encoding schemes.pdf

Chapter 2 Encoding Schemes and Number System 2.1 Introduction “We owe a lot to the Have you ever thought how the keys on the computer Indians, who taught us...

Chapter 2 Encoding Schemes and Number System 2.1 Introduction “We owe a lot to the Have you ever thought how the keys on the computer Indians, who taught us how keyboard that are in human recognisable form are to count, without which interpreted by the computer system? This section briefly no worthwhile scientific discusses text interpretation by the computer. discovery could have been We have learnt in the previous chapter that made.” computer understands only binary language of 0s and 1s. Therefore, when a key on the keyboard is pressed, it –Albert Einstein is internally mapped to a unique code, which is further converted to binary. Example 2.1 When the key ‘A’ is pressed (Figure 2.1), it is internally mapped to a decimal value 65 (code value), which is then converted to its equivalent binary value for the computer to understand. In this chapter »» Introduction to Figure 2.1: Encoding of data entered using keyboard Encoding Similarly, when we press alphabet ‘अ’ on hindi keyboard, »» UNICODE internally it is mapped to a hexadecimal value 0905, »» Number System whose binary equivalent is 0000100100000101. »» Conversion So what is encoding? The mechanism of converting Between Number Systems data into an equivalent cipher using specific code is 2024-25 Ch 2.indd 27 08-Apr-19 11:38:00 AM  28 Computer Science – Class xi called encoding. It is important to understand why code value 65 is used for the key “A” and not any other value? Is it same for all the keyboards irrespective of Cipher means something converted to a coded form their make? to hide/conceal it from Yes, it is same for all the keyboards. This has been others. It is also called possible because of standard encoding schemes where encryption (converted to each letter, numeral and symbol is encoded or assigned cipher) and sent to the receiver who in turn can a unique code. Some of the well-known encoding decrypt it to get back the schemes are described in the following sections. actual content. 2.1.1 American Standard Code for Information Interchange (ASCII) In the early 1960s, computers had no way of communicating with each other due to different ways of representing keys of the keyboard. Hence, the need for a common standard was realised to overcome this shortcoming. Thus, encoding scheme ASCII was developed for standardising the character representation. ASCII is still the most commonly used coding scheme. Initially ASCII used 7 bits to represent characters. Recall that there are only 2 binary digits (0 or 1). Therefore, total number of different characters on the English keyboard that can be encoded by 7-bit ASCII code is 27 = 128. Table 2.1 shows some printable characters for ASCII code. But ASCII is able to encode character set of English language only. Table 2.1 ASCII code for some printable characters Character Decimal Value Character Decimal Value Character Decimal Value Space 32 @ 64 ` 96 ! 33 A 65 a 97 “ 34 B 66 b 98 # 35 C 67 c 99 $ 36 D 68 d 100 % 37 E 69 e 101 & 38 F 70 f 102 ‘ 39 G 71 g 103 ( 40 H 72 h 104 ) 41 I 73 i 105 Example 2.2 Encode the word DATA and convert the encoded value into binary values which can be understood by a computer. 