1. CENG334 Introduction to Operating Systems
13/03/07
CENG334 Introduction to Operating Systems
Filesystems – Case studies
Topics:
FAT
Linux
Erol Sahin
Dept of Computer Eng.
Middle East Technical University
Ankara, TURKEY
URL:

2. FAT (MS-DOS) Filesystems
MS-DOS, Windows 98, Windows ME
Still supported in Windows NT, XP, Vista
Its use has been shifted towards embedded devices such as
Digital cameras
MP3 playes
iPod (the default filesystem)

3. Directories
Filenames are limited to 8+3 characters.. Smaller ones are left justified and padded with space.
Attributes: read-only, archive, hidden, system
Time represented with 2 bytes: correct upto +-2 second
Date: Counts in three fields: day (5 bit), month (4 bits), year (7 bits).
Contains: Y2108 problem
File size: 2 bytes. Theoretically 4GB limit, but the limit is 2GB due to other reasons.
10 bits reserved for future use.

4. File Allocation Table
MS-DOS keeps track of files through FAT table hold in memory.
First block number (2 bytes) used as the index to the FAT table which has 64K entries.
Block size can be set as multiple of byes.
Three versions of FAT depending on the number of bits a disk address contains:
FAT-12
FAT-16
FAT-32 (actually should be called as FAT-28)
FAT is also used for keeping track of free blocks.

5. FAT-12 Block size: 512 bytes (2^12-10) X 512 bytes =~ 2MB
10 disk addresses are used as special markers.
FAT table size: 4096 entries with 2 bytes each.
Worked well for floppy disks.
The limit wa extended using larger block sizes: 1KB, 2KB, 4KB ptoviding support for partititions 16MB.
Limited for hard disks.

6. The MS-DOS File System (2)
13/03/07
Figure Maximum partition size for different block sizes. The empty boxes represent forbidden combinations.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

7. The UNIX V7 File System (1)
13/03/07
Figure A UNIX V7 directory entry.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

8. The UNIX V7 File System (2)
13/03/07
Figure A UNIX i-node.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

9. The UNIX V7 File System (3)
13/03/07
Figure The steps in looking up /usr/ast/mbox.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved