# Levent EREN A-306 Office Phone: INTRODUCTION TO DIGITAL LOGIC

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1 Levent EREN A-306 Office Phone:

2 Number Systems Representation Positive radix, positional number systems A number with radix r is represented by a string of digits: A n - 1 A n - 2 A 1 A 0. A - 1 A - 2 A - m 1 A - m in which 0 A i < r and. is the radix point. The string of digits represents the power series: (Number) r = ( i = n - 1 ) ( j = - 1 ) i j i = 0 A i r j = - m A (Integer Portion) + (Fraction Portion) j r 2

3 Number Systems Examples General Decimal Binary Radix (Base) r 10 2 Digits 0 => r => 9 0 => 1 0 r r r r Powers of 4 r 4 10, Radix 5 r 5 100, r r r r r

4 Special Powers of (1024) is Kilo, denoted "K" 2 20 (1,048,576) is Mega, denoted "M" 2 30 (1,073, 741,824) is Giga, denoted "G" 2 40 (1,099,511,627,776) is Tera, denoted T" 4

5 Positive Powers of 2 Useful for Base Conversion Exponent Value Exponent Value , , , , , , , , , ,048, ,097,152 5

6 Converting Binary to Decimal To convert to decimal, use decimal arithmetic to form S (digit respective power of 2). Example:Convert to N 10 : 6

7 Converting Decimal to Binary Method 1 Subtract the largest power of 2 that gives a positive remainder and record the power. Repeat, subtracting from the prior remainder and recording the power, until the remainder is zero. Place 1 s in the positions in the binary result corresponding to the powers recorded; in all other positions place 0 s. Example: Convert to N 2 7

8 Commonly Occurring Bases Name Radix Digits Binary 2 0,1 Octal 8 0,1,2,3,4,5,6,7 Decimal 10 0,1,2,3,4,5,6,7,8,9 Hexadecimal 16 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F The six letters (in addition to the 10 integers) in hexadecimal represent: 8

9 Numbers in Different Bases Good idea to memorize! Decimal (Base 10) Binary (Base 2) Octal (Base 8) A B C D E F Hexadecimal (Base 16) 9

10 Conversion Between Bases Method 2 To convert from one base to another: 1) Convert the Integer Part 2) Convert the Fraction Part 3) Join the two results with a radix point 10

11 Conversion Details To Convert the Integer Part: Repeatedly divide the number by the new radix and save the remainders. The digits for the new radix are the remainders in reverse order of their computation. If the new radix is > 10, then convert all remainders > 10 to digits A, B, To Convert the Fractional Part: Repeatedly multiply the fraction by the new radix and save the integer digits that result. The digits for the new radix are the integer digits in order of their computation. If the new radix is > 10, then convert all integers > 10 to digits A, B, 11

12 Example: Convert To Base 2 Convert 46 to Base 2 (101110) 2 Convert to Base 2: (0.1011) 2 Join the results together with the radix point: ( ) 2 12

13 Additional Issue - Fractional Part Note that in this conversion, the fractional part became 0 as a result of the repeated multiplications. In general, it may take many bits to get this to happen or it may never happen. Example: Convert to N = The fractional part begins repeating every 4 steps yielding repeating 1001 forever! Solution: Specify number of bits to right of radix point and round or truncate to this number. 13

14 Checking the Conversion To convert back, sum the digits times their respective powers of r. From the prior conversion of = = = = 1/2 + 1/8 + 1/16 = =

15 Why Do Repeated Division and Multiplication Work? Divide the integer portion of the power series on slide 11 by radix r. The remainder of this division is A 0, represented by the term A 0 /r. Discard the remainder and repeat, obtaining remainders A 1, Multiply the fractional portion of the power series on slide 11 by radix r. The integer part of the product is A -1. Discard the integer part and repeat, obtaining integer parts A -2, This demonstrates the algorithm for any radix r >1. 15

16 Octal to Binary and Back Octal to Binary: Restate the octal as three binary digits starting at the radix point and going both ways. Binary to Octal: Group the binary digits into three bit groups starting at the radix point and going both ways, padding with zeros as needed in the fractional part. Convert each group of three bits to an octal digit. 16

17 Hexadecimal to Binary and Back Hexadecimal to Binary: Restate the hexadecimal as four binary digits starting at the radix point and going both ways. Binary to Hexadecimal: Group the binary digits into four bit groups starting at the radix point and going both ways, padding with zeros as needed in the fractional part. Convert each group of four bits to a hexadecimal digit. 17

