Disk structure & File Handling
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Transcript of Disk structure & File Handling
DISK STRUCTURES&
FILE HANDLING
FLOPPY DISK3 ½ INCH 5 ¼ INCH
HARD DISK
FILE ALLOCATION
DISK STRUCTURES:Tracks, sectors, and cylinders.Disk partitions.Disk capacity.
DISK STRUCTURES
FILE FILE HANDLINGFILE HANDLEFILE ERRORS
FILE POINTER
OPERATIONS OF FILE• 3CH• 3DH• 3EH• 3FH• 40H• 41H• 42H
FILE HANDLING
DISK STRUCTURES
The actual physical details of disk
FLOPPY DISK3 ½ INCH 5 ¼ INCH
HARD DISK
FILE ALLOCATION
DISK STRUCTURES:Tracks, sectors, and cylinders.Disk partitions.Disk capacity.
DISK STRUCTURES
TWO KINDS OF DISK
Floppy diskHard disk
FLOPPY DISK
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.
3 ½ INCH 5 ¼ INCH
HARD DISK
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.
WORKING OF HARD DISK
STRUCTURE OF DISK
CAPACITY OF THE 5 ¼ INCH DOUBLE DENSITY FLOPPY DISK
CAPACITY OF HARD DISK
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.
FILE DIRECTORY:
ATTRIBUTE BYTE
CLUSTER“A fixed number of sector (depends on the kind of
disk) is called cluster”
(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.”
FAT
DOSHOW 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 it finds 000 in entry 9.
If the data will fit in a cluster, DOS stores them in cluster 9 and places FFFh in FAT entry 9. If there are more data, DOS looks for the next available next entry in the FAT; for example, AH. DOS stores more data in cluster AH, and places 00Ah in FAT entry 9. This process of finding unused cluster from the FAT, storing data in those clusters, making each FAT entry point to the next cluster continuous until all the data have been stored. The last FAT entry for the file contains FFFh.
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 DTA as needed.
Since entry 2 contains 4 the next cluster in the file cluster is 4. If the program need more data DOS reads cluster 4 into the DTA.
Entry 4 in the FAT contain FFFh, which indicates the last cluster in the file. In general, the process of obtaining cluster numbers from the FAT and reading data into the DTA continues until a FAT entry contains FFFh.
FILE HANDLING
FILE FILE HANDLINGFILE HANDLEFILE ERRORS
FILE POINTER
OPERATIONS OF FILE• 3CH• 3DH• 3EH• 3FH• 40H• 41H• 42H
FILE HANDLING
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
FILE HANDLES
When file is created or open in a program, DOS assign it a unique number called file handle.
FILE ERRORS
OPERATIONS OF FLE
3CH
3DH
3EH
3FH40H
41H
42H
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 FILEOPEN AN EXISTING
A FILE
Creating File & Writing
FILE DB "c:\example_new\hello.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
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;
Close File
MOV AH, 3EH ; 3EH FOR CLOSE FILE
MOV BX, HANDLE ; COPY RESOURCES HANDLE
INT 21H
Delete File
System Call AH BX CX DX
File Delete 41H Offset OldFilename
.DATA
FILE DB "C:\EXAMPLE_NEW\WORLD.TXT",0
.CODE
MOV DX, OFFSET FILE ; OLD FILENAME
MOV AH, 41H ; 41H FOR DELETE FILE
INT 21H
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.
File pointer
System Call AH BX CX:DX
File pointer 42H File handle
Number ofbyte to move
Move File pointer INT 21H, FUNCTION 42H
INPUT AH = 42HAL = 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).
Caption
