CHAPTER 2 NUMBER SYSTEMS The Decimal Number System : We begin our study of the number systems with the familiar decimal number system. The decimal system contains ten unique symbol 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. Since counting in decimal involves ten symbols, we say that its base or radix is ten. There is no symbol for its base, i.e. for ten. It is a positional weighted system. It means that the value attached to a symbol depends on its location with respect to the decimal point. In this system, any number (integer, fraction or mixed) of any magnitude can be represented by the use of these ten symbols only. Each symbol in the number is called a digit. The left most digit in any number representation, which has the greatest positional weight out of all the digits present in the number, is called the most significant digit (MSD) and the right most digit which has the least positional weight out of all the digits present in that number, is called the least significant digit (LSD). The digits on the left side of the decimal point from the integer part of a decimal number and those on the right side from the fractional part. The digits to the right of the decimal point have weights which are negative powers of 10 and the digits to the left of the decimal point have weights which are positive powers of 10. The value of a decimal number is the sum of the products of the digits of that number with their respective column weights. The weight of each column is 10 times greater than the weight of the column to its right. The first digit to the left of the decimal point has a weight of unity or 10 0, the second digit to the left of the decimal point has a weight of 10 or 10 1, the third has a weight of 100 or 10 2, and so on. The first digit to the right of the decimal point has a weight of 1/1 or 10 1, the second digit to the right has a weight of 1/100 or 10-2, the third has a weight of 1/1000 or 10-3, and so on.
In general, the value of any mixed decimal number d n d n-1 d n-2... d 1 d 0. d -1 d -2 d -3... d -k is given by (d n x 10 n ) + (d n 1 x 10 n 1 ) +... + (d 1 x 10 1 ) + (d 0 x 10 0 ) + (d -1 x 10-1 ) + (d -2 x 10-2 ) +... Consider the decimal number 9256.26 using digit 2, 5, 6, 9. This is mixed number. Hence, 9256.26 = 9 x 1000 + 2 x 100 + 5 x 10 + 6 x 1 + 2 x (1/10) + 6 x (1/100) = 9 x 10 3 + 2 x 10 2 + 5 x 10 1 + 6 x 10 0 + 2 x 10-1 + 6 x 10-2 Consider another number 6592.69, using the same digits 2, 5, 6, 9. Here, 6592.69 = 6 x 10 3 + 5 x 10 2 + 9 x 10 1 + 2 x 10 0 + 6 x 10-1 + 9 x 10-2 Note the difference in position values of the same digits, when placed in different positions. Nine s and Ten s Complements The 9 s complement of a decimal number is obtained by subtracting each digit of that decimal number from 9. The 10 s complement of a decimal number is obtained by adding a 1 to its 9 s complement. To perform decimal subtraction using the 9 s complement method, obtain the 9 s complement of the subtrahend and add it to the minuend. Call this number the intermediate result. If there is a carry, it indicates that the answer is positive. Add the carry to the LSD of this result to get the answer. The carry is called the end around carry. If there is no carry, it indicates that the answer is negative and the intermediate result is its 9 s complement. Take the 9 s complement of this result and place a negative sign in front to get the answer. To perform decimal subtraction using the 10 s complement method, obtain the 10 s complement of the subtrahend and add it to the minuend. Call this number the intermediate result. If there is a carry, it indicates that the answer is positive and ignore that carry.the result obtained is itself a result. If there is no carry, it indicates that the answer is negative and the intermediate
result is its 10 s complement. Take the 10 s complement of this result and place a negative sign in front to get the answer. Example : Find the 9 s complement of the following decimal numbers. (a) 3465 (b) 782.54 (c) 4526.075 (a) 9 9 9 9-3 4 6 5 6 5 3 4 (9 s complement of 3465) (b) 9 9 9. 9 9-7 8 2. 5 4 2 1 7. 4 5 (9 s complement of 782.54) (c) 9 9 9 9. 9 9 9-4 5 2 6. 0 7 5 5 4 7 3. 9 2 4 (9 s complement of 4526.075) Example : Find 10 s complement of the following decimal numbers. (a) 4069 (b) 1056.074 (a) 9 9 9 9-4 0 6 9 5 9 3 0 (9 s complement of 4069) + 1 5 9 3 1 (10 s complement of 4069) (b) 9 9 9 9. 9 9 9-1 0 5 6. 0 7 4 8 9 4 3. 9 2 5 (9 s complement of 4069)
