Dos

MS-DOS OPERATING SYSTEM
Prepared by: Mr. Emmanuel R. Mercado

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

MS-DOS Operating System
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

History
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

History (continued)
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

History (continued)
6
Table 13.1: The evolution of
MS-DOS

Design Goals
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

Design Goals (continued)
8
Figure 13.2: The three layers of MS-DOS

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
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
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
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

Memory Management
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

Memory Management
(continued)
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.

Main Memory Allocation
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

Main Memory Allocation
(continued)
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

Memory Block Allocation
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

(continued)
18
Table 13.2: First five bytes of a memory block define
block’s structural characteristics

(continued)
19
Table 13.3: A sample memory block with first five bytes
containing 7700000004h

(continued)
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

(continued)
21
Figure 13.4: The linked list of memory blocks

Processor Management
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

Interrupt Handlers
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

Interrupt Handlers (continued)
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
 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
 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
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

Device Management
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

File Management
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

Filename Conventions
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

Filename Conventions
(continued)
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

Managing Files
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

Managing Files (continued)
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
 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
 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
 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
Figure 13.5: An example of directory listing of a root
directory

38
Figure 13.6: Typical directory system

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
Figure 13.7: A typical FAT

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

User Interface
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

User Interface (continued)
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
Table 13.4: MS-DOS user commands

45
Table 13.4 (continued): MS-DOS user commands

Batch Files
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

Redirection
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

Filters
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

Pipes
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

Additional Commands
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

Additional Commands
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

Summary
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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