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  • 1. MS-DOS OPERATING SYSTEM Prepared by: Mr. Emmanuel R. Mercado
  • 2. Objectives Understanding Operating Systems, Fourth Edition 2 You should be able to describe:  The historical significance of MS-DOS  How MS-DOS provided a foundation for early Microsoft Windows releases  The basics of command-driven systems and how to construct simple batch files  How one processor can be shared among multiple processes  The limitations of MS-DOS for many of today’s computer users
  • 3. MS-DOS Operating System Understanding Operating Systems, Fourth Edition 3  Developed to run single-user, stand-alone desktop computers  Manages jobs sequentially from a single user  Advantages:  Fundamental operation  Straightforward user commands  Disadvantages:  Lack of flexibility  Limited ability to meet the needs of programmers and experienced users
  • 4. History Understanding Operating Systems, Fourth Edition 4  MS-DOS was successor of CP/M operating system that ran first PC  Microsoft discovered an innovative operating system, called 86-DOS, designed by Tim Patterson of Seattle Computer Products  Microsoft bought it, renamed it MS-DOS, and made it available to IBM  IBM chose MS-DOS in 1981, called it PC- DOS, and proclaimed it the standard for their line of PCs
  • 5. History (continued) Understanding Operating Systems, Fourth Edition 5  MS-DOS became standard operating system for most 16-bit personal computers  Each version of MS-DOS is a standard version  Later versions are compatible with earlier versions  Early versions of Windows (versions 1.0 through 3.1) were merely GUIs that ran on top of the MS-DOS operating system  Although MS-DOS is no longer widely used, many Windows OSs offer a DOS emulator
  • 6. History (continued) Understanding Operating Systems, Fourth Edition 6 Table 13.1: The evolution of MS-DOS
  • 7. Design Goals Understanding Operating Systems, Fourth Edition 7  Designed to accommodate single novice user in single-process environment  Standard I/O support includes keyboard, monitor, printer, and secondary storage unit  User commands are based on English words or phrases, interpreted by command processor  Layering approach is fundamental to design of the whole MS-DOS system
  • 8. Design Goals (continued) Understanding Operating Systems, Fourth Edition 8 Figure 13.2: The three layers of MS-DOS
  • 9. Design Goals (continued) Understanding Operating Systems, Fourth Edition 9 BIOS (Basic Input/Output System):  Interfaces directly with various I/O devices  Contains device drivers that control flow of data to and from each device except disk drives  Receives status information of each I/O operation and passes it on to processor  Takes care of small differences among I/O units  Example: Allows user to purchase a printer from any manufacturer without having to write a device driver
  • 10. Design Goals (continued) Understanding Operating Systems, Fourth Edition 10 DOS kernel:  Contains routines that interface with disk drives  Read into memory at initialization time from MSDOS.SYS file residing in boot disk  Accessed by application programs and provides collection of hardware-independent services, such as:  Memory management and file and record management  Compensates for variations from manufacturer to manufacturer
  • 11. Design Goals (continued) Understanding Operating Systems, Fourth Edition 11 DOS kernel: (continued)  Makes disk file management transparent to user  Manages storage and retrieval of files  Dynamically allocates and deallocates secondary storage as it’s needed
  • 12. Design Goals (continued) Understanding Operating Systems, Fourth Edition 12 Command processor (the shell):  Sends prompts to user  Accepts commands that are typed in  Executes commands, and issues appropriate responses  Resides in a file called COMMAND.COM, which consists of two parts, stored in two different sections of main memory  Only part of OS that appears on the public directory  Weakness: It isn’t interpretive
  • 13. Memory Management Understanding Operating Systems, Fourth Edition 13  Memory Manager manages single job for single user  To run second job, user must close or pause first file before opening second  Uses first-fit memory allocation scheme  Main memory comes in two forms:  ROM: Very small in size and contains a program, a section of BIOS, with the the startup process (bootstrapping)  RAM: Part of the main memory where programs are loaded and executed
  • 14. Memory Management (continued) Understanding Operating Systems, Fourth Edition 14 Figure 13.3: One megabyte of RAM main memory in MS-DOS. The interrupt vectors are located in low- addressable memory and COMMAND.COM overlay is located in high addressable memory.
