Ms dos

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Microsoft disk Operating System

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  • Figure are the layer structured of a ms-dos
  • Ms dos

    1. 1. Mercy Lou H. Yecla
    2. 2. MS-DOSo Historyo Design Goalso Memory Managemento Processor Managemento Device Managemento File Managemento User Interfaceo Additional Commands
    3. 3. MS-DOS, PC-DOS or DOS: Microsoft Disk Operating Systemo Developed to run single-user, stand-alone desktop computers.  Exemplifies early Operating Systems because it manages jobs sequentially from single user.o Advantages: Simple operation & straight-forward user commands.o Disadvantages: 1. Lack of flexibility & limited ability to meet needs of programmers & experienced users. 2. Written for a single family of microprocessors (Intel family of chips: 8086, 8088, 80186, and 80286).
    4. 4. History of DOS
    5. 5. History of DOS (cont)
    6. 6. Design Goalso Accommodate single novice user in single- process environment.o Standard I/O support includes keyboard, monitor, printer, & secondary storage unit.o User commands are based on English words/phrases indicative of action to be performed.o Commands are interpreted by command processor.o Layering approach “protects” user from hardware.
    7. 7. MS-DOS Architecture User COMMAND.COM DOS Kernel MS-DOS BIOS Hardware
    8. 8. First 2 DOS Layers : BIOS & Kernel• BIOS (Basic Input/Output System) -- interfaces directly with various I/ O devices.  Device drivers (control flow of data to/from each device).  Receives status info about success/failure of each I/O operation & passes it on to the processor.• DOS kernel -- routines needed to interface with the disk drives.  Read into memory at initialization time from MSDOS.SYS file on boot disk.  Accessed by application programs.  Provides a collection of hardware-independent services, such as memory management, & file & record management (system functions).
    9. 9. Third DOS Layer : Command Processor3. Command processor (shell) -- sends prompts to user, accepts commands, executes commands, & issues appropriate responses.  Resides in a file called COMMAND.COM, which consists of 2 parts stored in 2 different sections of main memory.  Not interpretive.  MS-DOS Version 4.0 -- menu-driven DOS shell.  OS/2 -- designed to replace MS-DOS.o MS-DOS ran enormous collection of software packages making it difficult to discontinue.
    10. 10. DOS Memory Managemento Memory Manager – Relatively simple job because it’s managing single job for single user. – Uses a first-fit memory allocation scheme since it is most efficient strategy in a single-user environment.
    11. 11. DOS Main Memory Allocationo First versions had simple contiguous memory allocation scheme that gave all of the available memory to resident application program.  Applications couldn’t dynamically allocate memory blocks.  MS-DOS Version 2.0 supported dynamic allocation, modification, & release of main memory blocks by applications.o Amount of memory each application owns depends on type of file from which program is loaded & size of TPA.  Programs.COM -- given all of TPA, whether or not they need it.  Programs.EXE -- given amount of memory they need.
    12. 12. Memory Block Allocationo Allocates memory by using first-fit algorithm & linked list of memory blocks.o With Version 3.3, MS-DOS started using best-fit or last-fit strategy.o Size of a block can vary from 16 bytes (paragraph) to maximum available memory.
    13. 13. Free/Busy Block Listo Whenever request for memory comes in, DOS looks through free/busy block list to find free block that fits.  If list becomes disconnected, system stops & must be rebooted.o Well-designed application program releases memory block it no longer needed.
    14. 14. Process Managemento MS-DOS doesn’t support multitasking.  Programs can’t break out of middle of DOS internal routine & restart routine from somewhere else.  Theres no interleaving & no need for sophisticated algorithms or policies to determine which job will run next or for how long.
    15. 15. Interrupt Handlerso Interrupt Handlers - are Responsible for Synchronizing Processeso PC has 256 interrupts & interrupt handlers, & they are accessed via interrupt vector table.o Three types of interrupts:• Internal hardware interrupts -- generated by certain events occurring during program’s execution (e.g., division by zero).• External hardware interrupts -- caused by peripheral device controllers or by coprocessors & assigned by manufacturers.• Software interrupts -- generated by system & application programs to access DOS & BIOS functions, which, in turn, access system resources.