2024-25 Ch 2.indd 28 08-Apr-19 11:38:00 AM  Encoding Schemes and Number System 29 ASCII value of D is 68 and its equivalent 7-bit binary code = 1000100 ASCII value of A is 65 and its equivalent 7-bit binary code = 1000001 ASCII value of T is 84 and its equivalent 7-bit binary code = 1010100 ASCII value of A is 65 and its equivalent 7-bit binary code = 1000001 Replace each alphabet in DATA with its ASCII code value to get its equivalent ASCII code and with 7-bit binary code to get its equivalent binary number as shown in Table 2.2. Table 2.2 ASCII and Binary values for word DATA D A T A ASCII Code 68 65 84 65 Binary Code 1000100 1000001 1010100 1000001 Think and Reflect Do we need to install 2.1.2 Indian Script Code for Information Interchange some additional tool (ISCII) or font to type in an Indian language using In order to facilitate the use of Indian languages on UNICODE? computers, a common standard for coding Indian scripts called ISCII was developed in India during mid 1980s. It is an 8-bit code representation for Indian languages which means it can represent 28=256 characters. It retains all 128 ASCII codes and uses rest of the codes (128) for additional Indian language character set. Additional codes have been assigned in the upper region Activity 2.1 (160– 255) for the ‘aksharas’ of the language. Explore and list down two font names for 2.1.3 UNICODE typing in any three Indian languages in There were many encoding schemes, for character UNICODE. sets of different languages. But they were not able to communicate with each other, as each of them represented characters in their own ways. Hence, text created using one encoding scheme was not recognised by another machine using different encoding scheme. Therefore, a standard called UNICODE has been developed to incorporate all the characters of every Think and Reflect written language of the world. UNICODE provides a Why a character in unique number for every character, irrespective of UTF 32 takes more device (server, desktop, mobile), operating system space than in UTF 16 (Linux, Windows, iOS) or software application (different or UTF 8? 2024-25 Ch 2.indd 29 08-Apr-19 11:38:00 AM  30 Computer Science – Class xi browsers, text editors, etc.). Commonly used UNICODE encodings are UTF-8, UTF-16 and UTF-32. It is a superset of ASCII, and the values 0–128 have the same character as in ASCII. Unicode characters for Devanagari script is shown in Table 2.3. Each cell of the table contains a character along with its equivalent hexadecimal value. Table 2.3 Unicode table for the Devanagari script ऀ ँ ं ः ऄ अ आ इ ई उ ऊ ऋ ऌ ऍ ऎ ए 0900 0901 0902 0903 0904 0905 0906 0907 0908 0909 090A 090B 090C 090D 090E 090F ऐ ऑ ऒ ओ औ क ख ग घ ङ च छ ज झ ञ ट 0910 0911 0912 0913 0914 0915 0916 0917 0918 0919 091A 091B 091C 091D 091E 091F ठ ड ढ ण त थ द ध न ऩ प फ ब भ म य 0920 0921 0922 0923 0924 0925 0926 0927 0928 0929 092A 092B 092C 092D 092E 092F र ऱ ल ळ ऴ व श ष स ह ऺ ऻ ़ ऽ ा ि 0930 0931 0932 0933 0934 0935 0936 0937 0938 0939 093A 093B 093C 093D 093E 093F ी ु ू ृ ॄ ॅ ॆ े ै ॉ ॊ ो ौ ् ॎ ॏ 0940 0941 0942 0943 0944 0945 0946 0947 0948 0949 094A 094B 094C 094D 094E 094F ॐ ॑ ॒ ॓ ॔ ॕ ॖ ॗ क़ ख़ ग़ ज़ ड़ ढ़ फ़ य़ 0950 0951 0952 0953 0954 0955 0956 0957 0958 0959 095A 095B 095C 095D 095E 095F ॠ ॡ ॢ ॣ । ॥ ० १ २ ३ ४ ५ ६ ७ ८ ९ 0960 0961 0962 0963 0964 0965 0966 0967 0968 0969 096A 096B 096C 096D 096E 096F ॰ ॱ ॲ ॳ ॴ ॵ ॶ ॷ ॸ ॹ ॺ ॻ ॼ ॽ ॾ ॿ 0970 0971 0972 0973 0974 0975 0976 0977 0978 0979 097A 097B 097C 097D 097E 097F 2.2 Number System Till now, we have learnt that each key (representing character, special symbol, function keys, etc.) of the keyboard is internally mapped to an ASCII code following an encoding scheme. This encoded value is further converted to its equivalent binary representation so that the computer can understand it. In Figure 2.1, the code for character “A” belongs to the decimal number system and its equivalent binary value belongs to the binary number system. A number system is a method to represent (write) numbers. Every number system has a set of unique characters or literals. The count of these literals is called the radix or base of the number system. The four different number systems used in the context of computer are shown in Figure 2.2. These number systems are explained in subsequent sections. 