  • 15. Main Memory Allocation Understanding Operating Systems, Fourth Edition 15  MS-DOS Version 1.0 gave all available memory to resident application program  MS-DOS Version 2.0 began supporting dynamic allocation, modification, and release of main memory blocks by application programs  Amount of memory each application program actually owns depends on:  Type of file from which program is loaded  Size of TPA
  • 16. Main Memory Allocation (continued) Understanding Operating Systems, Fourth Edition 16  Programs with COM extension are given all of the TPA, whether or not they need it  Programs with EXE extension are only given amount of memory they need  Except for COM files, there can be any number of files in TPA at one time  Two programs can’t be run at same time  Shrinking and expanding of memory allocation during execution can be done only from programs written in either assembly language or C
  • 17. Memory Block Allocation Understanding Operating Systems, Fourth Edition 17  Memory Manager allocates memory by using first-fit algorithm and linked list of memory blocks  Best-fit or last-fit strategy can be selected with Version 3.3 and beyond  When using last-fit, DOS allocates highest addressable memory block big enough to satisfy program’s request  Size of a block can vary from as small as 16 bytes (called a “paragraph”) to as large as maximum available memory
  • 18. Memory Block Allocation (continued) Understanding Operating Systems, Fourth Edition 18 Table 13.2: First five bytes of a memory block define block’s structural characteristics
  • 19. Memory Block Allocation (continued) Understanding Operating Systems, Fourth Edition 19 Table 13.3: A sample memory block with first five bytes containing 7700000004h
  • 20. Memory Block Allocation (continued) Understanding Operating Systems, Fourth Edition 20  When a memory request comes in:  DOS looks through free/busy block list until it finds a free block that fits request  A well-designed application program releases memory block it no longer needs  If two free memory blocks are contiguous, they are merged immediately into one block and linked to the list
  • 21. Memory Block Allocation (continued) Understanding Operating Systems, Fourth Edition 21 Figure 13.4: The linked list of memory blocks
  • 22. Processor Management Understanding Operating Systems, Fourth Edition 22  MS-DOS doesn’t support reentrant code (basis for multitasking)  Programs can’t break out of middle of DOS internal routine and then restart routine from somewhere else  Each job runs in complete segments and is not interrupted midstream  Interrupt handlers allows the saving of all information about parent program that allows its proper restart after child program has finished
  • 23. Interrupt Handlers Understanding Operating Systems, Fourth Edition 23  Responsible for synchronizing processes  A personal computer has 256 interrupts and interrupt handlers, accessed via interrupt vector table  Interrupts can be divided into three groups:  Internal hardware interrupts  External hardware interrupts  Software interrupts
  • 24. Interrupt Handlers (continued) Understanding Operating Systems, Fourth Edition 24  Internal hardware interrupts: Generated by certain events occurring during program’s execution, e.g., division by zero  Assignment of such events to specific interrupt numbers is electronically wired into processor  Not modifiable by software instructions
  • 25. Interrupt Handlers (continued) Understanding Operating Systems, Fourth Edition 25  External hardware interrupts: Caused by peripheral device controllers or by coprocessors  Assignment of external devices to specific interrupt levels is done by manufacturer  Can’t be modified by software  Implemented as physical electrical connections  Software interrupts: Generated by system and application programs  Access DOS and BIOS functions
  • 26. Interrupt Handlers (continued) Understanding Operating Systems, Fourth Edition 26  Software interrupts: (continued)  Some are used to activate specialized application programs that take over control of computer  Example: Borland’s SideKick (type of TSR)  Terminate and Stay Resident (TSR) interrupt handler:  Terminates process without releasing its memory  Usually used by subroutine libraries  When running, it sets up memory tables and prepares for execution by connecting to DOS interrupt
  • 27. Interrupt Handlers (continued) Understanding Operating Systems, Fourth Edition 27 Interrupts synchronization:  When CPU senses interrupt, it does two things:  Puts contents of PSW (program status word), code segment register, and instruction pointer register on a stack  Disables interrupt system so that other interrupts will be put off until current one has been resolved  CPU uses 8-bit number to get address of appropriate interrupt handler  Interrupt handler reenables interrupt system to allow higher-priority interrupts to occur