+ 1 8 9 4 3. 9 2 5 (10 s complement of 4069) Example : Subtract the following numbers using the 9 s complement method. (a) 745.81 436.62 (b) 436.62 745.81 (a) 7 4 5. 8 1-4 3 6. 6 2 3 0 9. 1 9 7 4 5. 8 1 + 5 6 3. 3 7 3 0 9. 1 8 + 1 3 0 9. 1 9 (b) 4 3 6. 6 2 4 3 6. 6 2-7 4 5. 8 1 + 2 5 4. 1 8-3 0 9. 1 9 6 9 0. 8 0 The 9 s complement of 690.80 is 309.19 Therefore, the answer is 309.19. (9 s complement of 436.62) (Intermediate result) (End around carry) (Answer) (9 s complement of 745.81) (Intermediate result with no carry) Example: Subtract the following numbers using the 10 s complement method (a) 2928.54 416.73 (b) 416.73 2928.54 : (a) 2 9 2 8. 5 4-0 4 1 6. 7 3 2 5 1 1. 8 1 (a) 2 9 2 8. 5 4 + 9 5 8 3. 2 7 2 5 1 1. 8 1 (10 s complement of 416.73) (Ignore the carry)
The answer is 2511.81. The Binary Number System : The binary number system is a positional weighted system. the base or radix of this number system is 2. Hence, it has two independent symbols. The base itself cannot be a symbol. The symbols used are 0 and 1. A binary digit is called a bit. A binary number consists of a sequence of bits, each of which is either a 0 or a 1. The binary point separates the integer and fraction parts. Each digit (bit) carries a weight based on its position relevant to the binary point. The weight of each bit position is one power of 2 greater than the weight of the position to its immediate right. The first bit to the left of the binary point has weight of 2 0 and the column is called the units column. The second bit to the left has a weight 2 1 and it is in the 2 s column. The third bit to the left has a weight of 2 2 and it is in the 4 s column, and so on. The first bit to the right of the binary point has a weight of 2-2 is in the ¼ s column, and so on. The decimal value of the binary number is the sum of the products of all its bits multiplied by the weights of their respective positions. In general, a binary number with an integer part of (n + 1) bits and a fraction part of k bits can be written as d n d n-1 d n- 2.... d 1 d 0. d -1 d -2 d -3... d -k
Its decimal equivalent is (d n x 2 n ) + (d n-1 x 2 n-1 ) +... + (d 1 x 2 1 ) + (d 0 x 2 0 ) + (d -1 x 2-1 ) + (d -2 x 2-2 )... In general, the decimal equivalent of the number d n d n-1... d 1 d 0.d -1 d -2... in any number system with base b is given by (d n x b n ) + (d n-1 x b n-1 ) +... + (d 1 x b 1 ) + (d 0 x b 0 ) + (d -1 x b -1 ) + (d -2 x b -2 ) +... The binary number system is used in digital computers because the switching circuits used in these computers use two-state devices such as transistors, diodes, etc. A transistor can be OFF or ON, a switch can be OPEN or CLOSED, a diode can be OFF or ON, etc. These devices have to exist in one of the two possible states. So, these two states can be represented by the symbols 0 and 1, respectively. Counting in Binary :- Counting in binary is very much similar to decimal counting as shown in Table. Start counting with 0, the next count is 1. We have now exhausted all symbols; therefore we put a 1 in the column to left and continue to get 10, 11. Thus, 11 is the maximum we can count using two bits. So, put a 1 in the next column to the left and continue counting; we can count 100, 101, 111. The largest number we can count using three bits is 111. Put a 1 to the left and continue; we get, 1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111. The maximum count we can get using four bits is 1111. Continue counting with 5, 6,... bi ts as shown in Table. Table Counting in binary Decimal number Binary number Decimal number Binary number 0 0 20 10100 1 1 21 10101 2 10 22 10110 3 11 23 10111
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 100 101 110 111 1000 1001 1010 1011 1100 1101 1110 1111 10000 10001 10010 10011 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 11000 11001 11010 11011 11100 11101 11110 11111 100000 100001 100010 100011 100100 100101 100110 100111 An easy way to remember to write a binary sequence of n bits is : The right most column is the binary number begins with a 0 and alternates between 0 and 1 The second column begins with 2 (=2 1 ) zeros and alternates between the groups of 0 0 and 1 1 The third column begins with 4(= 2 2 ) zeros and alternates between the groups of 0 0 0 0 and 1 1 1 1. The n th column begins with 2 n-1 zeros and alternates between the groups of 2 n-1 zeros and 2 n-1 ones. Binary to Decimal Conversion Binary numbers may be converted to their decimal equivalents by the positional weights method. In this method, each binary digit of the number is multiplied by its position weight and the product terms and added to obtain the decimal number.
