1. File System A file system provides efficient and convenient access to the disk The operating system imposes one or more file systems on a disk A file system poses two different design problems – Defining how the file system should look to the user A file and its attributes The operations allowed on a file The directory structure for organizing files – Creating algorithms and data structures to map the logical file system onto the physical secondary- storage devices

2. File Structure Four terms are commonly used when discussing files: FieldRecordFile Database

3. Structure Terms Field – basic element of data – contains a single value – fixed or variable length File collection of related fields that can be treated as a unit by some application program One field is the key – a unique identifier Record Database collection of similar records treated as a single entity may be referenced by name access control restrictions usually apply at the file level collection of related data relationships among elements of data are explicit designed for use by a number of different applications consists of one or more types of files

4. 4 File 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

5. A file system Architecture 1.Record can be maintained in the form of a table 2.There should be optimized performance 3.Response time should be minimum 4.Throughput should be maximum 5.File management system should support verities of devices 6.There should be some standard set of I/O routines 7.Simultaneous access of file system/multiuser support User Program Basic I/O Supervisor Basic File System Device Drivers Devices

6. File Organization The Pile – Data are collected in the order they arrive – Purpose is to accumulate a mass of data and save it – Records may have different fields – No structure – Record access is by exhaustive search

7. File Organization(Pile)

8. File Organization The Sequential File Fixed format used for records Records are the same length All fields have the same (order and length) Field names and lengths are attributes of the file One field is the key filed (Primary key)

9. The Sequential File Most common form of file structure A fixed format is used for records Key field uniquely identifies the record & determines storage order Typically used in batch applications Only organization that is easily stored on tape as well as disk

10. File Organization Indexed Sequential File New records are added to an overflow file Record in main file that precedes it is updated to contain a pointer to the new record The overflow is merged with the main file Multiple indexes for the same key field can be set up to increase efficiency

11. Indexed File Records are accessed only through their indexes Variable-length records can be employed Exhaustive index contains one entry for every record in the main file Partial index contains entries to records where the field of interest exists Used mostly in applications where timeliness of information is critical Examples would be airline reservation systems and inventory control systems

12. Explain how disk space is allocated using indexing Each file has its own index block, which is an array of disk-block addresses. The ith entry in the index block points to the ith block of the file. The directory contains the address of the index block. In order to find and read the ith block, we use the pointer in the ith index block entry. This scheme is similar to paging scheme. When the file is created, all pointers in the index block are set to nil. When the ith block is first written, a block is obtained from the free space manager and its address is put in the ith index- block entry. Index allocation support direct access without suffering from external fragmentation.

13. 13 Directories 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

14. 14 Two-level Directory Systems Naming conflicts between different users are eliminated

15. 15 Hierarchical Directory Systems A hierarchical directory system

16. 16 A UNIX directory tree Path Names

17. Overview (on-disk structures) A directory structure per file system is used to organize the files – In unix file systems(UFS), includes the file names and associated inode numbers(inode is a data structure used to represent a file system object, which can be one of various things including a file or a directorydata structure – In windows 2000(NTFS), it is stored in the master file table A File Control Block (FCB) contains many details about the file – In UFS, this is called the inode – In NTFS, this information is stored in the master file table, with one row per file

18. File Access Methods Sequential Access read next write next reset no read after last write(rewrite) Direct Access read n write n position to n read next write next rewrite n n = relative block number

19. 19 File Access Methods Random access – bytes/records read in any order – essential for data base systems – read can be … move file marker (seek), then read or … read and then move file marker

20. Chapter 4: Operating Systems and File Management 20 The Boot Process

21. Chapter 4: Operating Systems and File Management 21 The Boot Process During the boot process, the operating system kernel is loaded into RAM – The kernel provides essential operating system services Your computer’s small bootstrap program is built into special ROM circuitry housed in the computer’s system unit

22. MBR The master Boot Record(MBR) is the small bit of code that the BIOS loads and executes to start the boot process. This code when fully executed, transfer control to the boot program stored on the boot partition to load the operating system.

23. Allocation Methods Allocation methods address the problem of allocating space to files so that disk space is utilized effectively and files can be accessed quickly Three methods exist for allocating disk space – Contiguous allocation – Linked allocation – Indexed allocation

24. File Allocation Contiguous: A contiguous set of blocks is allocated to a file at the time of file creation consolidation to improve locality Chained and linked allocation: Each block contains a pointer to the next one in the chain good for sequential files file size must be known at the time of file creation Suffer from external fragmentation Indexed allocation: good both for sequential and direct access (UNIX)

25. Contiguous Allocation Requires that each file occupy a set of contiguous blocks on the disk Accessing a file is easy – only need the starting location (block #) and length (number of blocks) Problems – Finding space for a new file (first fit, best fit, etc.) – External fragmentation (free space is broken into small unusable chunks) – Need for compaction, which requires file system down time – Determining space for a file, especially if it needs to grow

