Chapter 13
MS-DOS Operating System
Understanding Operating Systems,
Fourth Edition
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 today’s
computer users
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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
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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
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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 MSDOS operating system
• Although MS-DOS is no longer widely used, many
Windows OSs offer a DOS emulator
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History (continued)
Table 13.1: The evolution of MS-DOS
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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
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Design Goals (continued)
Figure 13.2: The three layers of MS-DOS
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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
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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
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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 it’s needed
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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 isn’t interpretive
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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
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Memory Management (continued)
Figure 13.3: One megabyte of RAM
main memory in MS-DOS. The
interrupt vectors are located in lowaddressable memory and
COMMAND.COM overlay is located in
high addressable memory.
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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
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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 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
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Memory Block Allocation
• Memory Manager allocates memory by using firstfit 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
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Memory Block Allocation (continued)
Table 13.2: First five bytes of a memory block define
block’s structural characteristics
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Memory Block Allocation (continued)
Table 13.3: A sample memory block with first five bytes
containing 7700000004h
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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
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Memory Block Allocation (continued)
Figure 13.4: The linked list of memory blocks
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Processor Management
• 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
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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
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Interrupt Handlers (continued)
• 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
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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
– Can’t be modified by software
• Implemented as physical electrical connections
• Software interrupts: Generated by system and
application programs
– Access DOS and BIOS functions
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Interrupt Handlers (continued)
• 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
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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
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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 MSDOS design
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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
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Filename Conventions
• 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
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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
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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
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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)
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Managing Files (continued)
• 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
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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 file’s last modification
Starting cluster number for the file
File attribute codes
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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 isn’t
hidden
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Managing Files (continued)
Figure 13.5: An example of directory listing of a root
directory
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Managing Files (continued)
Figure 13.6: Typical directory system
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Managing Files (continued)
• 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
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Managing Files (continued)
Figure 13.7: A typical FAT
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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
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User Interface
•
•
•
•
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
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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
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User Interface (continued)
Table 13.4: MS-DOS user commands
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User Interface (continued)
Table 13.4 (continued): MS-DOS user commands
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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 system’s root directory
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Redirection
• 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
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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
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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
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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
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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 that’s duplicated on
several different directories
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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 singleuser/single-task systems
• Can’t support multitasking, networking, and other
sophisticated applications
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