18 Octal to Hexadecimal via Binary Convert octal to binary. Use groups of four bits and convert as above to hexadecimal digits. Example: Octal to Binary to Hexadecimal Why do these conversions work? 18

19 A Final Conversion Note You can use arithmetic in other bases if you are careful: Example: Convert to Base 10 using binary arithmetic: Step / 1010 = 100 r 0110 Step / 1010 = 0 r 0100 Converted Digits are or (4 6) 10 19

20 Binary Numbers and Binary Coding Flexibility of representation Within constraints below, can assign any binary combination (called a code word) to any data as long as data is uniquely encoded. Information Types Numeric Must represent range of data needed Very desirable to represent data such that simple, straightforward computation for common arithmetic operations permitted Tight relation to binary numbers Non-numeric Greater flexibility since arithmetic operations not applied. Not tied to binary numbers 20

21 Non-numeric Binary Codes Given n binary digits (called bits), a binary code is a mapping from a set of represented elements to a subset of the 2 n binary numbers. Example: A binary code for the seven colors of the rainbow Code 100 is not used Color Red Orange Yellow Green Blue Indigo Violet Binary Number

22 Number of Bits Required Given M elements to be represented by a binary code, the minimum number of bits, n, needed, satisfies the following relationships: 2 n > M > 2 (n 1) n = log 2 M where x, called the ceiling function, is the integer greater than or equal to x. Example: How many bits are required to represent decimal digits with a binary code? 22

23 Number of Elements Represented Given n digits in radix r, there are r n distinct elements that can be represented. But, you can represent m elements, m < r n Examples: You can represent 4 elements in radix r = 2 with n = 2 digits: (00, 01, 10, 11). You can represent 4 elements in radix r = 2 with n = 4 digits: (0001, 0010, 0100, 1000). This second code is called a "one hot" code. 23

24 Binary Codes for Decimal Digits The usual way of expressing a decimal number in terms of a binary number is known as pure binary coding There are over 8,000 ways that you can chose 10 elements from the 16 binary numbers of 4 bits. A few are useful: Decimal 8,4,2,1 Excess3 8,4,-2,-1 Gray

25 Binary Coded Decimal (BCD) In the 8421 Binary Coded Decimal (BCD) representation each decimal digit is converted to its 4-bit pure binary equivalent This code is the simplest, most intuitive binary code for decimal digits and uses the same powers of 2 as a binary number, but only encodes the first ten values from 0 to 9. For example: (57) dec = ( ) bcd 25

26 Excess 3 Code and 8, 4, 2, 1 Code Decimal Excess 3 8, 4, 2, What interesting property is common to these two codes? 26

27 Binary to Gray Code Conversion What special property does the Gray code have in relation to adjacent decimal digits? To convert binary to a Graycoded number then follow this method : 1. The binary number and the Gray-coded number will have the same number of bits 2. The Gray code MSB (left-hand bit) and binary MSB will always be the same 3. To get the Gray code next-to- MSB (i.e. next digit to the right) add the binary MSB and the binary next-to-msb. Record the sum, ignoring any carry. 4. Continue in this manner right through to the end. Decimal 8421 Gray

28 Gray Code to Binary conversion To convert a Gray-coded number to binary then follow this method : 1. The binary number and the Gray-coded number will have the same number of bits 2. The binary MSB (left-hand bit) and Gray code MSB will always be the same 3. To get the binary next-to-msb (i.e. next digit to the right) add the binary MSB and the gray code next-to-msb. Record the sum, ignoring any carry. 4. Continue in this manner right through to the end. Decimal 8421 Gray

29 Binary to Gray Conversion Example Question: Convert the binary number to its Gray code equivalent. Answer: Binary Gray CodeDigit 1 = 1 (same as binary) Binary Gray Code Digit 2 = = 0 (carry 1) Binary Gray Code Digit 3 = = 0 (carry 1) Binary Gray Code Digit 4 = = 1 Binary Gray Code Digit 5 = = 1 Binary Gray Code Digit 6 = = 0 (carry 1) Binary Gray Code Digit 7 = = 1 Binary Gray Code Digit 8 = = bin = gray 29