Example :Convert 10101 2 to decimal : (Positional weights) 2 4 2 3 2 2 2 1 2 0 (Binary number) 1 0 1 0 1 = (1 x 2 4 ) + (0 x 2 3 ) + (1 x 2 2 ) + (0 x 2 1 ) + (1 x 2 0 ) = 16 + 0 + 4 + 0 + 1 = 21 10 Example : Convert 11011.101 2 to decimal : 2 4 2 3 2 2 2 1 2 0 2-1 2-2 2-3 1 1 0 1 1 1 0 1 = (1 x 2 4 ) + (1 x 2 3 ) + (0 x 2 2 ) + (1 x 2 1 ) + (1 x 2-1 ) + (1 x 2-2 ) + (1 x 2-3 ) = 16 + 8 + 0 + 2 + 1 + 0.5 + 0 + 0.125 = 27.625 10 Decimal to Binary Conversion : method : In this method, the decimal integer number is converted to binary integer number by successive division by 2, and the decimal fraction is converted to binary fraction by successive multiplication by 2. This is also known as double-dabble method. In the successive division by 2 method, the give decimal integer number is successively divided by 2 till the quotient is zero. The last remainder is the MSB. The remainders read from bottom to top give the equivalent binary integer number. In the successive multiplication by-2 method, the given decimal fraction and the subsequent decimal fractions are successively multiplied by 2, till the fraction part of the product is 0 or till the desired accuracy is obtained. The first
integer obtained is the MSB. Thus, the integers and read from top to bottom give the equivalent binary fraction. To convert a mixed number to binary, convert the integer and fraction parts separately to binary and then combine them. In general, these methods can be used for converting a decimal number to an equivalent number in any other base system by just replacing 2 by the base b of that number system. That is, any decimal number can be converted to an equivalent number in any other base system by the sum-of-weights method, or by the double-dabble method repeated division-by-b for integers and multiplication by b for fractions. Example : Convert 52 10 to binary using the double-dabble method We divide the given decimal number successively by 2 and read the remainders upwards to get the equivalent binary number. Successive division Remainder 2 52 2 26 0 2 13 0 2 6 1 2 3 1 2 1 1 0 1 Reading the remainder from bottom to top, the result is 52 10 = 111100 2 Example : Convert 0.75 10 to binary using the double-dabble method Multiply the given fraction by 2. Keep the integer in the product as it is and multiply the new fraction in the product by 2. Continue this process and read the integers in the products from top to bottom. Given Fraction 0.75 Multiply 0.75 by 2 1.50
Multiply 0.50 by 2 1.00 Reading the integers from top to bottom 0.75 10 = 0.11 2 Example : Covert 105.15 10 to binary. Conversion of integer 105. Successive division Remainder 2 105 2 52 1 2 26 0 2 13 0 2 6 1 2 3 0 1 1 0 1 Reading the reminders from bottom to top, 105 10 = 1101001 2 Conversion of fraction 0.15 10 Given Fraction 0.15 Multiply 0.15 by 2 Multiply 0.30 by 2 Multiply 0.60 by 2 Multiply 0.20 by 2 Multiply 0.40 by 2 Multiply 0.80 by 2 0.30 0.60 1.20 0.40 0.80 1.60 This particular fraction can never be expressed exactly in binary. This process may be terminated after a few steps. Reading the integers from top to bottom, 0.15 10 = 0.001001 2 Therefore, the final result is, 105.15 10 = 1101001.001001 2 Binary Addition : The rules for binary addition are the following: 0 + 0 = 0; 0 + 1 = 1; 1 + 0 = 1; 1 + 1 = 10 i.e. 0 with a carry of 1
Example : Add the binary numbers 1010 and 1111 16 8 4 2 1 (Column number) 1 0 1 0 + 1 1 1 1 0 0 0 1 In the 1 s column 0 + 1 = 1 In the 2 s column 1 + 1 = 0 with a carry of 1 to the 4 s column In the 4 s column 0 + 1 + 1 = 0 with a carry of 1 to the 8 s column In the 8 s column 1 + 1 = 0 with a carry of 1 to the 16 s column Example : Add the binary numbers 1101.101 and 111.011 8 4 2 1. 2-1 2-2 2-3 (Column number) 1 1 0 1. 1 0 1 + 1 1 1. 0 1 1 1 0 1 0 1. 0 0 0 In the 2 3, s column 1 + 1 = 0, with a carry of 1 to the 2-2 column In the 2 2, s column 0+ 1 + 1 = 0, with a carry of 1 to the 2-1 column In the 2 1, s column 1+ 0 + 1 = 0, with a carry of 1 to the 1 s column In the 1 s column 1+ 1 + 1 = 1, with a carry of 1 to the 2 s column In the 2 s column 0+ 1 + 1 = 0, with a carry of 1 to the 4 s column In the 4 s column 1+ 1 + 1 = 1, with a carry of 1 to the 8 s column In the 8 s column 1 + 1 = 0, with a carry of 1 to the 16 s column Binary Subtraction :
The binary subtraction is performed in a manner similar to that in decimal subtraction. The rules for binary subtraction are : 0 0 = 0; 1 1 = 0; 1 0 = 1; 0 1 = 1, with a borrow of 1. Example :Subtract 10 2 from 1000 2 : In the 1 s column 0 0 = 0 In the 2 s column, a 1 cannot be subtracted from 0; so, a 1 must be borrowed from the 4 s column. But the 4 s column also has a 0; so, a 1 must be borrowed from the 8 s column, making the 8 s column 0. the 1 borrowed from the 8 s column becomes 10 in the 4 s column. Keep one 1 in the 4 s column and bring the remaining 1 to the 2 s column. This becomes 10 in the 2 s column, so 10 1 = 1 in the 2 s column. In the 4 s column 1 0 = 1 8 4 2 1 (Column number) In the 8 s column 0 0 = 0 1 0 0 0-1 0 Hence, the result is 0110 2. 0 1 1 0 Example : Subtract 111.111 2 from 1010.01 2 : 8 4 2 1 2-1 2-2 2-3 (Column number) 1 0 1 0. 0 1 0 1 1 1. 1 1 1 0 0 1 0. 0 1 1 In the 2-3 column, a 1 cannot be subtracted from a 0. So, borrow a 1 from the 2 2 column making the 2-2 column 0. The 1 borrowed from the 2-2 column becomes 10 in the 2-3 column. Therefore, in the 2-3 column 10 1 = 1. In the 2-2 column, as 1 cannot be subtracted from a 0. So, borrow a 1 from the 2-1 column, but it is also a 0. So, borrow a 1 from the 1 s column. That is also a 0, so borrow a 1 from the 2 s column making the 2 s column 0. this 1 borrowed from the 2 s column becomes 10 in the 1 s column. Keep one 1 in the 1 s column, bring the other 1 to the 2-1 column, which becomes 10 in this