  • 28. Device Management Understanding Operating Systems, Fourth Edition 28  Requests are handled first-come, first-served  Does not support reordering requests, though in Version 3.0, BIOS can support spooling  MS-DOS Device Manager can work with magnetic tape, floppy disks, or hard disks  BIOS handles device driver software  Device drivers are only items needed by Device Manager to make system work  Installable device drivers are salient feature of MS-DOS design
  • 29. File Management Understanding Operating Systems, Fourth Edition 29  MS-DOS supports following file organizations:  Sequential  Can have either variable or fixed-length records  Direct  Can only have fixed-length records  Indexed sequential  Can only have fixed-length records
  • 30. Filename Conventions Understanding Operating Systems, Fourth Edition 30  A filename:  Contains no spaces  Consists of drive designation, directory, any subdirectory, a primary name, and an optional extension  DOS isn’t case-sensitive  Drive name is followed by a colon (:)  Directories or subdirectories can be from one to eight characters long and preceded by a backslash  Primary filename can be from one to eight characters long
  • 31. Filename Conventions (continued) Understanding Operating Systems, Fourth Edition 31  Extension can be from one to three characters long and can have special meaning  File is assumed in current working directory if no directories or subdirectories are included in name  File is assumed on current drive if no drive is designated  Relative name consists of primary name and extension  Absolute name consists of drive designation and directory location
  • 32. Managing Files Understanding Operating Systems, Fourth Edition 32  Earliest versions kept every file in single directory  Slow and cumbersome file retrieval  Microsoft implemented hierarchical directory structure in Version 2.0  An inverted tree directory structure (root at top)  Disk tracks are divided into sectors of 512 bytes each when formatted  Corresponding to buffer size of 512 bytes  Concept of cylinders, applies to hard disks
  • 33. Managing Files (continued) Understanding Operating Systems, Fourth Edition 33  Sectors (from two to eight) are grouped into clusters  When a file needs additional space, DOS allocates more clusters to it  FORMAT creates three special areas on disk:  Boot record  Root directory  FAT(file allocation table)
  • 34. Managing Files (continued) Understanding Operating Systems, Fourth Edition 34  Boot records: First sector of every logical disk and contains:  Disk boot program  Table of disk’s characteristics  Root directory: Where system begins its interaction with user and contains:  List of system’s primary subdirectories and files  Any system-generated configuration files  Any user-generated booting instructions
  • 35. Managing Files (continued) Understanding Operating Systems, Fourth Edition 35  Root Directory (continued):  AUTOEXEC.BAT file: Batch file containing series of commands defined by user  Every time CPU is powered up, the commands in this file are executed automatically by system  The information kept in root directory include:  Filename, File extension  File size in bytes  Date and time of the file’s last modification  Starting cluster number for the file  File attribute codes
  • 36. Managing Files (continued) Understanding Operating Systems, Fourth Edition 36  Root Directory (continued):  Number of entries in root directory is fixed  Version 2.0 and onward versions allow users to avoid this limitation by creating subdirectories  Each subdirectory can contain its own subdirectories and/or files  MS-DOS supports hidden files  Files that are executable but not displayed in response to DIR commands  COMMAND.COM is the only system file that isn’t hidden
  • 37. Managing Files (continued) Understanding Operating Systems, Fourth Edition 37 Figure 13.5: An example of directory listing of a root directory
  • 38. Managing Files (continued) Understanding Operating Systems, Fourth Edition 38 Figure 13.6: Typical directory system
  • 39. Managing Files (continued) Understanding Operating Systems, Fourth Edition 39  File Allocation Table (FAT): Contains status information about disk’s sectors  Status includes, which sectors are allocated, free, and can’t be allocated because of formatting errors  All sectors except first are linked in a chain  Each FAT entry gives sector/cluster number of next entry  Last entry contains value FF to indicate end of chain
  • 40. Managing Files (continued) Understanding Operating Systems, Fourth Edition 40 Figure 13.7: A typical FAT
