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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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computer forensics bacs 371
Computer ForensicsBACS 371

Computer System Basics 1

Number Systems & Text Representation

computer system basics
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
    • Unicode
number systems
Number Systems
  • Decimal – base 10
  • Binary – base 2
  • Octal – base 8
  • Hexadecimal – base 16
decimal number system
Decimal Number System
  • 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

binary number system
Binary Number System
  • 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

octal number system
Octal Number System
  • 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

hexadecimal number system
Hexadecimal Number System
  • 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

number system representations
Number System Representations
  • Binary
    • 01001101b
    • 010011012
  • Octal
    • 115o – note: trailing charter is a lowercase ‘oh’
    • 1158
  • Hexadecimal
    • 0x4D -- note: leading character is a zero
    • 4Dh
    • 4D16
little endian vs big endian
Little Endian vs. Big Endian

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

Please read this.

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 representations
Text Representations
  • Text values stored in a computer can be in several formats
  • ASCII
  • EBCDIC
  • Unicode
ascii
ASCII
  • 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
text binary converters
Text <-> Binary Converters
  • 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

ebcdic
EBCDIC
  • 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
unicode
Unicode
  • 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
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