26. Linked Allocation Disadvantages – Can only be used effectively for sequential access of files Each access to a file block requires a disk access, and some may also require a disk seek It is inefficient to support direct access capability – Disk space is required to store the block pointers One solution is the clustering of a certain constant number of blocks – Relies on the integrity of the links – an error might result in a pointer value becoming corrupt and then pointing into the free-space list or to the blocks of another file A partial solution is a doubly linked list or storing a relative block number or the file name in each block (these schemes all require space and algorithm overhead)

27. Linked Allocation (continued)

28. File-Allocation Table (FAT)

29. Linked Allocation (continued) One variation on linked allocation is the file allocation table (FAT) used by MS-DOS – A section of the disk on each volume contains the FAT – The FAT has one entry for each disk block and is indexed by block number – The directory entry contains the block number of the first block of the file – The table entry indexed by the block number of the next block in the file – The last block contains a special EOF value as the table entry – Unused blocks are indicated by a zero table value – To allocate a new block to a file Find the first zero-valued table entry Replace the previous end-of-file value with the address of the new block – Disadvantage – can result in a significant number of disk head seeks – Advantage – random-access time is improved because the FAT can be checked

30. FAT Implementation It is a simple but efficient method of disk allocation used by MS-Dos, OS/2. A section of the disk on each volume contains the FAT table. The table has one entry for each disk block, and is indexed by block number. The FAT is used as a linked list. The directory entry contains the block number of the first block of the file. The table entry is indexed by that block number then contains the block number of the next block in the file. This chain continues until the last block, which has a special EOF value as the table entry. Unused blocks are indicated by a 0 table value. By locating a new block to a file is used to find the first 0 valued table entry, and replacing the previous end-of-file value with the address of the new block. The zero is then replaced with the EOF value.

31. FAT Implementation(Cont.) FAT allocation scheme can result in a significant number of head seeks, unless the FAT is cached. The disk head must move to the start of the partition to read the FAT and find the location of the block in question, then move to the location of the block itself. In the worst case, both move occurs for each block. Here random access is optimized because the disk head can find the location of any block by reading the information in the FAT.

32. Indexed Allocation

33. Security in Operating System Security breaches Security goals Protected objects of the general purpose operating system Protection of objects

34. Threats Interruption Interception Modification Fabrication

35. Protecting Memory and Addressing Fence Relocation Tagged Architecture Segmentation Paging

36. Fence A fence is a method to confine users to one side of a boundary. Usually, fence is implemented via a hardware register.

37. Relocation Relocation is the process of taking a program written as if it began at address 0 and changing all addresses to reflect the actual address at which the program is located in memory. Fence register can be used within relocation process. To each program address, the contents of the fence register are added. This both relocates the address and guarantees that no one can access a location lower than a fence address.

38. Paging An alternative to segmentation is paging. The program is divided into equal-sized pieces called pages, and memory is divided into the same sized units, called page frames. Each address is represented in a form. Operating system maintains a table of user page numbers and their true addresses in memory. The page portion of every reference is converted to a page frame address by a table lookup; the offset portion is added to the page frame address to produce the real memory address of the object referred to as.

39. Directory This technique works like a file directory. Imagine the set of objects to be files and the set of subjects to be users of a computing system. Every file has a unique owner who possesses “control” access rights, including the right to declare who has what access and to revoke access to any person at any time. Each user has a file directory, which lists all the files to which that user has access. OS maintains all directories. Each user has a list (directory) that contains all the objects that user is allowed to access.

40. File Protection Mechanisms Basic Forms of Protection Single Permissions

41. AUTHENTICATION It is the process of verifying a user’s identity (who you are) through the use of a shared secret (such as a password), a physical token or an artifact (such as a key or a smart card), or a biometric measure (such as a fingerprint). These three types of authentication are commonly referred to as something you have (physical token), something you know (shared secret), and something you are (biometric measure). The types and rigor of authentication methods and technologies vary according to the security requirements or policies associated with specific situations and implementations

42. User Authentication Use of Passwords Attacks on Passwords Password Selection Criteria The Authentication Process Flaws in the Authentication Process Authentication other than Passwords

43. Passwords &Protection The most common and least stringent form of authentication technology demands that users provide only a valid account name and a password to obtain access to a system or network. The password-based authentication is one-way and normally stores the user-id and password combination in a file that may be stored on the server in an encrypted or plaintext file. Most people using the public e- mail systems use this form of authentication.

44. Data Encryption Standard (DES) Encryption is based on one-way functions: functions that are cheap to compute but whose inverse is very expensive to compute. A still widely used, though older encryption algorithm is the Data Encryption Standard (DES), which uses a 56bit key. UNIX does not encrypt passwords with a secret key, instead, it uses the password as the key to encrypt a standard string. The latter method is not as vulnerable to attacks based on special properties of the “secret” key, which must nevertheless be known to many people.