column, keep one in the 2-1 column and bring the other 1 to 2-2 column, which becomes 10 in this column. Therefore, In the 2-2 s, column 10 1 = 1 In the 2-1 s, column 1 1 = 0 In the 1 s, column 1 1 = 0 Now in the 2 s column, a 1 cannot be subtracted from a 0, so, borrow a 1 from the 4 s column. But the 4 s column has a 0. So, borrow a 1 from the 8 s column, making the 8 s column 0, and bring it to the 4 s column. It becomes 10 in the 4 s column. Keep one 1 in the 4 s column and bring the second 1 to the 2 s column making it 10 in the 2 s column. Therefore, In the 2 s column 10 1 = 1 In the 4 s column 1 1 = 0 In the 8 s column 0 0 = 0 Hence, the result is 0010.011 2. Binary Multiplication There are two method of binary multiplication the paper method and the computer method. Both the methods obey the following multiplication rules: 0 x 0 = 0; 1 x 1 = 1; 1 x 0 = 0; 0 x 1 = 0 The paper method is similar to the multiplication of decimal numbers on paper. Multiply the multiplicand with each bit of the multiplier, and add the partial products. The partial product is the same as the multiplicand if the multiplier bit is a 1 and is zero if the multiplier bit is a 0. Example: Multiply 1101 2 by 110 2 The LSB of the multiplier is a 0. So, the first partial product is a 0. The next two bits of the multiplier are 1s. So, the next two partial products are equal to the multiplicand itself. The sum of the partial 1 1 0 1 x 1 1 0 0 0 0 0 1 1 0 1 1 1 0 1
products gives the answer. 1 0 0 1 1 1 0 Example: Multiply 1011.101 2 by 101.01 2 : 1 0 1 1. 1 0 1 x 1 0 1. 0 1 0 1 0 1 1 1 0 1 0 0 0 0 0 0 0 1 0 1 1 1 0 1 0 0 0 0 0 0 0 1 0 1 1 1 0 1 1 1 1 1 0 1. 0 0 0 0 1 Binary Division Like multiplication, division too can be performed by two method the paper method and the computer method. In the paper method, long division procedures similar to those in decimal are used. Example: Divide 101101 2 by 110 2 Divisor 110 cannot go in 101. So, consider the first 4 bits 1011 of the dividend. 110 can go in 1011, one time with a remainder of 101. Next, 110 can go in 1010, one time with a remainder of 100. Next 110 can go in 1001, one time with a remainder of 11. finally, 110 can go in 110 with a remainder of 0. 1 1 0 ) 1 0 1 1 0 1 ( 1 1 1. 1 1 1 0 1 0 1 0 1 1 0 1 0 0 1 1 1 0
1 1 0 1 1 0 0 0 0 Therefore, 101101 110 = 111.1 Example : Divide 110101.112 by 101 2. : 1 0 1 )1 1 0 1 0 1. 1 1 ( 1 0 1 0. 1 1 1 0 1 0 1 1 0 1 0 1 1 1 1 1 0 1 1 0 1 1 0 1 0 0 0 Therefore, 110101.11 101 = 1010.11 REPRESENTATION OF SIGNED NUMBERS AND BINARY ARITHMETIC IN COMPUTERS So far, we have considered only positive numbers. The representation of negative numbers is also equally important. There are two ways of representing signed numbers sign magnitude form and complement form. There are two complement forms: 1 s complement form and 2 s complement form. Most digital computers do subtraction by the 2 s complement method, but some do it by the 1 s complement method. The advantage of perfoming subtraction by the complement method is reduction in the hardware. Instead of having separate digital circuits are needed. That is, subtraction is also
performed by adders only. instead of subtracting one number from the other, the complement of the subtrahend is added to the minuend. In sign-magnitude form, an additional bit called the sign bit is placed in front of the number. If the sign bit is 0, the number is positive. If it is a 1, the number is negative for example, 0 1 0 1 0 0 1 = + 41 Sign bit Magnitude 1 1 0 1 0 0 1 = - 41 Sign bit Magnitude Under the signed magnitude system, a great deal of manipulation is necessary to add a positive number to a negative number. Thus, though the signed magnitude number system is possible, it is impractical. Representation of Signed Numbers using 2 s (or 1 s) Complement method The 2 s (or 1 s) complement system for representing signed numbers work like this: 1. If the number is positive, the magnitude is represented in its true binary from and a sign bit 0 is placed in from of the MSB 2. If the number is negative the magnitude is represented in its 2 s (or 1 s) complement form and a sign bit 1 is placed in front of the MSB. That is, to represent the numbers in sign 2 s (or 1 s) complement form, determine the 2 s (or 1 s) complement of the magnitude of the number and then attach the sign bit. The 2 s (or 1 s) complement operation on a signed number will change a positive number to a negative number and vice versa. The conversion of complement to true binary is the same as the process used to convert binary to complement. The representation of + 51 and 51 in both 2 s and 1 s complement forms is shown below: 0 1 1 0 0 1 1 + 51 (In sign magnitude form) Sign bit Magnitude (In 2 s Complement form)
