1. Machine level representation of data Character representation
Bits can represent anything!!
Characters?
26 letters  5 bits (25 = 32)
upper/lower case + punctuation  7 bits (in 8) (“ASCII”)
standard code to cover all the world’s languages  8,16,32 bits (“Unicode”)
Logical values?
0  False, 1  True
colors ? Ex:
locations / addresses? commands?
MEMORIZE: N bits  at most 2N things
Red (100)
Green (010)
Blue (001)

2. Characters representation in computers and devices
ASCII  ANSI  MultiByte  Unicode
American Standard Code for Information Interchange
This was the de facto world-wide standard for the code numbers used by computers to represent all the upper and lower-case Latin letters, numbers, punctuation, etc.
How many bits we need for 5 letters representation?
How many bits we need to represent ?
4 letters - A,B,C,D – 2 bits - 22 = 4 patterns
How many bits for
26 English uppercase letters ? - 5 bits – 25 = 32 patterns
26 English lowercase letters bits – 25 = 32 patterns
Decimal digits and special signs - 5 bits – 25 = 32 patterns
Special Control Characters - 5 bits – 25 = 32 patterns
How many bits we need for 128 patterns representation?

3. The Content of ASCII table
Contains 128 characters
ASCII needs only 7 bits for character representation (“A” )
The first printable character is SP (space) and corresponds to the bit pattern – 0x20.
The characters A and B correspond to
A – 0x41 B – 0x42
Find “z” ’s ASCII code
z – – 0x7A
1's place
16's place 1 2 3 4 5 6 7 8 9 A B C D E F
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL BS HT LF VT FF CR SO SI
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN EM
SUB
ESC FS GS RS US SP !
" # $ %
& ' ( ) * + , - . / : ;
<  =
>  ? @ G H I J K L M N O P Q R S T U V W X Y Z [ \ ] ^ _ ` a b c d e f g h i j k l m n o p q r s t u v w x y z { | } ~
DEL

4. Control characters 1 2 3 4 5 6 7 8 9 A B C D E F
Control characters are not shown or printed on the different devices. They control the devices.
These control characters have different meaning for different devices.
0x0A – Line Feed, x0D – Carriage Return
1's place
16's place 1 2 3 4 5 6 7 8 9 A B C D E F
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL BS HT LF VT FF CR SO SI
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN EM
SUB
ESC FS GS RS US SP !
" # $ %
& ' ( ) * + , - . / : ;
<  =
>  ? @ G H I J K L M N O P Q R S T U V W X Y Z [ \ ] ^ _ ` a b c d e f g h i j k l m n o p q r s t u v w x y z { | } ~
DEL
ASCII full chart

5. ASCII code Advantages, Disadvantages
Why this is an advantage?
How?
How?
1's place
16's place 1 2 3 4 5 6 7 8 9 A B C D E F
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL BS HT LF VT FF CR SO SI
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN EM
SUB
ESC FS GS RS US SP !
" # $ %
& ' ( ) * + , - . / : ;
<  =
>  ? @ G H I J K L M N O P Q R S T U V W X Y Z [ \ ] ^ _ ` a b c d e f g h i j k l m n o p q r s t u v w x y z { | } ~
DEL
The history of ASCII since 1967 is mostly a history of attempts to overcome its limitations and make it more applicable to languages other than American English.

6. ASCII code Extension. Character sets.
8 bits
ANSI chars
IBM chars
Other- 256 chars
ASCII 128 chars
Extra 128 chars
ASCII 128 chars
Extra 128 chars
ASCII 128 chars
Arme nian
7 bits
7 bits

7. 1 byte – 256 character Code pages
ANSI and similar code pages need 8 bits for character representation
(“A” )
(“ ” – 0xB ) Ա
(“ ” – 0xE ) Б
ANSI chars
IBM chars
Armenian code page #1
Armenian code page #2
Cyrillic code page #1
ASCII
Extra 128 chars
ASCII
Extra 128 chars
ASCII
Arme nian 1
ASCII
Arme nian 2
ASCII
Cyril lic 1

8. Chinese code page (lead byte) Chinese code page (trail byte)
Double (or multi) byte character sets
Chinese code page (lead byte)
Chinese code page (trail byte)
ASCII 128 chars
A DBCS starts off with 256 codes
Like any well-behaved code page, the first 128 of these codes are ASCII.
However, some of the codes in the higher 128 are always followed by a second byte. The two bytes together (called a lead byte and a trail byte) define a single character, usually a complex ideograph.

9. Double (or multi) byte character sets
Advantage: DBCS allows to create pages also for languages having more than 256 letters or signs.
Disadvantage:
Different documents created by different code pages even for the same language (Cyrillic or Armenian) are still be not compatible.
The problem with a double-byte character set is not that characters are represented by 2 bytes. The problem is that some characters (in particular, the ASCII characters) are represented by 1 byte. This creates odd programming problems. For example, the number of characters in a character string cannot be determined by the byte size of the string.

10. Unicode code point – Symbol’s identifier
The best thing about Unicode is that there's only one character set. There's simply no ambiguity.
The representation in bits is enough large to accommodate all the languages and signs.
Flavors – UTF-8, UTF-16, UTF-32.
code point – Symbol’s identifier

11. Unicode UTF-8 (like multibyte)
Code point – Symbol’s identifier
Leading Byte of the multi-byte sequence
Continuation Byte of the multi-byte sequence
Bits
Last code point
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
  7
U+007F
0xxxxxxx 11
U+07FF
110xxxxx
10xxxxxx 16
U+FFFF
1110xxxx 21
U+1FFFFF
11110xxx 26
U+3FFFFFF
111110xx 31
U+7FFFFFFF x
Single Byte
Header bits
Header bits – The # of “1”s = # of bytes
Header bit always = 0
Armenian character set uses the codes 0x0530 through 0x058F

12. Unicode UTF-16
The Unicode code space is divided into seventeen PLANES of 216 (65,536) code points each
The code points in each plane have the hexadecimal values xx0000 to xxFFFF
where xx is a hex value from 00 to 10
1st plane code points U+0000 to U+D7FF and U+E000 to U+FFFF - Basic Multilingual Plane – most frequently used characters.
Code points U+D800 to U+DFFF - Extensions

13. Unicode
Advantage:
Supports all languages and different signs by single code page.
UTF-8 is back compatible with the ASCII
UTF-16 Basic multilingual plane (first 16 bits- 2 bytes) and UTF-32 (4 bytes) allow to work with the characters like with the regular text.

14. Unicode Disadvantage:
Fixed 2 or 4 bytes UTF-16 and UTF-32 Unicode character strings occupy twice or four times as much memory as ASCII strings.
UTF-8 character set has variable length.
This is an advantage to have less size than the fixed Unicode character set.
And this is a disadvantage with the programming.
For example, the number of characters in a character string cannot be determined by the byte size of the string.