2024-25 Ch 2.indd 30 08-Apr-19 11:38:00 AM  Encoding Schemes and Number System 31 0–9 and A–F Figure 2.2: Four different number systems Number systems are also called positional number system because the value of each symbol (i.e., digit and alphabet) in a number depends upon its position within the number. Number may also have a fractional part similar to decimal numbers used by us. The symbol at the right most position in the integer part in a given number has position 0. The value of position (also called position value) in the integer part increases from right to left by 1. On the other hand, the first symbol in the fraction part of the number has position number –1, which decreases by 1 while reading fraction part from left to right. Each symbol in a number has a positional value, which is computed using its position value and the base value of the number system. The symbol at position number 3 in a decimal system with base 10 has positional value 103. Adding the product of positional value and the symbol Digit 1 2 3. 4 5 value results in the given number. Position Number 2 1 0 –1 –2 Figure 2.3 shows Positional Value (10) (10) (10) (10) (10)-2 2 1 0 -1 the computation Add the product of positional value and corresponding digit to get decimal number. of decimal number 1 × 102 + 2 × 101 + 3 × 100 + 4 × 10-1 + 5 × 10-2 = (123.45)10 123.45 using its positional value. Figure 2.3: Computation of decimal number using its positional value 2.2.1 Decimal Number System The decimal number system is used in our day-to-day life. It is known as base-10 system since 10 digits (0 to 9) are used. A number is presented by its two values — symbol value (any digit from 0 to 9) and positional value (in terms of base value). Figure 2.4 shows the integer and fractional part of decimal number 237.25 alongwith computation of the decimal number using positional values. 2024-25 Ch 2.indd 31 08-Apr-19 11:38:01 AM  32 Computer Science – Class xi Positive powers of 10 Negative powers of 10 (Integer part) (Fractional part) 2 3 7. 2 5 2×10 2 + 3×10 1 + 7×10 0 2×10-1 + 5×10-2 200 + 30 + 7 +.2.05 = (237.25)10 Integer part Fraction Part Figure 2.4: Positional value for digits of decimal number represented as power of base 10 2.2.2 Binary Number System The ICs (Integrated Circuits) in a computer are made up of a large number of transistors which are activated by Base value of a number the electronic signals (low/high) they receive. The ON/ system is used to high and OFF/low state of a transistor is represented distinguish a number in using the two digits 1 and 0, respectively. These two one number system from digits 1 and 0 form the binary number system. This another number system. Base value is written as system is also referred as base-2 system as it has two the subscript of the given digits only. Some examples of binary numbers are number. For example, 1001011, 1011.101, 111111.01. A binary number can (70)8 represents 70 as be mapped to an equivalent decimal number that can octal number and (70)10 denotes 70 as decimal be easily understood by the human. number. Table 2.4 Binary value for (0–9) digits of decimal number system Decimal Binary 0 0 1 1 2 10 3 11 4 100 5 101 6 110 7 111 8 1000 9 1001 2.2.3 Octal Number System With increase in the value of a decimal number, the number of bits (0/1) in its binary representation also increases. Sometimes, a binary number is so large that it becomes difficult to manage. Octal number system was devised for compact representation of the binary numbers. Octal number system is called base-8 system 2024-25 Ch 2.indd 32 08-Apr-19 11:38:01 AM  Encoding Schemes and Number System 33 as it has total eight digits (0-7), and Table 2.5 Decimal and binary positional value is expressed in powers equivalent of octal numbers 0 – 7 of 8. Three binary digits (8=23) are Octal Digit Decimal 3 -bit Binary sufficient to represent any octal digit. Value Number Table 2.5 shows the decimal and binary 0 0 000 equivalent of 8 octal digits. Examples of 1 1 001 octal numbers are (237.05)8, (13)8, and 2 2 010 (617.24)8. 