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Chapter 12Chapter 12

MSMS--DOSDOS

ICSICS--023 PC Operating Systems023 PC Operating Systems

Ahmer ZakirAhmer Zakir

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Chapter 12 - MS DOS 2

MSMS--DOS Operating SystemDOS Operating System

History

Design Goals

Memory Management

Processor Management

Device Management

File Management

User Interface Additional Commands

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MSMS--DOS, PCDOS, PC--DOS or DOSDOS or DOS

Developed to run single-user, stand-alone desktopcomputers. Exemplifies early Operating Systems because it manages jobs

sequentially from single user.

Advantages: Simple operation & straight-forward usercommands.

Disadvantages:1. Lack of flexibility & limited ability to meet needs of programmers &

experienced users.2. Written for a single family of microprocessors (Intel family of

chips: 8086, 8088, 80186, and 80286).

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History of DOSHistory of DOS

Version Release

date

Features

1.0 1981 CP/M compatible; supported only 1 directory

1.1 1982 Allowed double-sided 5 inch disks

2.0 1983 Eliminated some defects in version 13.0 1984 Increased memory requirement to 36K,

supported PC/AT

3.1 1984 First release to support networking

3.2 1986 Supported token ring and 3 inch disks

3.3 1987 IBM PS/2 computer

4.0 1988 Supported hard disks larger than 32 megabytes5.0 1991 Better use of extended memory

6.0 1993 Better use of conventional memory

6.22 1994 Provided users with capabilities previouslyavailable only as third party applications

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Design GoalsDesign Goals

User

Hardware

COMMAND.COM

DOS Kernel

BIOS

MS-DOS

Accommodate single novice userin single-process environment.

Standard I/O support includeskeyboard, monitor, printer, &secondary storage unit.

User commands are based onEnglish words/phrases indicative ofaction to be performed.

Commands are interpreted bycommand processor.

Layering approach protects userfrom hardware.

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Chapter 12 - MS DOS6

First 2 DOS Layers : BIOS &First 2 DOS Layers : BIOS &

KernelKernel

1. 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.

2. 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 : CommandThird DOS Layer : Command

ProcessorProcessor

3. Command processor (shell) -- sends prompts to user,accepts commands, executes commands, & issuesappropriate 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 packagesmaking it difficult to discontinue.

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DOS Memory ManagementDOS Memory Management

Relatively simple job because its managing single jobfor 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 ofCommand.COM

Transient program

Area (user memory)

TSR programs

Resident part ofCommand.COM

Installable Drivers

Buffer Cache

MS-DOS Kernel

BIOS Interface

Interrupt Vectors (256) 0

RAM Layout for

1MB of Memory

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DOS Main Memory AllocationDOS Main Memory Allocation

First versions had simple contiguous memory allocation schemethat gave all of the available memory to resident applicationprogram. Applications couldnt dynamically allocate memory blocks.

MS-DOS Version 2.0 supported dynamic allocation, modification, & release ofmain memory blocks by applications.

Amount of memory each application owns depends on type of filefrom 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 AllocationMemory 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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Free/Busy Block ListFree/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.

Busy Free Busy Free Busy

Block 1 Block 2 Block 3 Block 4 Block 5

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Process ManagementProcess 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 ResponsibleInterrupt Handlers Are Responsible

for Synchronizing Processesfor Synchronizing Processes

PC has 256 interrupts & interrupt handlers, & they are accessed viainterrupt vector table.

Three types of interrupts:

1. Internal hardware interrupts -- generated by certain events occurring

during programs execution (e.g., division by zero).2. External hardware interrupts -- caused by peripheral device

controllers or by coprocessors & assigned by manufacturers.

3. Software interrupts -- generated by system & application programs toaccess DOS & BIOS functions, which, in turn, access system

resources.

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Device ManagementDevice 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 itsinterrupts.

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Managing FilesManaging 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 ThreeFORMAT Creates Three

Special Areas on DiskSpecial Areas on Disk

1. Boot record-- first sector of every logical disk & contains disk bootprogram & table of disk's characteristics.

2. Root directory-- Lists systems primary subdirectories & files (name,

extension, size, date & time of modification, starting cluster #, fileattribute codes).

3. FAT (File Allocation Table) -- contains status info about diskssectors: which are allocated, which are free, and which cant be

allocated because of errors.

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Managing Files in DOSManaging 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 InterfaceUser 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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Command Stands for ction to be performed

DIR Directory List whats in this directory.

CD or CHDIR Change Directory Change working directory.

COPY Copy Copy a file. Append one to another.

DEL or ERASE Delete Delete the following file orfiles.

RENAME Rename Rename a file.

TYPE Type Display text file on screen.

PRINT Print Print one or more files on printer.DATE Date Display and/or change system date.

TIME Time Display and/or change system time.

MD or MKDIR Make Directory Create a new directory or subdirectory.

FIND Find Find a string. Search files for a string.

FORMAT Format Disk Logically prepare diskforfile storage.

CHKDSK Check Disk Check diskfor disk/file/directory

status.

PROMPT System Prompt Change system prompt symbol.DEFRAG Defragment Disk Compact fragmented files.

(filename) Run, execute, file.

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Batch Files & RedirectionBatch Files & Redirection

By creating customized batch files, users can quicklyexecute combinations of DOS commands to configuretheir system, perform routine tasks, or make it easier fornon-technical users to run software.

MS-DOS can redirect output from one standard input oroutput device to another.

command > destination

E.g., dir > PRN

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Filter Commands (SORT, MORE)Filter Commands (SORT, MORE)

Filter commands accept input from default device, manipulate data insome 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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ics-023 chapter 12

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