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# Number system and codes

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A digital system can understand positional number system only where there are only a few symbols called digits and these symbols represent different values depending on the position they occupy in the …

A digital system can understand positional number system only where there are only a few symbols called digits and these symbols represent different values depending on the position they occupy in the number.

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• 1. The basis for conveying & quantifying information For exact description of any parameter, use quantities e.g. 250 C or 300 C
• 2. • An ordered set of symbols – which utilizes positional notations to represent VALUE. • The radix (or base) – is the total number of SYMBOLS or characters • Example, decimal number system: – Radix, r = 10 – Digits allowed = 0,1, 2, 3, 4, 5, 6, 7, 8, 9
• 3. RadixRadix (base)(base) Characters in setCharacters in set ExampleExample 22 (binary)(binary) 0,10,1 (11111111)(11111111)22 33 0,1,20,1,2 44 01,2,301,2,3 55 0,1,2,3,40,1,2,3,4 .... .... 88 (octal)(octal) 0,1,2,3,4,5,6,70,1,2,3,4,5,6,7 (377)(377)88 .... .... 1010 (decimal)(decimal) 0,1,2,3,4,5,6,7,8,90,1,2,3,4,5,6,7,8,9 (255)(255)1010 .... .... 1616 (hexadecimal)(hexadecimal) 0,1,2,3,4,5,6,7,8,9,A,B,C,0,1,2,3,4,5,6,7,8,9,A,B,C, D,E,FD,E,F (FF)(FF)1616
• 4. Distinguish-ability of symbols Arithmetic operation capability Number of different symbols is restricted by the base or radix. A special symbol is used to describe nothing (‘0’- Zero). Every symbol has positional value in an ordered list. The position of the symbol within the number indicates multiplication by a relevant power of the base.
• 5.  The general representation of an unsigned number is with integer and fraction portions number in a number system with radix r: Decimal point Integer part Fractional part d3 d2 d1 d0. d-1 d-2 d-3  Digit Position 3 2 1 0 -1 -2 -3  Weight 103 102 101 100 10-1 10-2 10-3 (Exponent notation)  Weight 1000 100 10 1 1/10 1/100 1/1000 (Value) e.g. 273.5 = 2x103 + 7x102 + 3x101 + 5 10-1 = 200+70+3+0.5
• 6. Well known? Easy to use?? Every body is using??? ???? Hint – Human Anatomy
• 7. A number representation dp−1dp−2 ⋅⋅⋅ d1d0 . d−1d−2 ⋅⋅⋅ d−n The value of the number is given by: i p ni i rdD ⋅= ∑ − −= 1
• 8. The general form of a binary number of p+n binary digits (bits) is: bp−1bp−2 ⋅⋅⋅ b1b0 . b−1b−2 ⋅⋅⋅ b−n and its value is: i p ni ibB 2 1 ⋅= ∑ − −=
• 9. Base- 2 Symbols – 0 & 1 Binary digIT = BIT Binary system is popular for computers low voltage level (0-state) and high voltage level (1-state). On the other hand, it is impossible to represent any other number system physically in terms of many voltage levels and design digital system to identify these distinct levels.
• 10. Positive Power of 2Positive Power of 2 Decimal numberDecimal number Negative power of 2Negative power of 2 Decimal valueDecimal value 00 11 -- -- 11 22 -1-1 0.50.5 22 44 -2-2 0.250.25 33 88 -3-3 0.1250.125 44 1616 -4-4 0.06250.0625 55 3232 -5-5 0.031250.03125 66 6464 -6-6 0.0156250.015625 77 128128 -7-7 0.00781250.0078125 88 256256 -8-8 0.003906250.00390625 99 512512 -9-9 0.0019531250.001953125 1010 1024 ~ 1 K (Kilo)1024 ~ 1 K (Kilo) -10-10 0.0009765625 ~ 1 mili0.0009765625 ~ 1 mili 2020 222020 ~ 1 Mega~ 1 Mega -20-20 22-20-20 ~ 1 micro~ 1 micro 3030 223030 ~ 1 Giga~ 1 Giga -30-30 22-30-30 ~ 1 nano~ 1 nano 4040 224040 ~ 1 Tera~ 1 Tera -40-40 22-40-40 ~ 1 pico~ 1 pico
• 11. i p ni ibB 2 1 ⋅= ∑ − −=  Method: summation Example: 101110110012 = 1 ⋅ 210 + 0 ⋅ 29 + 1 ⋅ 28 + 1 ⋅ 27 + 1 ⋅ 26 + 0 ⋅ 25 + 1 ⋅ 24 + 1 ⋅ 23 + 0 ⋅ 22 + 0 ⋅ 21 + 1 ⋅ 20 = 149710
• 12. Method: successive divisions Example:
• 13. EXAMPLE: convert 5310 to binary EXAMPLE: convert .625ten to binary EXAMPLE: convert 2314 to base 7.
