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# Computer Forensics BACS 371 - PowerPoint PPT Presentation

Computer Forensics BACS 371. Computer System Basics 1 Number Systems & Text Representation. Computer System Basics. Number Systems Decimal (base 10) Binary (base 2) Octal (base 8) Hexadecimal (base 16) Conversions Little Endian vs. Big Endian Text Representation ASCII EBCDIC

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Presentation Transcript
Computer ForensicsBACS 371

Computer System Basics 1

Number Systems & Text Representation

• Number Systems

• Decimal (base 10)

• Binary (base 2)

• Octal (base 8)

• Conversions

• Little Endian vs. Big Endian

• Text Representation

• ASCII

• EBCDIC

• Unicode

• Decimal – base 10

• Binary – base 2

• Octal – base 8

• Base 10

• Uses digits 0~9

• Based on powers of 10

3 * 105 = 300,000

2 * 104 = 20,000

7 * 103 = 7,000

1 * 102 = 100

9 * 101 = 90

4 * 100 = 4

-------------------------------

TOTAL = 327,194

• Base 2

• Uses digits 0~1

• Based on powers of 2

1 * 25 = 32

1 * 24 = 16

0 * 23 = 0

1 * 22 = 4

0 * 21 = 0

1 * 20 = 1

-------------------------------

1101012 = 5310

• Base 8

• Uses digits 0~7

• Based on powers of 8

7 * 84 = 28,672

0 * 83 = 0

2 * 82 = 128

6 * 81 = 48

5 * 80 = 5

-------------------------------

702658 = 28,85310

• Base 16

• Uses digits 0~9 and A, B, C, D, E, F

• Based on powers of 16

3 * 165 = 3,145,728

F * 164 = 983,040

7 * 163 = 28,672

A * 162 = 2560

0 * 161 = 0

E * 160 = 14

-------------------------------

3F7A0E16 = 10,451,47010

• Binary

• 01001101b

• 010011012

• Octal

• 115o – note: trailing charter is a lowercase ‘oh’

• 1158

• 0x4D -- note: leading character is a zero

• 4Dh

• 4D16

http://www.noveltheory.com/TechPapers/endian.asp

Deals with the order that bytes are stored in Intel-based versus non Intel-based computers.

• Intel-based are normally PC-type computers

• Non Intel-based are normally mainframe computers

• Little Endian – stored left-to-right (Intel-based)

• Big Endian – stored right-to-left (mainframe)

• Text values stored in a computer can be in several formats

• ASCII

• EBCDIC

• Unicode

• ASCII, pronounced "ask-key", is the common code for microcomputer equipment

• American Standard Code for Information Interchange

• Proposed by ANSI in 1963, and finalized in 1968

• The standard ASCII character set consists of 128 decimal numbers ranging from zero through 127 assigned to letters, numbers, punctuation marks, and the most common special characters

• The first 32 codes are reserved for “non-printing” or “control” characters – supported original teletype systems

• The Extended ASCII Character Set also consists of 128 decimal numbers and ranges from 128 through 255 representing additional special, mathematical, graphic, and foreign characters

• http://students.washington.edu/cwei/tools/binary.shtml

• http://www.sitinthecorner.com/binary/binary.php

TEXT

Hello World

BINARY

01001000 01100101 01101100 01101100 01101111 00100000 01010111 01101111 01110010 01101100 01100100

Hex

48 65 6C 6C 6F 20 57 6F 72 6C 64

WinHex View

• Extended Binary Code Decimal Interchange Code

• Originally used by IBM-based mainframes

• Totally different encoding scheme from ASCII and Unicode

• Still used, but not as prevalent as in the past

• Character coding standard used in NTFS

• “Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language.” http://www.unicode.org

• Three varieties of Unicode Transformation Format

• UTF-8 – identical to ASCII for western languages

• UTF-16 – 16-bits per character

• UTF-32 – 32-bits per character