Recording a File on Disk.

Recording a File on Disk.

DOS Database WP Letter.txt memo.txt Report.txt

1 - When you use commands or presses the mouse button to save a file, use the program sends a
DOS command to asking the operating system to take the necessary measures so that the file is written to
disk. As an example, suppose you are using a word processor to write a file called LETTER.TXT.
2 - The DOS modifies the directory structure recorded in FAT to indicate that a file called LETTER.TXT will be recorded in
current directory (or another directory if you have specified a different path).

File allocation table file cluster lwksht.wk1 report.jan 1 2 3 available budget.wk1 4 cluster address cluster 3 track 1 sector 2,3,4,5
3 - The DOS checks the FAT (File Allocation Table), seeking the number of a cluster that can save the file without
overwrite any existing data. At this point, the FAT tells DOS that cluster 3 is free to receive data.
4 - Through the FAT, DOS also determines the location of the cluster 3 includes sections 2,3,4 and 5 from track 1. The DOS
sends this information to the PC's BIOS.

5 - The BIOS release details of the program file write. It takes the data that make up the file of LETTER.TXT
where the processor was using the RAM. At the same time, he instructs the controller to the disk drive
record the data that is sending the BIOS, starting at sector 2 through 5 on track 1.

File allocation table file available available 12 13 14 memo2.txt cluster address cluster 3 track 12 sector 6,7,8,9

6 - If the file is larger than the number of bytes contained in one cluster, calls to DOS FAT indicating the position
another cluster where the file can still be recorded. The clusters are not necessarily adjacent in
disk. The FAT keeps a log chain around clusters that make up the file. The process repeats until the DOS
find a special code called end of file marker.

File allocation table file report.txt cluster letter.txt 3 4 5 budget.wk1

7 - Finally, DOS asks the FAT that mark the clusters that make up the file so that later he LETTER.TXT
can know which blocks are already being used.

Source: Evolution of Computers