Chapter I: Digital System and Binary Numbers

Transcription

1 Chapter I: Digital System and Binary Numbers 1-1Digital Systems Digital systems are used in: - Communication - Business transaction - Traffic Control - Medical treatment - Internet The signals in digital systems use just two discrete values: a binary digit. Binary digit called a bit, has two values: 0 or 1. The decimal digits a through 9 are represented in digital system with a code of four bits (e.g. is represented by 0111, 8 is represented by 1000, 9 is represented by 1001). 1-2 Binary Numbers Decimal number: The decimal number system is said to be of base, or radix, 10 because it uses 10 digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9). Example1: The decimal number 245 may be written as 2x x x10 0 where 2, 4, and 5 are the coefficients. Example2: The decimal number may be written as 2x x x x x10-2 Binary Number System: The coefficients of the binary number have only two possible values: 0 or 1. (111) 2 its equivalent decimal number is : 1x2 2 +1x2 1 +1x , where 1, 1, and 1 are the coefficients. (1001) 2 its equivalent decimal number is : 1x2 3 +0x2 2 +0x2 1 +1x where 1, 0, 0, and 1 are the coefficients. ( ) 2 its equivalent decimal number is :

2 1x x x x x x , In general, a number expressed in a base r system has coefficients multiplied by powers of r. a n.r n + a n-1.r n a 2.r 2 + a 1.r 1 + a 0 + a -1.r -1 + a -2.r a -m.r -m The coefficient a j coefficient rang in value from 0 to r-1. To distinguish between numbers of different bases, the coefficients are enclosed in parentheses and write a subscript equal to the based used (except sometimes for decimal number). Hexadecimal number system (Base 16): It uses 16 digits : (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F). The letter A, B, C, D, E, and F are used for the digits 10,11, 12, 13, 14, and 15 respectively. (124) 16 1x x x (B32A) 16 11x x x x Number-Base conversions The conversion of a number in base r is done by expanding the number in a power series and adding all terms. Example1: (24) 8 2x x8 0 (18) Conversion of a decimal integer The conversion of a decimal integer to a number in base r is done by dividing the number and all successive quotients by r and accumulation the remainders. Example2: Convert decimal 41 to binary Dividing by (41) 10 (101001) result reading

3 Example3: Convert decimal 153 to octal. 153 Dividing by (153) 10 (231) result reading Conversion of a decimal fraction The conversion of a decimal to a binary is accomplished by a method similar to that used for integers. However, multiplication is used instead of division, and integers instead of remainders are accumulated. Example4: Convert (0.6875) 10 to binary, Integer fraction coefficient x a x a x a x a -4 1 Therefore, the answer is (0.6875) 10 (0.1011) Conversion from binary to hexadecimal and to octal - The conversion from and to binary, octal, and hexadecimal plays an important role in digital computers. Each octal digit corresponds to three binary digits. Each hexadecimal digit corresponds to four binary digits. - The first 16 numbers in decimal, binary, octal, and hexadecimal number systems are listed in table1.

4 Decimal Binary Octal Hexadecimal A B C D E F Table1: Numbers with different bases 9 Example1: Convert binary to octal and to hexadecimal. - Binary to octal: Starting from the left and partitioning the binary number into groups of three digits each, then assign the corresponding octal digit to each group. ( ) 2 (17463) 8 - Binary to Hexadecimal: Conversion from binary to Hexadecimal is similar, except thr binary number is divided into groups of four digits. ( ) 2 (1F33) 16 Example2: Convert the following numbers. ( ) 2 (? ) 8 ( ) 2 (? ) 16 Solution: ( ) 2 (671.67) 8 ( ) 2 (1B9.37) 16

5 1-5 Conversion from octal or hexadecimal to binary This conversion is done by reversing the preceding procedure. Each octal digit is converted to its three digits binary and each hexadecimal digit is converted to its four digits binary. (436) 8 ( ) 2 (52.67) 8 ( ) 2 (B79A) 16 ( ) Complements Complements are used in digital computers to simplify the subtraction operation and for logical manipulation. Each base-r-system has two types of complements: - The radix complement: r's Complement. - The diminished complement: (r-1)'s complement. For base 2, the two types are: 2's Complement and 1's Complement. For base 10, the two types are: 10's Complement and 9's Complement. - Diminished radix Complement Given a number N in base r having n digits, the (r-1)'s Complement of N is defined as (r n -1) N. - For decimal numbers, r 10 and r-1 9, so, the 9's Complement of N is (10 n -1) N Find the 9's Complement of The number has 5 digits (n 5), the 9's Complement is : (10 5-1) For binary numbers, r 2 and r-1 1, so, the 1's Complement of N is (2 n -1) N. Important: 2 n is represented by a binary number that consists of a 1 followed by n 0's. e.g. : 2 5 (100000) 2 ; 2 4 (10000) 2. 2 n -1 is binary number represented by n 1's. e.g. : (11111) 2 ; 2 4 (1111) 2. Find the 1's Complement of Solution:

6 N8 so, the 1's Complement of is: (2 8-1) - ( ) ( ) ( ) The 1's Complement can be obtained more easily by changing 1's to 0's and 0's to 1's as follow: The 1's Complement of is The 1's Complement of is Radix Complement: The r's Complement of an n digits number N in base r is defined as r n N for N 0 and as 0 for N 0. Comparing with (r-1)'s Complement, we can write: r's Complement (r-1)'s Complement + 1. Thus the 2's Complement of binary is obtained by adding 1 to the 1's Complement value. The 2's Complement of binary is : 's Complement The 2's Complement can be obtained by leaving all least significant 0's and the first 1 unchanged and replacing 1's with 0's and 0's with 1's in all other significant digits. The 2's Complement of is: All other significant bits are changed - Subtraction with Complement: First 1 unchanged Two least significant 0: unchanged The subtraction of two n digits unsigned numbers M-N in base r can be done as follow. 1- Add the minuend M to the r's Complement of the subtrahend N. M + (r n N) M N + r n 2- If M > N, the Sum will produce an end carry r n, which can be discarded, what is left is the result M N. 3- If M < N, the sum does not produce an end carry and is equal to r n (M N) which is the r's Complement of (M N). To obtain the r's complement of the sum and place a negative sign in front.

7 Given the two binary numbers X and Y , perform the substraction: a- X - Y b- Y X by using 2's Complement a- Solution: X 2' s Complement of Y Sum L Discard end carry 2 AnswerX Y b- Y 2' s Complement of X Sum There is no end carry. Therefore, the answer is: Y X - (2's Complement of ) Substraction of unsigned numbers can also be done by means of the 1's Complement: a- X Y X 1' s Complement of Y Sum End around carry Answer X Y b- Y X Y ' s Complement of Sum X There is no carry. Therefore, the answer is: Y X - (1's Complement of )

8 1-6 Signed Binary Numbers - Positive Integer can be represented as unsigned numbers. - Negative integers, are represented by using a signed complement system which can use either 1's complement or 2's complement but the 2's complement is the most common. The convention is to make the sign bit the sign bit 0 for positive and 1 for negative. - As an example, consider the number 9, represented in binary with eight bits (By using 2's complement) Arithmetic Addition: To add two signed binary numbers, the negative number must be initially in 2's complement form and that if the sum obtained after the addition is negative, it's in 2's complement form