(In 1 s Complement form) 1 1 1 0 0 1 1-51 (In sign magnitude form) Sign bit Magnitude Sign bit 1 0 0 1 1 0 1-51 (In 2 s sign magnitude form) Magnitude 1 0 0 1 1 0 0-51 (In 1 s sign magnitude form) Sign bit Magnitude Example : Express 45 in 8 bit 2 s complement form. + 45 in 8 bit form is 00101101. Obtain the 1 s complement of 00101101 and then add 1. Positive expression of the given number 0 0 1 0 1 1 0 1 1 s complement of it 1 1 0 1 0 0 1 0 Add 1 + 1 Thus, the 2 s complement form of 45 is 1 1 0 1 0 0 1 1 Example : Express 73.75 in 12 bit 2 s complement form. + 73.75 = N = 01001001.1100 Positive expression of the given number 0 1 0 0 1 0 0 1. 1 1 0 0 1 s complement of it 1 0 1 1 0 1 1 0. 0 0 1 1 Add 1 1 Thus, the 2 s complement of 73.75 is 1 0 1 1 0 1 1 0. 0 1 0 0 Two s Complement Arithmetic
The 2 s complement system is used to represent negative numbers using modulus arithmetic. The word length of a computer is fixed. That means, if a 4- bit number is added to another 4-bit number, the result will be only of 4 bits. Carry, if any, from the fourth bit will overflow. This is called the modulus arithmetic. For example : 1100 + 1111 = 1011. In the 2 s complement subtraction, add the 2 s complement of the subtrahend to the minuend. If there is a carry out, ignore it. Look at the sign bit, i.e. MSB of the sum term. If the MSB is a 0, the result is positive and is in true binary form. If the MSB is a 1 (whether there is a carry or no carry at all) the result is negative and is in its 2 s complement form. Take its 2 s complement to find its magnitude in binary. Example : Subtract 14 from 46 using the 8-bit 2 s complement arithmetic. + 1 4 = 0 0 0 0 1 1 1 0-1 4 = 1 1 1 1 0 0 1 0 (In 2 s complement from) + 4 6 0 0 1 0 1 1 1 0-1 4 + 1 1 1 1 0 0 1 0 (2 s complement form of 14) + 3 2 0 0 1 0 0 0 0 0 (Ignore the carry) There is a carry, ignore it. The MSB is 0; so, result is positive and is in normal binary form. Therefore, the result is + 00100000 = +32 Example : Add 75 to +26 using the 8 bit 2 s complement form + 7 5 = 0 1 0 0 1 0 1 1-7 5 = 1 0 1 1 0 1 0 1 (In 2 s complement from)
+ 2 6 0 0 0 1 1 0 1 0-7 5 + 1 0 1 1 0 1 0 1 (2 s complement form of 75) + 4 9 1 1 0 0 1 1 1 1 (No carry) There is no carry, the MSB is a 1. So, the result is negative and is in 2 s complement form. The magnitude is 2 s complement of 11001111, that is, 00110001 = 49. Therefore, the result is 49 Example : Add 45.75 to + 87.5 using the 12-bit 2 s complement arithmetic. : + 8 7. 5 0 1 0 1 0 1 1 1. 1 0 0 0-4 5. 7 5 + 1 1 0 1 0 0 1 0. 0 1 0 0 (-45.75 in 2 s complement form) + 4 5. 7 5 0 0 1 0 1 0 0 1. 1 1 0 0 (Ignore the carry) There is a carry, ignore it. The MSB is 0; so, the result is positive and is in normal binary form. Therefore, the is + 41.75 Example : Add 27.125 to 79.625 using the 12-bit 2 s complement arithmetic. + 2 7. 1 2 5 0 0 0 1 1 0 1 1. 0 0 1 0-7 9. 6 2 5 + 1 0 1 1 0 0 0 0. 0 1 1 0 ( - 79.625 in 2 s - 5 2. 5 0 0 1 1 0 0 1 0 1 1. 1 0 0 0 complement form) (No carry) The is no carry, indicating that the result is negative and is in its 2 s complement form. The 2 s complement of 11001011.1000 is 00110100.1000. Therefore, the result is 52.5 Example :Add 47.25 to 55.75 using the 2 s complement method. - 9 3. 1 2 5 1 0 1 0 0 0 1 0. 1 1 1 0 (-93.25 in 2 s complement form) - 3 1. 5 0 0 + 1 1 1 0 0 0 0 0. 1 1 1 0 (-31.5 in 2 s complement form) - 1 2 4. 6 2 5 1 0 0 0 0 0 1 1. 0 1 1 0 (Ignore the carry)
There is a carry, ignore it. The MSB is a 1; so, the result is negative and is in its 2 s complement form. The 2 s complement of 1000 0011.0110 is 0111 1100.1010. Therefore, the result is 124.625 Example : Add 47.25 to 55.75 using the 2 s complement method. 4 7. 2 5 0 0 1 0 1 1 1 1. 0 1 0 0 5 5. 7 5 + 0 0 1 1 0 1 1 1. 1 1 0 0 1 0 3. 0 0 0 1 1 0 0 1 1 1. 0 0 0 0 (No entry) There is no carry, and the MSB is 0. Therefore the result is positive and is in its true binary form. Hence, it is equal to + 103.0 Example : Add +47.25 to -40.75 using the 12 bit 2 s complement arithmetic. + 4 0. 7 5 0 0 1 0 1 0 0 0. 1 1 0 0-4 0. 7 5 + 1 1 0 1 0 1 1 1. 0 1 0 0 (-47.75 in 2 s complement form) 0 0. 0 0 0 0 0 0 0 0 0 0. 0 0 0 0 (No entry) There is a carry, ignore it. The result is 0 One s Complement Arithmetic The 1 s complement of a number is obtained by simply complementing each bit of the number, that is, by changing all the 0s to 1s and all the 1s to 0s. we can also say that the 1 s complement of a number is obtained by subtracting each bit of the number from 1. This complemented value represents the negative of the original number. This system is very easy to implement in hardware by simply feeding all bits thought inverters. One of the difficulties of using 1 s complement is its representation of zero. Both 00000000 and its 1 s complement 11111111 represent zero. The 00000000 is called positive zero and the 11111111 is called negative zero. Example : Represent 99 and 77.25 in 8-bit 1 s complement form.
