Chapter 6

Disk Operating System

Hardware

ROM BIOS

Operating System

Applications

Memory

I/O Device’s

Internal Commands

External Commands

Interrupt HandlerIO SystemMS-DOS

Command Resident

Program Memory

Video Area

0000

640 K

Figure 2 Memory Map of 8088 Processor

Flat File System

Flat Issues

Hierarchical File System

Payroll Inventory Payroll Inventory Payroll Inventory

Files Files Files Files Files Files

C:DOS (ROOT dir)

Chicago dir Atlanta dir Portland dir

Path statement

• Absolute pathname• C:\US\sales\salesreps\smith

• Relative pathname• CD \smith

• “Program not found”

8.3

File Specification A:\DIRNAME\FILENAME.EXT

Drive location

Directory location

File Name

Extension

Can be up to 8 eight characters

The following characters are not allowed:

. ” / \ [ ] < > + = ; ‘ Spacebar

Commands

• Name Options Parameters

• Keyword Drive Options

• Keyword Source file File switches

• Do what? To who? How?

Command Essentials

• You must enter words shown in capital letters y).• You must supply any items shown in italic letters• Items in square brackets [] are optional.• Items separated by a bar ( | ) means you can

enter one of the other ( Example OFF | ON ) • An Ellipsis (…) indicates you can repeat as

many time as you want.• You must include all punctuation, except square

brackets and vertical bars.

Global Filename Characters

• ? = can be any character• * = any character can occupy that position

and all remaining positions• DIR A:INPUT.??? Or DIR A:INPUT.*• ******WARNING******* • DELETE C:*.*• ******WARNING*******

Batch Commands

• ECHO,FOR,GOTO,IF,PAUSE,REM

• A batch file is a file containing 1 or more commands that DOS executes. ext = .bat

• ECHO ON• REM This is an example of a batch cmd.• Copy C:prog1.exe to C:prog2.exe• DIR• Autoexec.bat = start program at DOS start

Redirection

• DIR >PRN Sends cmd to printer• DIR >catalog Sends cmd to filename• MYPROG <INPUT.TXT • DIR >> filename appends to file

Piping & Filters

• Filters = SORT, FIND,MORE

• DIR|SORT>FILE

• SORT <MYFILE >RESULT

• TYPE MYFILE|MORE

Help /?

• Command Summary C:>Help• Help command-name (C:>help copy)• Copy /?

C:>copy /?

Physical Structure of a Diskette• Formatting

– concentric circles called tracks– pie-shaped wedges called sectors

• Data is written to the tracks as bits,0’s and 1’s– bits become magnetized spots on the disk– spacing prevents one spot from affecting other spots

• Read/write heads– above and below the diskette move in unison

• Diskette made up of Mylar – covered with either cobalt oxide or iron oxide to hold

magnetic charge– Formatting writes the division symbol ÷ F6 1111 0110 to

each track

Logical Structure of a Diskette

• The BIOS manages the disk in sectors, physical terms– a sector holds 512 bytes of data

• The OS manages the disk in clusters, logical terms– A cluster is defined as the smallest unit of data that can

be read from or written to at one time– File allocation unit (FAT)– One cluster equals the amount of storage space per two

sectors, top and bottom of the diskette512 bytes X 2 = 1024 (one cluster, double-density 3.5”

diskette )80 X 9 = 720 (one side of diskette in sectors, the whole

diskette in terms of clusters)1024 X 720 = 737,280

The Standard PC Diskette Configuration:

Diskette size Tracks per side

Number of sectors per tracks

Capacity

5.25" Single side SD8

40 8 40 X 8 X 512 bytes = 160 KB

5.25" Double side DD9

40 9 2 X 40 X 9 X 512 bytes = 360 KB

5.25" Double side High Density

DQ15

80 15 2 X 80 X 15 X 512 bytes = 1.2 MB

3.5" DD DQ9

80 9 2 X 80 X 9 X 512 bytes = 720 KB

3.5" HD DQ18

80 18 2 X 80 X 18 X 512 bytes = 1.44 MB

3.5" XD ( IBM only)

DG36

80 36 2 X 80 X 36 X 512 bytes = 2.88 MB

Diskette drives turn at 300 RPM. That results in an average search time (½ revolution) of 100 ms.

IDE/ATA 3 ½ Hard Disk

Tracks, Cylinders, and Sectors

$64 Million Question

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What does this have to do with security?

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