Lecture notes on Computer operating System

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Published Date:11-07-2017
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CSE 380 Computer Operating Systems Instructor: Insup Lee and Dianna Xu University of Pennsylvania Fall 2003 Lecture Notes: File Systems 1File Systems  Computer applications need to store and retrieve information:  need to store large amount of information  need to store information permanently  need to share information  A file is a collection of data records grouped together for purpose of access control and modification  A file system is software responsible for creating, destroying, organizing, reading, writing, modifying, moving, and controlling access to files; and for management of resources used by files. 2User-Level View  Naming convention  File structures  File types  File access: sequential vs random access  File operations:  system calls (file/directory operations)  Memory-mapped files  Directory structure (single-level vs. two-level vs. hierarchical  path names  directory operations 3File Naming  Naming convention  number of characters (e.g. limited to 8+3 in MS-DOS)  case sensitive or not, Which chars are allowed  special prefixes/extensions (.txt, .ps, .gif, .mpg, …..)  The family of MS-DOS  Win3.1, Win95, Win98  NT, Win2000 (supports MS-DOS, but have native file system NTFS)  In Unix, many extensions are just conventions  exceptions are for example compilers  Windows assigns meaning to extensions 4File Naming Typical file extensions. 5File Structure  Three kinds of files  byte sequence both Unix and Windows  record sequence: when 80-column punch cards were king  tree: data processing on large mainframe 6File Types  Unix  regular files • ASCII • binary  directory files  character special files (I/O devices that operate on streams)  block special files (disk I/O)  Every OS must at least recognize its own executables  Unix: header, text and data  magic numbers 7File Types (a) An executable file (b) An archive 8File Access  Sequential access  read all bytes/records from the beginning  cannot jump around, could rewind or back up  convenient when medium was magnetic tape  Random access  bytes/records read in any order  essential for many applications  read can be … • move file pointer (seek), then read or … • read and then move file marker  all modern OS have all files as random access 9File Attributes  File name  Size information (current, limit)  Physical address  File type  ASCII vs binary  Temporary vs Permanent  Access rights: owner, protection (who can access it)  Access type: Sequential/Random  History: Creator, time of last access/modification, other usage data  Info for managing links 10File Attributes Possible file attributes 11File Operations 1. Create (creat) 7. Append (write) 2. Delete (unlink) 8. Seek (lseek) 3. Open 9. Get attributes (stat, lstat, fstat, fcntl) 4. Close 10.Set Attributes (fcntl) 5. Read 11.Rename 6. Write 12Memory Mapped Files  Instead of making a series of system calls that involve I/O, map files into the address space of a running process  Just two system calls, map and unmap  File I/O can be done in simple instructions that address memory as usual  Problems:  files must fit in memory  modifications will not be written to disk until unmapped 13Memory-Mapped Files (a) Segmented process before mapping files into its address space (b) Process after mapping existing file abc into one segment creating new segment for xyz 14Directories Single-Level Directory Systems  A single level directory system  contains 4 files  owned by 3 different people, A, B, and C  ownerships are shown, not file names 15Two-level Directory Systems Naming conflicts between different users are eliminated 16Hierarchical Directory Systems A hierarchical directory system 17Path Names A UNIX directory tree 18Directory Operations 5. Readdir 1. Create 6. Rename 2. Delete 7. Link 3. Opendir 8. Unlink 4. Closedir 19File System Implementation  Sector 0 is called the Master Boot Record  used to boot the computer  contains partition table at the end  one partition must be marked as active/primary  BIOS (located on the parentboard) reads and executes MBR (after trying to boot from floppy or CD- ROM)  MBR locates the active partition and reads in its first block  Every partitions comes with a boot block 20

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