3 3 011 4 4 100 2.2.4 Hexadecimal Number System 5 5 101 Hexadecimal numbers are also used 6 6 110 for compact representation of binary 7 7 111 numbers. It consists of 16 unique symbols (0 – 9, A–F), and is called base- 16 system. In hexadecimal system, each alphanumeric digit is represented as a group of 4 binary digits because 4 bits (24=16) are sufficient to represent 16 alphanumeric symbols. Note here that the decimal numbers 10 through 15 are represented by the letters A through F. Examples of Hexadecimal numbers are (23A.05)16, (1C3)16, (619B.A)16. Table 2.6 shows decimal and binary equivalent of 16 alphanumeric symbols used in hexadecimal number system. Table 2.6 Decimal and binary equivalent of hexadecimal numbers 0–9, A–F Hexadecimal Symbol Decimal Value 4-bit Binary Number 0 0 0000 1 1 0001 2 2 0010 3 3 0011 4 4 0100 5 5 0101 6 6 0110 7 7 0111 8 8 1000 9 9 1001 A 10 1010 B 11 1011 C 12 1100 D 13 1101 E 14 1110 F 15 1111 2024-25 Ch 2.indd 33 08-Apr-19 11:38:01 AM  34 Computer Science – Class xi 2.2.5 Applications of Hexadecimal Number System Main memory is made up of memory locations where each location has a unique address. Usually, size of a memory address is 16-bit or 32-bit. To access 16-bit memory address, a programmer has to use 16 binary bits, which is difficult to deal with. To simplify the address representation, hexadecimal and octal numbers are used. Let us consider a 16- bit memory address 1100000011110001. Using the hexadecimal notation, this address is mapped to C0F1 which is more easy to remember. The equivalent octal representation for this 16-bit value is 140361. Hexadecimal numbers are also used for describing the colours on the webpage. Each colour is made up of three primary colours red, green and blue, popularly called RGB (in short). In most colour maps, each colour is usually chosen from a palette of 16 million colours. Therefore, 24 bits are required for representing each colour having three components (8 bits for Red, 8 bits for Green, 8 bits for Blue component). It is difficult to remember 24-bit binary colour code. Therefore, colour codes are written in hexadecimal form for compact representation. For example, 24-bit code for RED colour is 11111111,00000000,00000000. The equivalent hexadecimal notation is (FF,00,00), which can be easily remembered and used. Table 2.7 shows Table 2.7 Colour codes in decimal, binary examples of and hexadecimal numbers some colours Colour Name Decimal Binary Hexadecimal represented Black (0,0,0) (00000000,00000000,00000000) (00,00,00) with decimal, binary and White (255,255,255) (11111111,11111111,11111111) (FF,FF,FF) hexadecimal Yellow (255,255,0) (11111111,11111111,00000000) (FF,FF,00) numbers. Grey (128,128,128) (10000000,10000000,10000000) (80, 80, 80) 2.3 Conversion between Number Systems In the previous section, we learnt about different number systems used in computers. Now, let us learn how to convert a number from one number system to another number system for better understanding of the number representation in computers. Decimal number 2024-25 Ch 2.indd 34 21-May-19 11:34:53 AM  Encoding Schemes and Number System 35 system is most commonly used by humans, but digital Activity 2.2 systems understand binary numbers; whereas Octal Convert the following and hexadecimal number systems are used to simplify decimal numbers in the binary representation for us to understand. the form understood by computer. 