• 14. Conversion between Number Systems • Radix-r to decimal: § Multiply digits with their corresponding weights and add • Decimal to binary (radix 2) § Whole numbers: repeated division by 2 § Fractions: repeated multiplication by 2 • Decimal to radix-r § Whole numbers: repeated division by r § Fractions: repeated multiplication by r • Binary to Octal § Substitute groups of three bits with corresponding octal digit. • Binary to Hexadecimal § Substitute groups of four bits with corresponding hexadecimal digit.
• 15. A code is a symbol or group of symbols that stand for something Although the binary system of numbers is most appropriate for use in computers but has several disadvantages  Binary machine code is long  Difficult to assimilate  Tedious to convert to decimal Simpler way to represent binary numbers for conversion to decimal representation  Binary coded decimal (BCD) – 4 bit code  Binary coded octal (BCO) – 3 bit code  Binary coded hexadecimal (BCH) – 4 bit code
• 16. • Binary codes for simplification in representation - BCD, BCO, BCH • Binary codes for driving display – BCD • Binary codes for arithmetic operations –BCD, 9’s & 10’s complement (Self-complementing codes) • Binary codes for creation of digital transducer – Gray code (Unit-distance code or reflective code) • Binary codes for transmission & reception – ASCII • Binary codes for representing signed numbers – Sign- magnitude, 1’s & 2’s complement
• 17.  The octal number system uses radix 8, while the hexadecimal number system uses radix 16  The octal and hex number systems are useful for representing multibit numbers
• 18. Binary Arithmetic Operations Addition • Similar to decimal number addition, two binary numbers are added by adding each pair of bits together with carry propagation. • Addition Example: 1 0 1 1 1 1 0 0 0 Carry X 190 1 0 1 1 1 1 1 0 Y 141 1 0 0 0 1 1 0 1 X + Y 331 1 0 1 0 0 1 0 1 1 + +
• 19. Negative Binary Number Representations • Signed-Magnitude Representation: – For an n-bit binary number: Use the first bit (most significant bit, MSB) position to represent the sign where 0 is positive and 1 is negative. Ex. 1 1 1 1 1 1 1 12 = - 12710 – Remaining n-1 bits represent the magnitude which may range from: -2(n-1) + 1 to 2(n-1) - 1 – This scheme has two representations for 0; i.e., both positive and negative 0: for 8 bits: 00000000, 10000000 – Arithmetic under this scheme uses the sign bit to indicate the nature of the operation and the sign of the result, but the sign bit is not
• 20. Negative Binary Number Representations • One’s-Complement representation • MSB is the sign (MSB = 1 indicates a negative number) • Negative numbers are found by complementing all bits • ex. 11910 = 01110111 -11910 = 10001000 • The range of values for an n-bit binary number in 1’s complement representation is -2(n-1) +1 to 2(n-1) - 1 • One’s-complement addition/subtraction: If there is a carry out of the sign position add 1 Ex. -2 1101 + -5 1010 -7 10111 + 1 1000
• 21. Negative Binary Number Representations • Two’s complement representation: • MSB is the sign (MSB = 1 indicates a negative number) •The range for an n-bit binary number in 2’s complement representation is from -2(n-1) to 2(n-1) - 1 • To negate a number complement all bits and add 1 • ex. 11910 = 01110111 complement bits 10001000 + 1 add 1 100010012 = - 11910 Two’ complement addition/subtraction Examples: 4 0100 -2 1110 + -7 1001 + -6 1010 -3 1101 -8 1 1000 Ignore carry out from MSB
• 22. Binary Arithmetic Operations Subtraction • Two binary numbers are subtracted by subtracting each pair of bits together with borrowing, where needed. • Subtraction Example: 0 0 1 1 1 1 1 0 0 Borrow X 229 1 1 1 0 0 1 0 1 Y 46 0 0 1 0 1 1 1 0 183 1 0 1 1 0 1 1 1 - -
• 23. Binary Multiplication • Multiplication is achieved by adding a list of shifted multiplicands according to the digits of the multiplier. • Ex. (unsigned) 11 1 0 1 1 multiplicand (4 bits) X 13 X 1 1 0 1 multiplier (4 bits) -------- ------------------- 33 1 0 1 1 11 0 0 0 0 1 0 1 1 143 1 0 1 1 --------------------- 1 0 0 0 1 1 1 1 Product (8 bits)
• 24. Binary Division • Shift and subtract Example: 101 15/3 =5 0011 } 1111 11 001 000 0011 0011 0000
• 25. Sign- Magnitude method 1’s complement 2’s complement
