Chapter 12 MSDOS

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Chapter 12MS-DOS

• ICS-023 PC Operating Systems
• Ahmer Zakir

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MS-DOS Operating System

• History
• Design Goals
• Memory Management
• Processor Management
• Device Management
• File Management
• User Interface
• Additional Commands

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MS-DOS, PC-DOS or DOS

• Developed to run single-user, stand-alone desktop
  computers.
• Exemplifies early Operating Systems because it
  manages jobs sequentially from single user.
• Advantages Simple operation straight-forward
  user commands.
• Disadvantages
• Lack of flexibility limited ability to meet
  needs of programmers experienced users.
• Written for a single family of microprocessors
  (Intel family of chips 8086, 8088, 80186, and
  80286).

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History of DOS
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Design Goals

• Accommodate single novice user in single-process
  environment.
• Standard I/O support includes keyboard, monitor,
  printer, secondary storage unit.
• User commands are based on English words/phrases
  indicative of action to be performed.
• Commands are interpreted by command processor.
• Layering approach protects user from hardware.

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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).

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Third DOS Layer Command Processor

• 3. 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.
• MS-DOS ran enormous collection of software
  packages making it difficult to discontinue.

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DOS Memory Management

• Relatively simple job because its managing
  single job for single user.
• Uses a first-fit memory allocation scheme since
  it is most efficient strategy in a single-user
  environment.

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• Reserved for BIOS 1M
• Unused 640K
• Transient part of
• Command.COM
• Transient program
• Area (user memory)
• TSR programs
• Resident part of
• Command.COM
• Installable Drivers
• Buffer Cache
• MS-DOS Kernel

RAM Layout for 1MB of Memory
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DOS Main Memory Allocation

• First versions had simple contiguous memory
  allocation scheme that gave all of the available
  memory to resident application program.
• Applications couldnt dynamically allocate memory
  blocks.
• MS-DOS Version 2.0 supported dynamic allocation,
  modification, release of main memory blocks by
  applications.
• 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.
  

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Memory Block Allocation

• Allocates memory by using first-fit algorithm
  linked list of memory blocks.
• With Version 3.3, MS-DOS started using best-fit
  or last-fit strategy.
• Size of a block can vary from 16 bytes
  (paragraph) to maximum available memory.

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First 5 Bytes of Memory Block Define Blocks
Structural Characteristics
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Free/Busy Block List

• 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.
• Well-designed application program releases memory
  block it no longer needed.

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Process Management

• MS-DOS doesnt support multitasking.
• Programs cant break out of middle of DOS
  internal routine restart routine from somewhere
  else.
• There's no interleaving no need for
  sophisticated algorithms or policies to determine
  which job will run next or for how long.

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Interrupt Handlers Are Responsible for
Synchronizing Processes

• PC has 256 interrupts interrupt handlers,
  they are accessed via interrupt vector table.
• Three types of interrupts
• Internal hardware interrupts -- generated by
  certain events occurring during programs
  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.

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Device Management

• Ability to reorder requests to optimize seek
  search time is not a feature of DOS because its
  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.
• 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.

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Managing Files

• Earliest versions of MS-DOS kept every file in a
  single directory.
• Version 2.0 implemented hierarchical directory
  structure.
• When disk is formatted, its tracks are divided
  into sectors of 512 bytes.
• 2-8 sectors are grouped into clusters that's
  how File Manager allocates space to files.

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FORMAT Creates Three Special Areas on Disk

• Boot record -- first sector of every logical disk
  contains disk boot program table of disk's
  characteristics.
• Root directory -- Lists systems primary
  subdirectories files (name, extension, size,
  date time of modification, starting cluster ,
  file attribute codes).
• FAT (File Allocation Table) -- contains status
  info about disks sectors which are allocated,
  which are free, and which cant be allocated
  because of errors.

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Managing Files in DOS

• Supports noncontiguous file storage dynamically
  allocates disk space to a file, provided theres
  enough disk room.
• Compaction (MS-DOS Version 6.0) available via
  utility used to defragment disk (DEFRAG.EXE).
• CHKDSK command used to determine need for
  compaction.
• Restricting user access to computer system
  resources isnt built into MS-DOS.

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User Interface

• Command-driven operating system.
• When user presses Enter key, shell (COMMAND.COM)
  interprets command calls on next lower level
  routine to satisfy request.
• User commands include some or all of these
  elements in this order
• command source-file destination-file
  switches

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Batch Files Redirection

• 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.
• MS-DOS can redirect output from one standard
  input or output device to another.
• command gt destination
• E.g., dir gt PRN

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Filter Commands (SORT, MORE)

• Filter commands accept input from default device,
  manipulate data in some fashion, send results
  to default output device.
• SORT accepts input from keyboard, sorts that
  data, displays it on screen.  
• Sort the file by column.
• 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.

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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 lt INVENTRY.DAT
• DIR SORT MORE
• DIR SORT gt SORTFILE
• MORE lt SORTFILE