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: http://kovan.ceng.metu.edu.tr/ceng334

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)

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.

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 512 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.

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.

The MS-DOS File System (2) 13/03/07 Figure 4-32. 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. 0-13-6006639

The UNIX V7 File System (1) 13/03/07 Figure 4-33. A UNIX V7 directory entry. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

The UNIX V7 File System (2) 13/03/07 Figure 4-34. A UNIX i-node. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

The UNIX V7 File System (3) 13/03/07 Figure 4-35. The steps in looking up /usr/ast/mbox. Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639