    16. 16. Interrupt Handlers (cont)• Software interrupts (continued) – Some activate specialized application programs • Take control of computer • Example: Borland’s SideKick (type of TSR) – Terminate and Stay Resident (TSR) interrupt handler • Terminates process without releasing memory • Used by subroutine libraries • Sets up memory tables • Execution preparation via DOS interrupt connection • Determines memory required • Sends return code back to parent
    17. 17. Interrupt Handlers (cont)• Interrupt synchronization – CPU senses interrupt • Puts on stack: contents of PSW (program status word), code segment register, and instruction pointer register • Disables interrupt system • Uses eight-bit number to obtain interrupt handler address • Interrupt handler reenables interrupt system: allows higher-priority interrupts to occur • Saves registers and processes interrupt
    18. 18. Device Managemento Ability to reorder requests to optimize seek & search time is not a feature of DOS because it’s designed for a single-user environment.  All requests are handled on a first-come first-served basis.  Since version 3.0, BIOS can support spooling so users can schedule several files to be printed one after the other.o MS-DOS written for simple systems using keyboard, monitor, printer, mouse, 1-2 serial ports, & maybe second printer.  Devices do not require special management from OS.  Device drivers are the only items needed by Device Manager to make system work.  A device driver is a software module that controls an I/O device and handles its interrupts.
    19. 19. Managing Fileso Earliest versions of MS-DOS kept every file in a single directory.o Version 2.0 implemented hierarchical directory structure.o When disk is formatted, its tracks are divided into sectors of 512 bytes.o 2-8 sectors are grouped into clusters & thats how File Manager allocates space to files.
    20. 20. FORMAT Creates Three Special Areas on Disk• Boot record -- first sector of every logical disk & contains disk boot program & table of disks characteristics.• Root directory -- Lists system’s primary subdirectories & files (name, extension, size, date & time of modification, starting cluster #, file attribute codes).• FAT (File Allocation Table) -- contains status info about disk’s sectors: which are allocated, which are free, and which can’t be allocated because of errors.
    21. 21. Managing Files in DOSo Supports noncontiguous file storage & dynamically allocates disk space to a file, provided there’s enough disk room.o Compaction (MS-DOS Version 6.0) available via utility used to defragment disk (DEFRAG.EXE).o CHKDSK command used to determine need for compaction.o Restricting user access to computer system & resources isn’t built into MS-DOS.
    22. 22. User Interfaceo Command-driven operating system.o When user presses Enter key, shell (COMMAND.COM) interprets command & calls on next lower level routine to satisfy request.o User commands include some or all of these elements in this order:command source-file destination-file switches
    23. 23. Batch Files & Redirectiono By creating customized batch files, users can quickly execute combinations of DOS commands to configure their system, perform routine tasks, or make it easier for non-technical users to run software.o MS-DOS can redirect output from one standard input or output device to another. command > destination E.g., dir > PRN
    24. 24. Filter Commands (SORT, MORE)o Filter commands accept input from default device, manipulate data in some fashion, & send results to default output device.o SORT accepts input from keyboard, sorts that data, & displays it on screen.  Sort the file by column.o MORE causes output to be displayed on screen in groups of 24 lines, one screen at a time, & waits until user presses Enter key before displaying next 24 lines.
    25. 25. Pipe• Cause standard output from one command to be used as standard input to another command. – Symbol is a vertical bar, |. – Alphabetically sort directory & display sorted list on screen: DIR | SORT• Combine pipes and other filters. TYPE INVENTRY.DAT | MORE MORE < INVENTRY.DAT DIR | SORT | MORE DIR | SORT > SORTFILE MORE < SORTFILE
    26. 26. Summary• MS-DOS – Written to serve 1980s personal computer users• Limitation – Limited flexibility – Operating system unusable as hardware evolved• First standard operating system – Adopted by personal computing machine manufacturers – Supported by legions of software design groups
    27. 27. Summary (continued)• Advantages – Fundamental operation – Straightforward user commands• Weakness – Design • Single-user/single-task systems – No multitasking, networking, sophisticated applications support

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