: We first write the positive representation of the given number in binary form and then complement each of its bits to represent the negative of the number. (a) + 9 9 = 0 1 1 0 0 0 1 1-9 9 = 1 0 0 1 1 1 0 0 (In 1 s complement form) (b) + 7 7. 2 5 = 0 1 0 0 1 1 0 1. 0 1 0 0-7 7. 2 5 = 1 0 1 1 0 0 1 0. 1 0 1 1 (In 1 s complement form) In 1 s complement subtraction, add the 1 s complement of the subtrahend to the minuend. If there is a carry out, bring the carry around and add it to the LSB. This is called the end around carry. Look at the sign bit (MSB); if this is a 0, the result is positive and is in true binary. If the MSB is a 1 (whether there is a carry or no carry at all), the result is negative and is in its 1 s complement form. Take its 1 s complement to get the magnitude in binary. Example: Subtract 14 from 25 using the 8-bit 1 s complement arithmetic. + 2 5 0 0 0 1 1 0 0 1-1 4 + 1 1 1 1 0 0 0 1 (In 1 s complement form) 1 1 0 0 0 0 1 0 1 0 + 1 (Add the end around carry) 0 0 0 0 1 0 1 1 = + 11 10 Example : Add 25 to +14 using the 8-bit 1 s complement method. + 1 4 0 0 0 0 1 1 1 0-2 5 + 1 1 1 0 0 1 1 0 (In 1 s complement form) - 1 1 1 1 1 1 0 1 0 0 (No carry) There is no carry and the MSB is a 1. So, the result is negative and is in its 1 s complement form. The 1 s complement of 111101000 is 00001011. The result is, therefore, -11 10 Example : Add 25 to 14 using the 8-bit 1 s complement method.
+ 2 5 0 0 0 1 1 0 0 1 + 1 4 + 0 0 0 0 1 1 1 0 (In 1 s complement form) 3 9 0 0 1 0 0 1 1 1 There is no carry. The MSB is a 0. So, the result is positive and is in pure binary. Therefore, the result is, 00100111 = + 39 Example :Add + 25 to 25 using the 8-bit 1 s complement method + 2 5 0 0 0 1 1 0 0 1-2 5 + 1 1 1 0 0 1 1 0 (In 1 s complement form) 0 0 1 1 1 1 1 1 1 1 There is no carry. The MSB is a 1. So, the result is negative and is in its 1 s complement form. The 1 s complement of 111111111 is 000000000. therefore, the result is 0. Example: Subtract 27.50 from 68.75 using the 12 bit 1 s complement arithmetic + 6 8. 7 5 0 1 0 0 0 1 0 0. 1 1 0 0-2 7. 5 0 + 1 1 1 0 0 1 0 0. 0 1 1 1 (In 1 s complement form) 4 1. 2 5 0 0 1 0 1 0 0 1. 0 0 1 1 + 1 (Add the end around carry) 0 0 1 0 1 0 0 1. 0 1 0 0 The MSB is a 0. So, the result is positive and is in its normal binary form. Therefore, the result is +41.25 Example : Add 89.75 to +43.25 using the 12-bit 1 s complement method. + 4 3. 2 5 0 0 1 0 1 0 1 1. 0 1 0 0-8 9. 7 5 + 1 0 1 0 0 1 1 0. 0 0 1 1 (In 1 s complement form)
- 4 6. 5 0 1 1 0 1 0 0 0 1. 0 1 1 1 There is no carry. The MSB is a 1. So, the result is negative and is in its 1 s complement form. The 1 s complement of 1101001.0111 is 00101110.1000. Therefore, the result is 46.50 The Octal Number System The octal number system was extensively used by early minicomputers. It is also a positional weighted system. its base or radix is 8. It has 8 independent symbols 0, 1, 2, 3, 4, 5, 6, and 7. Since its base 8 = 2 3, every 3-bit group of binary can be respected by an octal digit. An octal number is, thus, 1/3 rd length of the corresponding binary number. Usefulness of the Octal System The ease with which conversions can be made between octal and binary makes the octal system more attractive as a shorthand means of expressing large binary numbers. In computer work, binary numbers with up to 64-bits are not uncommon. These binary numbers do not always represent a numerical quantity; they often represent some type of code, which conveys non-numerical information. In computers, binary