dos

MS-DOS OPERATING SYSTEM

Prepared by: Mr. Emmanuel R. Mercado

Understanding Operating Systems, Fourth Edition

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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 MS-DOS 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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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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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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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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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 low-addressable 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 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


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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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Table 13.3: A sample memory block with first five bytes containing 7700000004h


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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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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 MS-DOS 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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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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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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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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Figure 13.5: An example of directory listing of a root directory


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Figure 13.6: Typical directory system


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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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Figure 13.7: A typical FAT


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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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Table 13.4: MS-DOS user commands


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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 single-user/single-task systems

Can’t support multitasking, networking, and other sophisticated applications

dos

Technology

version of msdos

msdos system

available memory

limitations of msdos

evolution of msdos

layers of msdos

main memory allocation

memory manager