2.3.1 Conversion from Decimal to other Number Systems (i) (593)10 (ii) (326)10 (iii) (79)10 To convert a decimal number to any other number system (binary, octal or hexadecimal), use the steps given below. Step 1: Divide the given number by the base value (b) of the number system in which it is to be converted Step 2: Note the remainder Step 3: Keep on dividing the quotient by the base value and note the remainder till the quotient is zero Step 4: Write the noted remainders in the reverse order (from bottom to top) (A) Decimal to Binary Conversion Since the base value of binary system is 2, the decimal number is repeatedly divided by 2 following the steps Activity 2.3 given in above till the quotient is 0. Record the remainder Express the following after each division and finally write the remainders in decimal numbers into reverse order in which they are computed. octal numbers. In Figure 2.1 you saw that the binary equivalent of (i) (913)10 65 is (1000001)2. Let us now convert a decimal value (ii) (845)10 (iii) (66)10 to its binary representation and verify that the binary equivalent of (65)10 is (1000001)2. Step 1: Divide the decimal number by 2. 2 65 Remainders Step 2: Write its remainder. 2 32 1 Step 3: Keep on dividing the quotient 2 16 0 by the base value 2 and note the 2 8 0 remainder till the quotient is zero. 2 4 0 2 2 0 2 1 0 2 0 1 Step 4: Collect the remainders from bottom (65)10 = (1000001)2 to top to get the binary equivalent. Figure 2.5: Conversion of a decimal number to its equivalent binary number 2024-25 Ch 2.indd 35 08-Apr-19 11:38:01 AM  36 Computer Science – Class xi Example 2.3 Convert (122)10 to binary number. 2 122 Remainders 2 61 0 2 30 1 2 15 0 2 7 1 2 3 1 2 1 1 0 1 Therefore, (122)10 = (1111010)2 (B) Decimal to Octal Conversion Since the base value of octal is 8, the decimal number is repeatedly divided by 8 to obtain its equivalent octal number. Step 1: Divide the decimal number by 8 Step 2: Write its remainder 8 65 Remainders 8 8 1 Step 3: Keep on dividing the 8 1 0 quotient by the base value 8 0 1 and note the remainder till the quotient is zero Step 4: Collect the remainder from bottom (65)10 = (101)8 to top to get the octal equivalent Figure 2.6: Conversion of a decimal number to its equivalent octal number The octal equivalent of letter “A” by using its ASCII code value (65)10 is calculated as shown in Figure 2.6. Example 2.4 Convert (122)10 to octal number. 8 122 Remainders 8 15 2 8 1 7 0 1 Therefore, (122)10= (172)8 (C) Decimal to Hexadecimal Conversion Since the base value of hexadecimal is 16, the decimal number is repeatedly divided by 16 to obtain its equivalent hexadecimal number. The hexadecimal 2024-25 Ch 2.indd 36 08-Apr-19 11:38:01 AM  Encoding Schemes and Number System 37 equivalent of letter ‘A’ using its ASCII code (65)10 is calculated as shown in Figure 2.7. Step 1: Divide the decimal number by 16 Step 2: Write its remainder 16 65 Remainders 16 4 1 Step 3: Keep on dividing the quotient 16 0 4 by the base value 16 and note the remainder till the quotient is zero Step 4: Collect the remainders from bottom to top to get the hexadecimal equivalent (65)10 = (14)16 Figure 2.7: Conversion of a decimal number to its equivalent hexadecimal number Example 2.5 Convert (122)10 to hexadecimal number. 16 122 Remainders 16 7 A (Hexadecimal symbol equivalent to decimal number 10) 0 7 Therefore, (122)10=(7A)16 2.3.2 Conversion from other Number Systems to Decimal Number System Activity 2.4 We can use the following steps to convert the given Convert the following number with base value b to its decimal equivalent, numbers into decimal where base value b can be 2, 8 and 16 for binary, octal numbers. and hexadecimal number system, respectively. (i) (110101)2 (ii) (1703)8 Step 1: Write the position number for each alphanumeric (iii) (COF5)16 symbol in the given number Step 2: Get positional value for each symbol by raising its position number to the base value b symbol in the given number Step 3: Multiply each digit with the respective positional value to get a decimal value Step 4: Add all these decimal values to get the equivalent decimal number (A) Binary