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INFORMATION IN DIGITAL DEVICES

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Digital Devices

• Most computers today are composed of digital devices.– Process electrical signals.– Can only have two states.– States are:

• A high level of voltage: 5V DC• A low level of voltage: 0V DC

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Digital Devices• PCs are digital electronic systems.• PCs process data in digital format.

• What a Digital Computer Understands:– Computer is a digital electronic device, so it

recognizes high voltage levels (1, or 5V DC), and low voltage levels (0, or 0V DC).

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BINARY LANGUAGE AND LOGIC

• Logic values used in computer corresponds to binary, Base-2 number system.– Uses two values, 0 and 1.

• Binary digit is referred to as a bit.• Representing information using bits:

– Very little can be conveyed by a single bit. True or false can be conveyed.

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5V

Information Light Voltageon Wire

Symbol,One bit

Not in class OFF 5 V 0

In class ON 0 V 1

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• To represent more than one person’s presence or absence, need more bits: i.e., 100 bits for 100 people.

• Or a group of bits used together could represent 100 people (27 = 128), plus one bit to represent state (present or absent).

Joe’s number is 4 or 000 0100. If Joe is present, the representative number is 1000 0100. If Joe is absent the number is 0000 0100.

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• Two bits can represent 22 = 4 different combinations.

• Any number, letter, or symbol can be described by organizing a group of related bits into coded bit patterns called words.

• A bit is a basic unit of information.• A nibble is a group of 4 bits and can

represent 16 things.• A byte is a group of 8 bits and can

represent 256 things.

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• Three types of computer words exist:– Binary data words - numeric– Coded data words - alphanumeric (text and

numbers)– Instruction words - programs

• Data: Raw, unprocessed facts.– Such as pay rate and hours worked.

• After the data is processed it becomes information.– Such as total wages.

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• To process data into information it must be transformed from the human understandable form to machine understandable form (digital).– Known as character encoding. Characters are

turned into a series of 1s and 0s.– Can be encoded according to a variety of

standards:• ASCII (American Standard Code for Information

Interchange) - uses 7 bits, used in PCs.• EBCDIC (Extended Binary Coded Decimal

Interchange Code) uses 8 bits, IBM mainframes.• Unicode and ISO10646 uses 16 bits, Windows NT

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• Representing Numbers– Digital electronics represent any number with

voltage (1) or absence of voltage (0) in an array of cables.

– This two state system known as binary.– Decimal system known as decimal because 10

symbols are used 0 - 9. To represent more than 9 more digits are used.

505 =5x100 = 5x1 = 50x101 = 0x10 = 05x102 = 5x100 = 500

Base-10 System

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• In binary, to represent more than 1, two or more digits are used.101 binary=

1x20 = 1x1 = 10x21 = 0x2 = 01x22 = 1x4 = 1

Total = 5

Base-2 System

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• Word Size:– Used to describe computers because all internal

devices are constructed to accommodate a given word length.

– The larger the word size, the more powerful the computer because:

• more information can be moved at once• there is a larger range of numbers and symbols

recognized.– Different word sizes, most common is 8 bit

(byte).• 16 & 32 bit words are referred to in bytes, ie 2-byte,

4-byte.

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• Common to divide bytes into two 4-bit nibbles. Makes it easier to convert binary to hexidecimal.

This would represent 9D in hexidecimal.

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DECIMAL (10) BINARY (2) HEXIDECIMAL (16)0 0000 01 0001 12 0010 23 0011 34 0100 45 0101 56 0110 67 0111 78 1000 89 1001 910 1010 A11 1011 B12 1100 C13 1101 D14 1110 E15 1111 F16 10000 1017 10001 11

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• Converting to decimal:

Computer words are rarely converted to decimal unless done so by hardware.

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• Numeric Data Words:– Contain only numbers and represent a quantity

in binary, hexidecimal, or binary coded decimal (BCD), or octal (base-8).

Can easily convert BCD to decimal.

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• Alphanumeric Words:– Computer must be capable of handling letters

and special characters.– Done with coded binary patterns.– In ASCII (American Standard Code for

Information Inter-change) characters of the alphabet, decimal numbers, special symbols, punctuation, are assigned a 7-bit pattern.

– Some codes are control codes and are used to modify functions associated with other characters. Used when working with peripheral devices.

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• ASCII code is 7 bits, but computer world is 8 bits. Some applications pad 8th bit (MSB) with 0, some applications pad 8th bit with 1 to produce extended ASCII code of 256 characters.

• More often the MSB is used for error checking. Error checking is done when computer sends alphanumeric data to peripherals or other computer.– Parity bit assumes certain value dependent

upon the number of 1’s in character.

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• Odd Parity– The parity bit assumes a value of ‘1’ or ‘0’ so that

the total number of ‘1’ bits in the character is odd.• Even Parity

– The parity bit assumes a value of ‘1’ or ‘0’ so that the total number of ‘1’ bits in the character is even.

• When the peripheral receives the byte it regenerates the parity bit and compares it with the parity bit sent. If they are the same then no error occurred. This only works for single bit errors.

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• Instruction Words:– Tells the computer what operation to perform

and where to find the data on which to perform.– Instruction set (operations) varies from

computer to computer. Format of words varies also. But convey same types of information.

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• First example - contains only operation code (op code) such as HALT.

• Second example - contains op code and the operand (data to be worked on) address.

• Third example - op code followed by two pieces of data such as the address of what to work on and where to store the result.

• These instruction formats as in machine language, 1s and 0s.

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• Moving Words Around - most frequent operation of a computer.

• Information is moved in the form of words.• Two modes for moving words:

– Parallel: entire word is moved at once.– Serial: word is moved a bit at a time.

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