam. Shaziya Saqib
Presented By:
Section-B
(4rth Semester)
Department of Computer Science
Lahore Garrison University
(Walton Campus)

2. Introduction to Disk I/O

3. Types of Storage
I. Hard Disk
II.Floppy Disk
III.There Models (Given on next slides)
IV.Working Area

4. Models of Floppy and Hard Disks
DD Disk holds 720k of data & HD Disk holds 2.0 MB of data.

5. Working of Floppy Disk ( 5 ¼ In)

6. Working of Floppy Disk (3 ¼ In)

7. Working of Hard Disk (General)
One cylinder: the same track on the top and bottom of a platter.

8. Disk Access
• The method of accessing information for both
Floppy and Hard Disk is similar.
• This is done with the help of moving heads on
the disk plates.

9. The File Directory
• DOS creates a 32-bit directory for each file. The format of an entry
is as follows:
• Byte 11 is the attribute file. Each bit specifies a file attribute.
(Fig:19.4 , Given on the next slide)
 Hidden files ( bit 1)  DIR
 Archive bit ( bit 5)  BACKUP

10. Attribute File

11. Clusters
• DOS sets aside space for files in clusters.
• Clusters always start with ‘0’ .
• Number of sectors in a cluster in n^2.
(Here ‘n’ is cluster number)
• Byte 26-27 in directory bits (starting positions
of clusters).
• Cluster Size (1024 bytes for 5 ¼ in for DD Disk)

12. The FAT
(File Allocation Table)
FAT  provides map for file storage.
FAT  entries are 12-bit long.
(for Floppy Disks & 10 MB Hard Disk)
FAT  for higher Hard Disks it is 16-bit size.

13. File Processing
• The group of INT 21h Functions are called File
Handle Functions.
• This basically involves following functions:
I. Reading
II.Writing
III.Re-writing

14. What is File Handle ?
• When a file opened or created in a program, DOS
assigns it a unique number called “File Handle”.
• Five Built-In File Handles:
I. Keyboard
II.Screen
III.Error output-screen
IV.Auxiliary device (A Piece of equipment that connect to your
computer)
V.Printer
NOTE: Three additional file handles are also present.

15. File Errors
• If error is occurs then CF=1, and code no
appear in AX.

16. Opening a File

17. Opening a File(Example)
• Write instructions to open a new read_only
file called FILE1.

18. Open an Existing File
• To open an existing file, there is an another
function:

19. Closing a File
• After a file has been processed, it should be
closed.

20. Reading a file
• The following function reads a specified
number of bytes from a file and stores them in
memory.

21. Reading a file(Example)
• Write some code to read a 512-byte sector
from a file.

22. Writing a File
• Function 40h writes a specified number of
bytes to a file or device.

23. Writing a File(Example)
• Use function 40h to display a message on the
screen.

24. File Pointer

25. File Pointer(Example)

26. Summary
• Today we discuss about:
Internal structure & working of both Floppy &
Hard Disk.
The File Directory.
Cluster & FAT
Disk File Functions
1) 3Ch 2) 3Dh 3) 3Eh
4) 3Fh 5) 40h 6) 42h

27. Any Question ???