Number to Decimal Number Since binary number system has base 2, the positional values are computed in terms of powers of 2. Using the above mentioned steps we can convert 2024-25 Ch 2.indd 37 08-Apr-19 11:38:01 AM  38 Computer Science – Class xi a binary number to its equivalent decimal value as shown below: Example 2.6 Convert (1101)2 into decimal number. Digit 1 1 0 1 Position Number 3 2 1 0 Positional Value 23 22 21 20 Decimal Number 1 × 23 + 1 × 22 + 0 × 21 + 1 × 20 = 8 + 4 + 0 + 1 = (13)10 Note: Add the product of positional value and corresponding digit to get decimal number. (B) Octal Number to Decimal Number The following example shows how to compute the decimal equivalent of an octal number using base value 8. Example 2.7 Convert (257)8 into decimal number. Why 3 bits in a Digit 2 5 7 binary number are Position 2 1 0 grouped together to Number get octal number? Positional 82 81 80 The base value of octal Value number system is 8. Decimal 2 × 82 5 × 81 + 7 × 80 =128 + 40 + 7 = (175)10 Convert value 8 in Number terms of exponent of 2, i.e., 8=23. Hence, (C) Hexadecimal Number to Decimal Number three binary digits are sufficient to represent For converting a hexadecimal number into decimal all 8 octal digits. number, use steps given in this section with base value Simply stated, count all 16 of hexadecimal number system. Use decimal value possible combinations of three binary digits, equivalent to alphabet symbol of hexadecimal number which are 2x2x2=8. in the calculation, as shown in Table 2.6. Therefore, 3 bits are sufficient to represent Example 2.8 Convert (3A5)16 into decimal number. any octal digit. Hence, Digit 3 A 5 3-bit groups in a binary Position 2 1 0 number are formed to Number get equivalent octal number. Positional 162 161 160 Value Decimal 3 × 162 10 × 161 + 5 × 160 =768 + 160 + 5 = (933)10 Number Note: Use Table 2.5 for decimal value of alphabets 2.3.3 Conversion from Binary Number to Octal/ Hexadecimal Number and Vice-Versa A binary number is converted to octal or hexadecimal number by making groups of 3 and 4 bits, respectively, and replacing each group by its equivalent octal/ hexadecimal digit. 2024-25 Ch 2.indd 38 21-May-19 11:41:57 AM  Encoding Schemes and Number System 39 (A) Binary Number to Octal Number Given a binary number, an equivalent octal number represented by 3 bits is computed by grouping 3 bits Why 4 bits in a from right to left and replacing each 3-bit group by the binary number are corresponding octal digit. In case number of bits in a grouped together to get hexadecimal binary number is not multiple of 3, then add required number? number of 0s on most significant position of the binary The base value of number. hexadecimal number system is 16. Write Example 2.9 Convert (10101100)2 to octal number. value 16 in terms of Make group of 3-bits of the given exponent of 2 i.e. 16 = 24. Hence, four binary binary number (right to left) 010 101 100 digits are sufficient Write octal number for to represent all 16 hexadecimal symbols. each 3-bit group 2 5 4 Therefore, (10101100)2 = (254)8 (B) Octal Number to Binary Number Each octal digit is an encoding for a 3-digit binary number. Octal number is converted to binary by replacing each octal digit by a group of three Think and Reflect binary digits. While converting the fractional part of Example 2.10 Convert (705)8 to binary number. a decimal number Octal digits 7 0 5 to another number system, why do we Write 3-bits binary write the integer part value for each digit 111 000 101 from top to bottom and not other way? Therefore, (705)8 = (111000101)2 (C) Binary Number to Hexadecimal Number Given a binary number, its equivalent hexadecimal number is computed by making a group of 4 binary digits from right to left and substituting each 4-bit group by its corresponding hexadecimal alphanumeric symbol. If required, add 0 