• 26.  Representation of positive numbers same in most systems  Major differences are in how negative numbers are represented  Three major schemes:  sign and magnitude  ones complement  twos complement  Assumptions:  we'll assume a 4 bit machine word  16 different values can be represented  roughly half are positive, half are negative
• 27. Sign and Magnitude Representation 0000 0111 0011 1011 1111 1110 1101 1100 1010 1001 1000 0110 0101 0100 0010 0001 +0 +1 +2 +3 +4 +5 +6 +7-0 -1 -2 -3 -4 -5 -6 -7 0 100 = + 4 1 100 = - 4 + - High order bit is sign: 0 = positive (or zero), 1 = negative Three low order bits is the magnitude: 0 (000) thru 7 (111) Number range for n bits = +/-2 -1 Representations for 0 n-1
• 28. Cumbersome addition/subtraction Must compare magnitudes to determine sign of result Sign and Magnitude Ones Complement N is positive number, then N is its negative 1's complement N = (2 - 1) - N n Example: 1's complement of 7 2 = 10000 -1 = 00001 1111 -7 = 0111 1000 = -7 in 1's comp.Shortcut method: simply compute bit wise complement 0111 -> 1000 4
• 29. Subtraction implemented by addition & 1's complement Still two representations of 0! This causes some problems Some complexities in addition Ones Complement 0000 0111 0011 1011 1111 1110 1101 1100 1010 1001 1000 0110 0101 0100 0010 0001 +0 +1 +2 +3 +4 +5 +6 +7-7 -6 -5 -4 -3 -2 -1 -0 0 100 = + 4 1 011 = - 4 + -
• 30. Only one representation for 0 One more negative number than positive number Twos Complement 0000 0111 0011 1011 1111 1110 1101 1100 1010 1001 1000 0110 0101 0100 0010 0001 +0 +1 +2 +3 +4 +5 +6 +7-8 -7 -6 -5 -4 -3 -2 -1 0 100 = + 4 1 100 = - 4 + - like 1's comp except shifted one position clockwise
• 31. Twos Complement Numbers N* = 2 - N n Example: Twos complement of 7 2 = 10000 7 = 0111 1001 = repr. of -7 Example: Twos complement of -7 4 2 = 10000 -7 = 1001 0111 = repr. of 7 4 sub sub Shortcut method: Twos complement = bitwise complement + 1 0111 -> 1000 + 1 -> 1001 (representation of -7) 1001 -> 0110 + 1 -> 0111 (representation of 7)
• 32. Addition and Subtraction of Numbers Sign and Magnitude 4 + 3 7 0100 0011 0111 -4 + (-3) -7 1100 1011 1111 result sign bit is the same as the operands' sign 4 - 3 1 0100 1011 0001 -4 + 3 -1 1100 0011 1001 when signs differ, operation is subtract, sign of result depends on sign of number with the larger magnitude
• 33. Addition and Subtraction of Numbers Ones Complement Calculations 4 + 3 7 0100 0011 0111 -4 + (-3) -7 1011 1100 10111 1 1000 4 - 3 1 0100 1100 10000 1 0001 -4 + 3 -1 1011 0011 1110 End around carry End around carry
• 34. Addition and Subtraction of Binary Numbers Ones Complement Calculations Why does end-around carry work? Its equivalent to subtracting 2 and adding 1 n M - N = M + N = M + (2 - 1 - N) = (M - N) + 2 - 1 n n (M > N) -M + (-N) = M + N = (2 - M - 1) + (2 - N - 1) = 2 + [2 - 1 - (M + N)] - 1 n n n n M + N < 2 n-1 after end around carry: = 2 - 1 - (M + N) n this is the correct form for representing -(M + N) in 1's comp!
• 35. Addition and Subtraction of Binary Numbers Twos Complement Calculations 4 + 3 7 0100 0011 0111 -4 + (-3) -7 1100 1101 11001 4 - 3 1 0100 1101 10001 -4 + 3 -1 1100 0011 1111 If carry-in to sign = carry-out then ignore carry if carry-in differs from carry-out then overflow Simpler addition scheme makes twos complement the most common choice for integer number systems within digital systems
• 36. Addition and Subtraction of Binary Numbers Twos Complement Calculations Why can the carry-out be ignored? -M + N when N > M: M* + N = (2 - M) + N = 2 + (N - M) n n Ignoring carry-out is just like subtracting 2 n -M + -N where N + M < or = 2 n-1 -M + (-N) = M* + N* = (2 - M) + (2 - N) = 2 - (M + N) + 2 n n After ignoring the carry, this is just the right twos compl. representation for -(M + N)! n n
• 37. Overflow Conditions Add two positive numbers to get a negative number or two negative numbers to get a positive number 5 + 3 = -9 -7 - 2 = +7 0000 0001 0010 0011 1000 0101 0110 0100 1001 1010 1011 1100 1101 0111 1110 1111 +0 +1 +2 +3 +4 +5 +6 +7-8 -7 -6 -5 -4 -3 -2 -1 0000 0001 0010 0011 1000 0101 0110 0100 1001 1010 1011 1100 1101 0111 1110 1111 +0 +1 +2 +3 +4 +5 +6 +7-8 -7 -6 -5 -4 -3 -2 -1
• 38. Overflow Conditions 5 3 -8 0 1 1 1 0 1 0 1 0 0 1 1 1 0 0 0 -7 -2 7 1 0 0 0 1 0 0 1 1 1 0 0 1 0 1 1 1 5 2 7 0 0 0 0 0 1 0 1 0 0 1 0 0 1 1 1 -3 -5 -8 1 1 1 1 1 1 0 1 1 0 1 1 1 1 0 0 0 Overflow Overflow No overflow No overflow Overflow when carry in to sign does not equal carry out
• 39. Thank You.