numbers might represent (a) the actual numerical data, (b) the numbers corresponding to a location (address) in memory, (c) an instruction code, (d) a code expressing alphabetic and other non-numerical characters, or (e) a group of bits representing the status of devices internal or external to the computer. When dealing with large binary numbers of many bits, it is convenient and more efficient for us to write the numbers in octal rather than binary. However, the digital circuits and systems work strictly in binary; we use octal only for the convenience of the operators of the system. Table shows octal counting. Table Octal counting Decimal number Octal number Decimal number Octal number Decimal number Octal number Decimal number Octal number 0 0 10 12 20 24 30 36
1 1 11 13 21 25 31 37 2 2 12 14 22 26 32 40 3 3 13 15 23 27 33 41 4 4 14 16 24 30 34 42 5 5 15 17 25 31 35 43 6 6 16 20 26 32 36 44 7 7 17 21 27 33 37 45 8 10 18 22 28 34 38 46 9 11 19 23 29 35 39 47 Octal to Binary Conversion To convert a given octal number to a binary, just replace each octal digit by tis 3-bit binary equivalent. Example : Convert 367.52 8 to binary. Given octal number is 3 6 7. 5 2 Convert each octal digit to binary 011 110 111. 101 010 011110111. 101010 2 Binary to Octal Conversion
To convert a binary number to an octal number, starting from the binary point make groups of 3 bits each, on either side of the binary point, and replace each 3-bit binary group by the equivalent octal digit. Example : Convert 110101.101010 2 to octal. Group of three bits are 110 101. 101 010 Convert each group to octal 6 5. 5 2 The result is 65. 52 8 Example :Convert 01101111001.0111 2 to octal. Group of three bits are 10 101 111 011. 011 1 = 010 101 111 011. 011 100 Convert each group into octal 2 5 7 1. 3 4 The result is 2571. 34 8 Octal to Decimal Conversion To convert an octal number to a decimal number, multiply each digit in the octal number by the weight of its position and add all the product terms. The decimal value of the octal number d n d n-1 d n-2... d 1 d 0. d -1 d -2... d -k is (d n x 8 n ) + (d n-1 x 8 n-1 ) +... + (d 1 x 8 1 ) + (d 0 x 8 0 ) + (d -1 x 8-1 ) + (d -2 x 8-2 ) +... Example : Convert 4057.06 8 to decimal. 4057.068 = 4 x 8 3 + 0 x 8 2 + 5 x 8 1 + 7 x 8 0 + 0 x 8-1 + 6 x 8-2 = 2048 + 0 + 40 + 7 + 0 + 0.0937 = 2095.0937 10
Decimal to Octal Conversion To convert a mixed decimal number to a mixed octal number, convert the integer and fraction parts separately. To convert the given decimal integer number to octal, successively divide the given number by 8 till the quotient is 0. The last remainder is the MSD. The remainders read upwards given the equivalent octal integer number. To convert the given decimal fraction to octal, successively multiply the decimal fraction by 8 till the product is 0 or till the required accuracy is obtained. The first integer from the top is the MSD. The integers to the left of the octal point read downwards give the octal fraction. Example : Convert 378.93 10 octal : Conversion of 378 10 octal Successive division Remainders 8 378 2 8 47 7 8 5 5 0 0 Read the remainders from bottom to top. Therefore, 378 10 = 572 8 Conversion of 0.93 10 to octal 0.93 x 8 7.44 0.44 x 8 3.52 0.52 x 8 4.16 0.16 x 8 1.28 Read the integers to the left of the octal point downwards. Therefore, 0.93 10 = 7.7341 8. Hence 378.93 10 = 572.7341 8 Conversion of large decimal numbers to binary and large binary numbers to decimal can be conveniently and quickly performed via octal as shown below.