30 Gray to Decimal Conversion Question: Convert the Gray coded number to its binary equivalent. Answer: Gray Code Binary Digit 1 = 1 (same as Gray code) Gray Code Binary Digit 2 = = 1 Gray Code Binary Digit 3 = = 1 Gray Code Binary Digit 4 = = 0 (carry 1) Gray Code Binary Digit 5 = = 1 Gray Code Binary Digit 6 = = 1 Gray Code Binary Digit 7 = = 0 (carry 1) Gray Code Binary Digit 8 = = gray = bin 30

31 Gray Code (Continued) Does this special Gray code property have any value? An Example: Optical Shaft Encoder B B 1 B G 0 G 1 G (a) Binary Code for Positions 0 through 7(b) Gray Code for Positions 0 through 7 31

32 Gray Code (Continued) How does the shaft encoder work? For the binary code, what codes may be produced if the shaft position lies between codes for 3 and 4 (011 and 100)? Is this a problem? 32

33 Gray Code (Continued) For the Gray code, what codes may be produced if the shaft position lies between codes for 3 and 4 (010 and 110)? Is this a problem? Does the Gray code function correctly for these borderline shaft positions for all cases encountered in octal counting? 33

34 4221 BCD Code The 4221 BCD code is another binary coded decimal code where each bit is weighted by 4, 2, 2 and 1 respectively. Unlike BCD coding there are no invalid representations. The 1's complement of a 4221 representation is important in decimal arithmetic. In forming the code remember the following rules Below decimal 5 use the rightmost bit representing 2 first Above decimal 5 use the left-most bit representing 2 first Decimal 5 = and not 4+1 Decimal 's complement

35 Warning: Conversion or Coding? Do NOT mix up conversion of a decimal number to a binary number with coding a decimal number with a BINARY CODE = (This is conversion) (This is coding) 35

36 Binary Arithmetic Single Bit Addition with Carry Multiple Bit Addition Single Bit Subtraction with Borrow Multiple Bit Subtraction Multiplication BCD Addition 36

37 Single Bit Binary Addition with Carry Given two binary digits (X,Y), a carry in (Z) we get the following sum (S) and carry (C): Carry in (Z) of 0: Z X Y C S Carry in (Z) of 1: Z X Y C S

38 Multiple Bit Binary Addition Extending this to two multiple bit examples: Carries 0 0 Augend Addend Sum Note: The 0 is the default Carry-In to the least significant bit. 38

39 Single Bit Binary Subtraction with Borrow Given two binary digits (X,Y), a borrow in (Z) we get the following difference (S) and borrow (B): Borrow in (Z) of Z0: X Y BS Borrow in (Z) of 1: Z X Y BS

40 Multiple Bit Binary Subtraction Extending this to two multiple bit examples: Borrows 0 0 Minuend Subtrahend Difference Notes: The 0 is a Borrow-In to the least significant bit. If the Subtrahend > the Minuend, interchange and append a to the result. 40

41 Binary Multiplication The binary multiplication table is simple: 0 0 = = = = 1 Extending multiplication to multiple digits: Multiplicand 1011 Multiplier x 101 Partial Products Product

42 BCD Arithmetic Given a BCD code, we use binary arithmetic to add the digits: Eight Plus is 13 (> 9) Note that the result is MORE THAN 9, so must be represented by two digits! To correct the digit, subtract 10 by adding 6 modulo Eight Plus is 13 (> 9) so add 6 carry = leaving 3 + cy Final answer (two digits) If the digit sum is > 9, add one to the next significant digit 42

43 BCD Addition Example Addition is analogous to decimal addition with normal binary addition taking place from right to left. For example, BCD for BCD for BCD for BCD for BCD for BCD for 69 43

44 BCD Addition Example-2 Add 2905 BCD to 1897 BCD showing carries and digit corrections

45 Error-Detection Codes Redundancy (e.g. extra information), in the form of extra bits, can be incorporated into binary code words to detect and correct errors. A simple form of redundancy is parity, an extra bit appended onto the code word to make the number of 1 s odd or even. Parity can detect all single-bit errors and some multiple-bit errors. A code word has even parity if the number of 1 s in the code word is even. A code word has odd parity if the number of 1 s in the code word is odd. 45

46 4-Bit Parity Code Example Fill in the even and odd parity bits: Even Parity Odd Parity Message - Parity Message - Parity The codeword "1111" has even parity and the codeword "1110" has odd parity. Both can be used to represent 3-bit data. 46