bit on the most significant position of the binary number to have number of bits in a binary number as multiple of 4. Example 2.11 Convert (0110101100)2 to hexadecimal number. Make group of 4-bits of the given binary number (right to left) 0001 1010 1100 Write hexadecimal symbol 2024-25 Ch 2.indd 39 21-May-19 4:33:42 PM  40 Computer Science – Class xi for each group 1 A C Therefore, (0110101100)2 = (1AC)16 (D) Hexadecimal Number to Binary Number Activity 2.5 Each hexadecimal symbol is an encoding for a 4-digit Write binary binary number. Hence, the binary equivalent of a representation of the following numbers. hexadecimal number is obtained by substituting (i) (F018)16 4-bit binary equivalent of each hexadecimal digit and (ii) (172)16 combining them together (see Table 2.5). (iii) (613)8 Example 2.12 Convert (23D)16 to binary number. Hexadecimal digits 2 3 D Write 4-bit binary value for each digit 0010 0011 1101 Therefore, (23D)16 = (001000111101)2 2.3.4 Conversion of a Number with Fractional Part Till now, we largely dealt with different conversions for whole number. In this section, we will learn about conversion of numbers with a fractional part. (A) Decimal Number with Fractional Part to another Number System To convert the fractional part of a decimal number to another number system with base value b, repeatedly multiply the fractional part by the base value b till the fractional part becomes 0. Use integer part from top to bottom to get equivalent number in that number system. If the fractional part does not become 0 in successive multiplication, then stop after, say 10 multiplications. In some cases, fractional part may start repeating, then stop further calculation. Example 2.13 Convert (0.25)10 to binary. Integer part 0.25 × 2 = 0.50 0 0.50 × 2 = 1.00 1 Since the fractional part is 0, the multiplication is stopped. Write the integer part from top to bottom to get binary number for the fractional part. Therefore, (0.25)10 = (0.01)2 2024-25 Ch 2.indd 40 08-Apr-19 11:38:01 AM  Encoding Schemes and Number System 41 Example 2.14 Convert (0.675)10 to binary. Integer part 0.675 × 2 = 1.350 1 0.350 × 2 = 0.700 0 0.700 × 2 = 1.400 1 0.400 × 2 = 0.800 0 0.800 × 2 = 1.600 1 0.600 × 2 = 1.200 1 0.200 × 2 = 0.400 0 Since the fractional part (.400) is the repeating value in the calculation, the multiplication is stopped. Write the integer part from top to bottom to get binary number for the fractional part. Therefore, (0.675)10= (0.1010110)2 Example 2.15 Convert (0.675)10 to octal. Integer part 0.675 × 8 = 5.400 5 0.400 × 8 = 3.200 3 0.200 × 8 = 1.600 1 0.600 × 8 = 4.800 4 0.800 × 8 = 6.400 6 Since the fractional part (.400) is repeating, the multiplication is stopped. Write the integer part from top to bottom to get octal number for the fractional part. Therefore, (0.675)10= (0.53146)8 Example 2.16 Convert (0.675)10 to hexadecimal form. Integer part 0.675 × 16 = 10.800 A (Hexadecimal symbol for 10) 0.800 × 16 = 12.800 C (Hexadecimal symbol for 12) Since the fractional part (.800) is repeating, the multiplication is stopped. Write the integer part from top to bottom to get hexadecimal equivalent for the fractional part. Therefore, (0.675)10=(0. AC)16 (B) Non-decimal Number with Fractional Part to Decimal Number System Compute positional value of each digit in the given number using its base value. Add the product of 2024-25 Ch 2.indd 41 08-Apr-19 11:38:01 AM  42 COMPUTER SCIENCE – CLASS XI positional value and the digit to get the equivalent decimal number with fractional part. Example 2.17 Convert (100101.101)2 into decimal. Digit 1 0 0 1 0 1. 1 0 1 Fractional 2 5 2 4 2 3 2 2 2 1 2 0 2 -1 2 -2 2-3 Value Decimal 125 +024 +023 +122 +021 +120 + 12-1 + 02-2 +12-3 Value = 32 + 0 + 0 + 4 + 0 + 1 + 0.5 + 0 + 0.125 37 0.625 = 37 + 0.625 Therefore, (100101.101)2= (37.625)10 Example 2.18 Convert (605.12)8 into decimal number. Octal Digits 6 0 5. 