Example : Convert 5497 10 to binary. Since the given decimal number is large, we first convert this number to octal and then convert the octal number to binary. Successive division Remainders 8 5497 8 687 1 8 85 7 8 10 5 8 1 2 0 1 Therefore, 549710 = 12571 8 = 001010101111001 2 Example : Convert 101111010001 2 to decimal. Since the given binary number is large, we first convert this number to octal and then convert he octal number to decimal. 101111010001 2 = 5721 8 = 5 x 8 3 + 7 x 8 2 + 2 x 8 1 + 1 x 8 0 = 2560 + 448 + 16 + 1 = 3025 10 The Hexadecimal number system Binary numbers are long. These numbers are fine for machines but are too lengthy to be handled by human beings. So, there is a need to represent the binary numbers concisely. One number system developed with this objective is the hexadecimal number system (or Hex). Although it is somewhat more difficult to interpret than the octal number system, it has become the most popular of direct data entry and retrieval in digital systems. The
hexadecimal number system is a positional weighted system. The base or radix of this number system is 16, that means, it has 16 independent symbols. The symbols used are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. Since its base is 16 = 2 4, every 4 binary digit combination can be represented by one hexadecimal digit. So, a hexadecimal number, yet it provides the same information as the binary number. A 4-bit group is called a nibble. Since computer words come in 8 bits, 16 bits, 32 bits and so on, that is, multiples of 4 bits, they can be easily represented in hexadecimal. The hexadecimal system is particularly useful for human communications will computers. By far, this is the most commonly used number system in computer literature. It is used both in large and small computers. Hexadecimal counting sequence : 0 1 2 3 4 5 6 7 8 9 A B C D E F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F................................ F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 FA FB FC FD FE FF 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115................................ 1F0 1F1 1F2 1F3 1F4 1F5 1F6 1F7 1F8 1F9 1FA 1FB 1FC 1FD 1FE 1FF Binary to Hexadecimal conversion To convert a binary number to a hexadecimal number, starting from the binary point, make groups of 4 bits each, on either side of the binary point and replace each 4-bit group by the equivalent hexadecimal digit as shown in Table. Table 2.3 Binary to hexadecimal conversion Hexadecimal Binary Hexadecimal Binary 0 0 0 0 0 8 1 0 0 0 1 0 0 0 1 9 1 0 0 1
2 0 0 1 0 A 1 0 0 1 3 0 0 1 1 B 1 0 1 1 4 0 1 0 0 C 1 1 0 0 5 0 1 0 1 D 1 1 0 1 6 0 1 1 0 E 1 1 1 0 7 0 1 1 0 F 1 1 1 1 Example 54 Convert 1011011011 2 to hexadecimal. Make groups of 4 bits, and replace each 4-bit group by a hex digit. Given binary number is 1011011011 Groups of four bits are 0010 1101 1011 Convert each group to hex 2 D B The result 2DB 16 Example: Convert 01011111011.011111 2 to hexadecimal. Given binary number is 01011111011.011111 Groups of four bits are 0010 1111 1011. 0111 1100 Convert each group to hex 2 F B. 7 C The result is 2FB.7C 16 Hexadecimal to Binary To convert a hexadecimal number to binary, replace each hex digit by its 4-bit binary group. Example : Convert 4BAC 16 to binary. Given hex number is 4 B A C Convert each hex digit to 4-bit binary 0100 1011 1010 1100 There result is 0100101110101100 2 Example : Convert 3A9E.B0D 16 to binary.
Given hex number is 3 A 9 E B 0 D Convert each hex digit to 4-bit binary 0011 1010 1001 1100 1011 0000 1101 The result is 0011101010011110.101100001101 2 Hexadecimal to Decimal To convert a hexadecimal number to decimal, multiply each digit in the hex number by its position weight and add all those product terms. If the hex number is d n d n-1... d 1 d 0. d -1 d -2... d -k, its decimal equivalent is (d n x 16 n ) + (d n-1 x 16 n-1 ) +... + (D 1 x 16 1 ) + (d 0 x 16 0 ) + (d -1 x 16-1 ) + (d -2 x 16-2 ) +... Example : Convert 5C7 by to decimal. Multiply each digit of 5C7 by its position weight and add the product terms. 5C716 = (5 X 16 2 ) + (12 X 16 1 ) + (7 X 16 0 ) = 1280 + 192 + 7 = 1479 10 Example : Convert A0F9.0EB 16 to decimal A0F9.0EB 16 = (10 x 16 3 ) + (0 x 16 2 ) + (15 x 16 1 ) + (9 x 16 0 ) + (0 x 16-1 ) + (14 x 16-2 ) + (11 x 16-3 ) = 40960 + 0 + 240 + 9 + 0 + 0.0546 + 0.0026 = 41209.0572 10 Decimal to Hexadecimal To convert a decimal integer number to hexadecimal, successively divide the given decimal number by 16 till the quotient is zero. The last remainder is the MSB. The remainders read from bottom to top give the equivalent hexadecimal integer.
To convert a decimal fraction to hexadecimal, successively multiply the given decimal fraction by 16, till the product is zero or till the required accuracy is obtain, and collect all the integers to the left of decimal point. The first integer is the MSB and the integers read from top to bottom give the hexadecimal fraction. This is known as the hex dabble method. Example : Convert 2598.675 10 to hex The given decimal number is a mixed number. Convert the integer and the fraction parts separately to hex. Conversion of 2598 10 Successive division Remainder Decimal Hex 16 2598 16 162 6 6 16 10 2 2 16 0 10 A Reading the remainders upwards, 2598 10 = A26 16 Conversion of 0.675 10 Given fraction is 0.675 0.675 x 16 0.800 x 16 0.800 x 16 0.800 x 16 10.8 12.8 12.8 12.8 Reading the integers to the left of hexadecimal point downwards, 0.675 10 = 0.ACCC 16 Therefore, 2598.675 10 = A26.ACCC 16 Conversion of very large decimal numbers to binary and very large binary numbers to decimal is very much simplified if it is done via the hex route.