47 ASCII Character Codes American Standard Code for Information Interchange (Refer to Table 1-4 in the text) This code is a popular code used to represent information sent as character-based data. It uses 7-bits to represent: 94 Graphic printing characters. 34 Non-printing characters Some non-printing characters are used for text format (e.g. BS = Backspace, CR = carriage return) Other non-printing characters are used for record marking and flow control (e.g. STX and ETX start and end text areas). 47

48 ASCII Properties ASCII has some interesting properties: Digits 0 to 9 span Hexadecimal values to Upper case A-Z span to 5A 16. Lower case a-z span to 7A 16. Lower to upper case translation (and vice versa) occurs by flipping bit 6. Delete (DEL) is all bits set, a carryover from when punched paper tape was used to store messages. Punching all holes in a row erased a mistake! 48

49 UNICODE UNICODE extends ASCII to 65,536 universal characters codes For encoding characters in world languages Available in many modern applications 2 byte (16-bit) code words See Reading Supplement Unicode on the Companion Website 49

50 Data types Our first requirement is to find a way to represent information (data) in a form that is mutually comprehensible by human and machine. Ultimately, we will have to develop schemes for representing all conceivable types of information - language, images, actions, etc. We will start by examining different ways of representing integers, and look for a form that suits the computer. Specifically, the devices that make up a computer are switches that can be on or off, i.e. at high or low voltage. Thus they naturally provide us with two symbols to work with: we can call them on & off, or (more usefully) 0 and 1. 50

51 Decimal Numbers decimal means that we have ten digits to use in our representation (the symbols 0 through 9) What is 3,546? it is three thousands plus five hundreds plus four tens plus six ones. i.e. 3,546 = How about negative numbers? we use two more symbols to distinguish positive and negative: + and - 51

52 Unsigned Binary Integers Y = abc = a b c.2 0 (where the digits a, b, c can each take on the values of 0 or 1 only) Problem: N = number of bits Range is: 0 i < 2 N - 1 How do we represent negative numbers? 3-bits 5-bits 8-bits

53 Signed Magnitude Leading bit is the sign bit Y = abc = (-1) a (b c.2 0 ) Range is: -2 N < i < 2 N-1-1 Problems: How do we do addition/subtraction? We have two numbers for zero (+/-)!

54 One s Complement Invert all bits If msb (most significant bit) is 1 then the number is negative (same as signed magnitude) Range is: -2 N < i < 2 N-1-1 Problems: Same as for signed magnitude

55 Two s Complement Transformation To transform a into -a, invert all bits in a and add 1 to the result Advantages: Range is: -2 N-1 < i < 2 N-1-1 Operations need not check the sign Only one representation for zero Efficient use of all the bits

56 Manipulating Binary numbers - 1 Binary to Decimal conversion & vice-versa A 4 bit binary number A = a 3 a 2 a 1 a 0 corresponds to: a a a a = a a a a 0.1 (where a i = 0 or 1 only) A decimal number can be broken down by iterative division by 2, assigning bits to the columns that result in an odd number: e.g. (13) 10 => ((((13-1)/2-0)/2-1)/2-1) = 0 => (01101) 2 In the 2 s complement representation, leading zeros do not affect the value of a positive binary number, and leading ones do not affect the value of a negative number. So: = = 13 and = = -5 56

57 Manipulating Binary numbers - 2 Binary addition simply consists of applying, to each column in the sum, the rules: = = = = 10 With 2 s complement representation, this works for both positive and negative integers so long as both numbers being added are represented with the same number of bits. e.g. to add the number 13 => (8 bits) to -5 => 1011 (4 bits): we have to sign-extend (SEXT) the representation of -5 to 8 bits: => 8 (as expected!) 57

58 Manipulating Binary numbers - 3 Overflow If we add the two (2 s complement) 4 bit numbers representing 7 and 5 we get : 0111 => => => -4 (in 4 bit 2 s comp.) We get -4, not +12 as we would expect!! We have overflowed the range of 4 bit 2 s comp. (-8 to +7), so the result is invalid. Note that if we add 16 to this result we get back 16-4 = 12 this is like stepping up to 5 bit 2 s complement representation In general, if the sum of two positive numbers produces a negative result, or vice versa, an overflow has occurred, and the result is invalid in that representation. 58