1 2 Positional Value 8 2 8 1 8 0 8 -1 8-2 Decimal number 682 + 0 81 + 5 80 + 1 × 8-1 + 2 × 8-2 = 384 + 0 + 5 +.125 +.03125 389 0.15625 = 389 + 0.156225 Therefore, (605.12)8 = (389.15625)10 (C) Fractional Binary Number to Octal or Hexadecimal Number To convert the fractional binary number into octal or hexadecimal value, substitute groups of 3-bit or 4-bit in integer part by the corresponding digit. Similarly, make groups of 3-bit or 4-bit for fractional part starting from left to right, and substitute each group by its equivalent digit or symbol in Octal or Hexadecimal number system. Add 0s at the end of the fractional part to make a perfect group of 3 or 4 bits. Example 2.19 Convert (10101100.01011)2 to octal number. Make perfect group of 3 bits 010 101 100. 010 110 Write octal symbol for each group 2 5 4. 2 6 Therefore, (10101100.01011)2 = (254.26)8 Note: Make 3-bit groups from right to left for the integer part and left to right for the fractional part. 2024-25 Ch 2.indd 16 17-Aug-2023 10:31:10 AM  Encoding Schemes and Number System 43 Example 2.20 Convert (10101100.010111)2 to Notes hexadecimal number Make perfect group of 4 bits 1010 1100. 0101 1100 Write hexadecimal symbol for each group A C. 5 C Therefore, (10101100.010111)2 = (AC.5C)16 Summary Encoding scheme maps text into the codes that facilitate communication among computers. Textual data is encoded using ASCII, ISCII or Unicode. Unicode scheme is a character encoding standard which can encode all the characters of almost all languages of the world. Computer being a digital system understands only binary numbers which are 0 and 1. Encoded text is converted to binary form for processing by the computer. Octal and hexadecimal number systems are used to simplify the binary coded representation as they allow grouping of 3 or 4 bits of binary numbers each, respectively. Exercise 1. Write base values of binary, octal and hexadecimal number system. 2. Give full form of ASCII and ISCII. 3. Try the following conversions. (i) (514)8 = (?)10 (iv) (4D9)16 = (?)10 (ii) (220)8 = (?)2 (v) (11001010)2 = (?)10 (iii) (76F)16 = (?)10 (vi) (1010111)2 = (?)10 4. Do the following conversions from decimal number to other number systems. (i) (54)10 = (?)2 (iv) (889)10 = (?)8 (ii) (120)10 = (?)2 (v) (789)10 = (?)16 (iii) (76)10 = (?)8 (vi) (108)10 = (?)16 5. Express the following octal numbers into their equivalent decimal numbers. (i) 145 (ii) 6760 (iii) 455 (iv) 10.75 2024-25 Ch 2.indd 43 08-Apr-19 11:38:02 AM  44 Computer Science – Class xi Notes 6. Express the following decimal numbers into hexadecimal numbers. (i) 548 (ii) 4052 (iii) 58 (iv) 100.25 7. Express the following hexadecimal numbers into equivalent decimal numbers. (i) 4A2 (ii) 9E1A (iii) 6BD (iv) 6C.34 8. Convert the following binary numbers into octal and hexadecimal numbers. (i) 1110001000 (ii) 110110101 (iii) 1010100 (iv) 1010.1001 9. Write binary equivalent of the following octal numbers. (i) 2306 (ii) 5610 (iii) 742 (iv) 65.203 10. Write binary representation of the following hexadecimal numbers. (i) 4026 (ii) BCA1 (iii) 98E (iv) 132.45 11. How does computer understand the following text? (hint: 7 bit ASCII code). (i) HOTS (ii) Main (iii) CaSe 12. The hexadecimal number system uses 16 literals (0 – 9, A– F). Write down its base value. 13. Let X be a number system having B symbols only. Write down the base value of this number system. 14. Write the equivalent hexadecimal and binary values for each character of the phrase given below. ‘‘ हम सब एक” 15. What is the advantage of preparing a digital content in Indian language using UNICODE font? 16. Explore and list the steps required to type in an Indian language using UNICODE. 17. Encode the word ‘COMPUTER’ using ASCII and convert the encode value into binary values. 2024-25 Ch 2.indd 44 08-Apr-19 11:38:02 AM