Page 233 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
Figure 6.10: Chained Allocation

File allocation table
File B File Name Start Block Length
0 1 2 3 4 ... ... ...
File B 1 5
5 6 7 8 9 ... ... ...
10 11 12 13 14
15 16 17 18 19
20 21 22 23 24
25 26 27 28 29

30 31 32 33 34

A small “pointer” of typically 32 or 64 bits is allocated within each file block to indicate the next
block in the chain. Thus seeking within a file requires a read of each block to follow the pointers.
New blocks may be allocated from any free block on the disk. In particular, a file’s blocks need
no longer be contiguous.
6.10.3 File Allocation Methods—Indexed

The file allocation method of choice in both Unix and Windows is the indexed allocation method.
This method was championed by the Multics operating system in 1966.

Figure 6.11: Indexed Allocation with Block Portions

File allocations table
File Name Index Block
0 1 2 3 4 ... ...
File B 24
5 6 7 8 9 ... ...
1 0 1 1 12 13 1 4
1 5 16 1 7 18 1 9
1
20 21 22 23 2 4 8
3
25 26 27 28 29 14
28
30 31 32 33 34

The file-allocation table contains a multi-level index for each file. Indirection blocks are introduced
each time the total number of blocks “overflows” the previous index allocation. Typically, the
indices are neither stored with the file-allocation table nor with the file, and are retained in
memory when the file is opened.

Before using memory allocation must be know index concept of memory.

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