Chapter 13 MSDOS Operating System

1
Chapter 13MS-DOS Operating System

• Understanding Operating Systems, Fourth Edition

2
Objectives

• 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 todays
  computer users

3
MS-DOS Operating System

• 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

• 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)

• 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)
Table 13.1 The evolution of MS-DOS
7
Design Goals

• 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)
Figure 13.2 The three layers of MS-DOS
9
Design Goals (continued)

• 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)

• 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)

• DOS kernel (continued)
• Makes disk file management transparent to user
• Manages storage and retrieval of files
• Dynamically allocates and deallocates secondary
  storage as its needed

12
Design Goals (continued)

• 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 isnt interpretive

13
Memory Management

• 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)
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

• 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)

• 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 cant 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

• 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
  programs 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)
Table 13.2 First five bytes of a memory block
define blocks structural
characteristics
19
Memory Block Allocation (continued)
Table 13.3 A sample memory block with first five
bytes containing 7700000004h
20
Memory Block Allocation (continued)

• 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)
Figure 13.4 The linked list of memory blocks
22
Processor Management

• MS-DOS doesnt support reentrant code (basis for
  multitasking)
• Programs cant 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

• 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)

• Internal hardware interrupts Generated by
  certain events occurring during programs
  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)

• External hardware interrupts Caused by
  peripheral device controllers or by coprocessors
• Assignment of external devices to specific
  interrupt levels is done by manufacturer
• Cant 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)

• Software interrupts (continued)
• Some are used to activate specialized application
  programs that take over control of computer
• Example Borlands 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)

• 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

• 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

• 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

• A filename
• Contains no spaces
• Consists of drive designation, directory, any
  subdirectory, a primary name, and an optional
  extension
• DOS isnt 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)

• 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

• 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)

• 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)

• Boot records First sector of every logical disk
  and contains
• Disk boot program
• Table of disks characteristics
• Root directory Where system begins its
  interaction with user and contains
• List of systems primary subdirectories and files
• Any system-generated configuration files
• Any user-generated booting instructions

35
Managing Files (continued)

• 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 files last modification
• Starting cluster number for the file
• File attribute codes

36
Managing Files (continued)

• 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 isnt
  hidden

37
Managing Files (continued)
Figure 13.5 An example of directory listing of
a root directory
38
Managing Files (continued)
Figure 13.6 Typical directory system
39
Managing Files (continued)

• File Allocation Table (FAT) Contains status
  information about disks sectors
• Status includes, which sectors are allocated,
  free, and cant 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)
Figure 13.7 A typical FAT
41
Managing Files (continued)

• 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

• MS-DOS uses command-driven interface
• Users type in commands at system prompt
• Default prompt is drive indicator and gt 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)

• 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)
Table 13.4 MS-DOS user commands
45
User Interface (continued)
Table 13.4 (continued) MS-DOS user commands
46
Batch Files

• 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 systems root directory

47
Redirection

• MS-DOS can redirect output from one standard
  input or output device to another
• Syntax command gt destination
• e.g., DIR gt PRN sends directory listing to
  printer instead of monitor screen
• Append Symbol (gtgt) redirect and append new output
  to an existing file
• e.g., DIR gtgt BDIRFILE
• Redirection works in opposite manner as well
• Symbol (lt) changes source to a specific device or
  file. e.g., INVENTRY lt BTEST.DAT

48
Filters

• 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

• 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

• 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

• 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 thats duplicated
  on several different directories

52
Summary

• 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
• Cant support multitasking, networking, and other
  sophisticated applications