Question

Exercise 8.4.1: Operations on files. 0 About Current directory File descriptors Open file table Symbolic Descriptor name index File length Disk block # Descriptor Current index position free 20 5 7 | free 8 | 16 9 free abc free 16100 free 18 free 192808 550 40 test free Disk xxx free free XXXXXXXX XXX 256 bytes • Each entry in the current directory contains the symbolic name of a file, together with the index into the array of file descriptors. • Each descriptor records the file length and the number of the first block on the disk. • The block length is 256 bytes. • Ex: The file named test has a descriptor at index 19, which shows that the file contains 280 bytes. The file starts at disk block 8 and, since the file length exceeds 1 block, the file continues into block 9. • The OFT keeps track of open files. Each entry contains the index into the array of file descriptors and the current position within the file for sequential access. Currently, file abc is open. The descriptor index is 16 and the current position is 55, which means the first 55 bytes of the file have already been accessed. For each operation, show all changes to the data structures

Answer 1

a) Only OFT is changed. We add the Descriptor Index of the file test (i.e. 19) and the position is 0 since the file is only opened and no bytes have been accessed yet.

It becomes:

Descriptor Index	Current Position
....	.....
free
16	55
19	0
free
....	....

b) Only OFT is changed. The position is updated from 0 to 60.

It becomes:

Descriptor Index	Current Position
....	.....
free
16	55
19	60
free
....	....

c) The Current Directory and File Descriptors are changed. Just to keep things sequential, we give the file new the descriptive index of 21 (after descriptive index 20 of xx) and Disc Block #43 (since 40,41 and 42 is already taken up by file xx). The file length of new is 0 since nothing has been written in it yet.

Current Directory becomes:

Symbolic Name	Descriptor Index
....	.....
free
xx	20
abc	16
test	19
new	21
free
....	....

File Descriptors becomes:

	File length	Disc Block #
	....	.....
15	free
16	100	5
17	free
18	free
19	280	8
20	550	40
21	0	43
	....	....

d) Only OFT is changed. We add the Descriptor Index of the file new (i.e. 21) and the position is 0 since the file is only opened and no bytes have been accessed yet.

It becomes:

Descriptor Index	Current Position
....	.....
free
16	55
19	0
21	0
free
....	....

e) OFT is changed. The position is changed to 30 since we have written 30 bytes.

It becomes:

Descriptor Index	Current Position
....	.....
free
16	55
19	0
21	30
free
....	....

File Descriptors Table is also updated to show file length of 30 for the file new at index 21. It becomes:

	File length	Disc Block #
	....	.....
15	free
16	100	5
17	free
18	free
19	280	8
20	550	40
21	30	43
	....	....

f) OFT is changed. The entry for Descriptor Index 21 is removed and space is now free.

It becomes:

Descriptor Index	Current Position
....	.....
free
16	55
19	0
free
free
....	....

g) The Current Directory and File Descriptors are changed. We remove the entry of the file new from the Current Directory and free up the space. We also remove the entry at the descriptive index of 21 from the File Descriptors Table.

Current Directory becomes:

Symbolic Name	Descriptor Index
....	.....
free
xx	20
abc	16
test	19
free
free
....	....

File Descriptors becomes:

	File length	Disc Block #
	....	.....
15	free
16	100	5
17	free
18	free
19	280	8
20	550	40
21	free
	....	....