59 Limitations of integer representations Most numbers are not integer! Even with integers, there are two other considerations: Range: The magnitude of the numbers we can represent is determined by how many bits we use: e.g. with 32 bits the largest number we can represent is about +/- 2 billion, far too small for many purposes. Precision: The exactness with which we can specify a number: e.g. a 32 bit number gives us 31 bits of precision, or roughly 9 figure precision in decimal repesentation. We need another data type! 59

60 Real numbers Our decimal system handles non-integer real numbers by adding yet another symbol - the decimal point (.) to make a fixed point notation: e.g. 3, = The floating point, or scientific, notation allows us to represent very large and very small numbers (integer or real), with as much or as little precision as needed: Unit of electric charge e = x Coul. Volume of universe = 1 x cm 3 the two components of these numbers are called the mantissa and the exponent 60

61 Real numbers in binary We mimic the decimal floating point notation to create a hybrid binary floating point number: We first use a binary point to separate whole numbers from fractional numbers to make a fixed point notation: e.g = => (2-1 = 0.5 and 2-2 = 0.25, etc.) We then float the binary point: => x 2 4 mantissa = , exponent = 4 Now we have to express this without the extra symbols ( x, 2,. ) by convention, we divide the available bits into three fields: sign, mantissa, exponent 61

62 32 bits: IEEE-754 fp numbers - 1 s biased exp. fraction 1 8 bits 23 bits (biased exp. 127) N = (-1) s x 1.fraction x 2 Sign: 1 bit Mantissa: 23 bits We normalize the mantissa by dropping the leading 1 and recording only its fractional part (why?) Exponent: 8 bits In order to handle both +ve and -ve exponents, we add 127 to the actual exponent to create a biased exponent : => biased exponent = (= 0) 2 0 => biased exponent = (= 127) => biased exponent = (= 254) 62

63 IEEE-754 fp numbers - 2 Example: => => x 2 4 sign bit = 0 (+ve) normalized mantissa (fraction) = biased exponent = = 131 => so => => x41ce0000 Values represented by convention: Infinity (+ and -): exponent = 255 ( ) and fraction = 0 NaN (not a number): exponent = 255 and fraction 0 Zero (0): exponent = 0 and fraction = 0 note: exponent = 0 => fraction is de-normalized, i.e no hidden 1 63

64 IEEE-754 fp numbers - 3 Double precision (64 bit) floating point 64 bits: 1 11 bits 52 bits s biased exp. fraction N = (-1) s x 1.fraction x 2 (biased exp. 1023) Range & Precision: 32 bit: mantissa of 23 bits + 1 => approx. 7 digits decimal 2 +/-127 => approx. 10 +/ bit: mantissa of 52 bits + 1 => approx. 15 digits decimal 2 +/-1023 => approx. 10 +/

65 Other Data Types Other numeric data types e.g. BCD Bit vectors & masks sometimes we want to deal with the individual bits themselves Text representations ASCII: uses 8 bits to represent main Western alphabetic characters & symbols, plus several control codes, Unicode: 16 bit superset of ASCII providing representation of many different alphabets and specialized symbol sets. EBCDIC: IBM s mainframe representation. 65

66 Hexadecimal Representation Base 16 (hexadecimal) More a convenience for us humans than a true data type 0 to 9 represented as such 10, 11, 12, 13, 14, 15 represented by A, B, C, D, E, F 16 = 2 4 : i.e. every hexadecimal digit can be represented by a 4-bit binary (unsigned) and vice-versa. Example (16AB) 16 x16ab (5803) 10 #5803 b

67 Another use for bits: Logic Beyond numbers logical variables can be true or false, on or off, etc., and so are readily represented by the binary system. A logical variable A can take the values false = 0 or true = 1 only. The manipulation of logical variables is known as Boolean Algebra, and has its own set of operations - which are not to be confused with the arithmetical operations of the previous section. Some basic operations: NOT, AND, OR, XOR 67

68 Basic Logic Operations Truth Tables of Basic Operations NOT AND OR A A' A B A.B A B A+B Equivalent Notations not A = A' = A A and B = A.B = A B = A intersection B A or B = A+B = A B = A union B 68

69 More Logic Operations XOR A B A B XNOR A B (A B)' Exclusive OR (XOR): either A or B is 1, not both A B = A.B' + A'.B 69

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