  • 41. Managing Files (continued) Understanding Operating Systems, Fourth Edition 41  MS-DOS views data in disk file as continuous string of bytes  I/O operations request data by relative byte (relative to beginning of file) rather than by relative sector  MS-DOS supports noncontiguous file storage  Dynamically allocates disk space to file  Compaction became feature of MS-DOS Version 6.0 with inclusion of DEFRAG.EXE  CHKDSK (filename) responds with number of noncontiguous blocks in which file is stored  Security feature is not built into MS-DOS
  • 42. User Interface Understanding Operating Systems, Fourth Edition 42  MS-DOS uses command-driven interface  Users type in commands at system prompt  Default prompt is drive indicator and > character  Default prompt can be changed using PROMPT command  User commands include some or all of following elements in this order:  Command, source- file, destination-file, switches
  • 43. User Interface (continued) Understanding Operating Systems, Fourth Edition 43  Switches are optional and give specific details about how command is to be carried out  Begin with slash (i.e., /P /V /F)  COMMAND.COM carries out commands  Resident portion of code: Stored in low section of memory  Contains command interpreter and routines needed to support an active program  Transient code: Stored in highest addresses of memory  Can be overwritten by application programs if they need to use its memory space
  • 44. User Interface (continued) Understanding Operating Systems, Fourth Edition 44 Table 13.4: MS-DOS user commands
  • 45. User Interface (continued) Understanding Operating Systems, Fourth Edition 45 Table 13.4 (continued): MS-DOS user commands
  • 46. Batch Files Understanding Operating Systems, Fourth Edition 46  Customized batch files allows users to quickly execute combinations of DOS commands to:  Configure systems  Perform routine tasks  Make it easier for nontechnical users to run software  For such programs to run automatically every time system is restarted:  File should be renamed AUTOEXEC.BAT and loaded into system’s root directory
  • 47. Redirection Understanding Operating Systems, Fourth Edition 47  MS-DOS can redirect output from one standard input or output device to another  Syntax: command > destination  e.g., DIR > PRN sends directory listing to printer instead of monitor screen  Append Symbol (>>) redirect and append new output to an existing file  e.g., DIR >> B:DIRFILE  Redirection works in opposite manner as well  Symbol (<) changes source to a specific device or file. e.g., INVENTRY < B:TEST.DAT
  • 48. Filters Understanding Operating Systems, Fourth Edition 48  Filter commands: Accept input from default device, manipulate data in some fashion, and send results to default output device  Example: SORT  Can read data from file and sort it to another file  Sorted in ascending order  SORT /R sorts file in reverse order  Files can be sorted by columns  Example: MORE  Causes output to be displayed on screen in groups of 24 lines, one screen at time
  • 49. Pipes Understanding Operating Systems, Fourth Edition 49  Causes standard output from one command to be used as standard input to another command  Symbol: Vertical bar (|)  e.g., DIR | SORT alphabetically sort directory and display sorted list on screen  Pipes and other filters can be combined  Possible to sort directory and display it one screen at a time by using pipe command: DIR | SORT | MORE
  • 50. Additional Commands Understanding Operating Systems, Fourth Edition 50  FIND: Searches for specific string in given file or files and displays all lines that contain the string from those files  e.g., FIND "AMNT-PAID" PAYROLL.COB display all lines in the file PAYROLL.COB that contain string AMNT-PAID  PRINT: Allows user to set up series of files for printing while freeing up COMMAND.COM  PRINT /B allows changing of internal buffer size  PRINT /Q specifies the number of files allowed in print queue
  • 51. Additional Commands Understanding Operating Systems, Fourth Edition 51  TREE: Displays directories and subdirectories in hierarchical and indented list  Options allow user to delete files while tree is being generated  TREE /F displays names of files in each directory  Can also be used to delete file that’s duplicated on several different directories
  • 52. Summary Understanding Operating Systems, Fourth Edition 52  MS-DOS was written to serve users of several generations of personal computers  First standard operating system to be adopted by manufacturers of personal computing machines  Advantages are its fundamental operation and its straightforward user commands  Weakness is that it was designed for single- user/single-task systems  Can’t support multitasking, networking, and other sophisticated applications

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