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CS1104 Computer Organisaton

34CHAPTER 2 ( NUMBER SYSTEMS AND CODES

CHAPTER 2 ( NUMBER SYSTEMS AND CODES 33

Chapter

2

Number Systems and Codes

OBJECTIVES

In this chapter, you will be introduced to the following topics:

The number systems on various bases, and the conversion between numbers in different bases.

Arithmetic operations on binary numbers.

Representations of negative numbers.

Floating-point representation for real numbers.

Decimal codes, alphanumeric code, and error-detection code.

The Gray code.

2.1Information Representation

Computers are information processing tools that give meaning to raw data. Data should be represented in a form that permits efficient storage and ease of manipulation (although these two goals might be conflicting at times). In data communications, we are also concerned with issues such as security, error detection and error correction.

Data are usually represented in some form of code for compactness and uniformity. For instance, a yes response may be represented by Y, and a no by N; the four seasons by S (spring), M (summer), A (autumn) and W (winter). We can see that the choices are rather arbitrary. In real-life, codes are used for identification purposes, such as the Singapore NRIC number and the NUS students matriculation number.

Computers are also number crunching devices. Numeric values are represented in a form that eases arithmetic computations and preserves accuracy as much as possible.

The atomic unit of data is the bit (binary digit), stemming from the fact that the elementary storage units in a computer are electronic switches where each switch holds one of the two states: on (usually represented by 1), or off (0), as shown below.

Ultimately, all data, numeric or non-numeric, are represented in sequence of bits of zeroes and ones. Eight bits constitute a byte (a nibble is half a byte, or four bits, but this is rarely used these days), and a word, which is a unit for data storage and transfer, is usually in multiples of byte, depending on the width of the system bus. Computers nowadays typically use 32-bit or 64-bit words.

A sequence of bits allows for a range of representations. Storage units can be grouped together to provide larger range of representations. For convenience, we shall refer to these representations as values, though they may represent non-numeric data. For example, the four seasons could be represented by two switches, to cover a range of four values 00, 01, 10 and 11, as shown below.

In general, N bits can represent up to 2N distinct values. Conversely, to represent a range of M values, the number of bits required is

M

2

log

.

1 bit

(

represents up to 2 values (0, 1)

2 bits

(

represents up to 4 values (00, 01, 10, 11)

3 bits

(

represents up to 8 values (000, 001, 010, 011, 100, 101, 110, 111)

4 bits

(

represents up to 16 values (0000, 0001, 0010, , 1110, 1111)

32 values

(

requires 5 bits

40 values

(

requires 6 bits

64 values

(

requires 6 bits

100 values

(

requires 7 bits

1024 values

(

requires 10 bits

2.2Positional Numerals

Different numeral systems have emerged in civilizations over the history. They can be roughly categorised as follows:

Non-positional numeral systems

Relative-position numeral systems

Positional numeral systems

Non-positional systems are position-independent and include those of the Egyptian and Greek. In the Greek numeral system, more than 27 symbols are used, such as ( for 1, ( for 30, and ( for 500, and so 531 would be written as (((. The Egyptians drew different symbols for powers of ten, for example, for 1, for 10, and for 100. Hence, 531 would be depicted as .

The Roman numeral system includes different symbols for some basic values: I (1), V (5), X (10), C (50), and M (100), but employs complicated rules on relative positioning of symbols to represent other values. For instance, IV is 4 but VI is 6.

The systems described above are not well suited for computations. The problem is overcome by the elegant positional notation, where each position carries an implicit weight. The familiar Arabic decimal numeral is one such system.

In the decimal numeral system, the positional weights are powers of ten. In general, a decimal number (an an-1 a0 . f1 f2 fm) has the value

(an ( 10n) + (an-1 ( 10n-1) ++ (a0 ( 100) + (f1 ( 10-1) + (f2 ( 10-2) ++ (fm ( 10-m)

For example, 28.75 = (2 ( 101) + (8 ( 100) + (7 ( 10-1) + (5 ( 10-2)

2.3Bases of Number Systems

The base or radix of a number system is the number of digits present. The decimal numeral system has a base or radix of 10, where the set of 10 symbols (digits) is {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}. The weights are in powers of ten.

Number systems on other bases are defined likewise. For instance, a base-four number system consists of the set of four symbols {0, 1, 2, 3} with weights in powers of four. Hence, a base-four number 123.1, also written as (123.1)4 for clarity, has the value (1 ( 42) + (2 ( 41) + (3 ( 40) + (1 ( 4-1) or 16 + 8 + 3 + 0.25 or 27.25 in decimal, or (27.25)10. Listing base-four integers in increasing value yields 0, 1, 2, 3, 10, 11, 12, 13, 20, 21, 22, 23, 30, 31, 32, 33, 100, 101, 102, 103, 110, .

In general, a base-b number (an an-1 a0 . f1 f2 fm)b has the value

(an ( bn) + (an-1 ( bn-1) ++ (a0 ( b0) + (f1 ( b-1) + (f2 ( b-2) ++ (fm ( b-m)

The point that separates the integer part and fraction part is known as the radix point. The weights are in powers of b. The above forms the basis for conversion of a base-b number to decimal.

We may also adopt the computationally more efficient Horners rule to convert a base-b integer to decimal. This method simply factors out the powers of b, by rewriting (an ( bn) + (an-1 ( bn-1) ++ (a0 ( b0) as (((an ( b) + an-1) ( b + ) ( b + a0. For example, (123)4 = ((1 ( 4) + 2) ( 4 + 3 = 27. For fractions, we may adopt the same method by replacing multiplication with division.

Special names are given to number systems on certain bases. If you think that the base is a small value, you are wrong. The Babylonians used a sexagesimal system with base 60!

Table 2-1Bases of Positional Numeral Systems

Base

Name

Base

Name

2

Binary

9

Nonary

3

Ternary

10

Decimal (or Denary)

4

Quaternary

12

Duodecimal

5

Quinary

16

Hexadecimal

6

Senary

20

Vigesimal

7

Septimal

60

Sexagesimal

8

Octal (or Octonary)

For a base b that is less than ten, the symbols used are the first b Arabic symbols: 0, 1, 2, , b1. For bases that exceed ten, more symbols beyond the ten Arabic symbols must be introduced. The hexadecimal system uses the additional symbols A, B, C, D, E and F to represent 10, 11, 12, 13, 14 and 15 respectively.

The binary number system is of particular interest to us as the two symbols 0 and 1 correspond to the two bits that represent the states of a switch. The octal and hexadecimal number systems are also frequently encountered in computing.

Examples: Convert the following numbers into their decimal equivalent.

(1101.101)2 = (1 ( 23) + (1 ( 22) + (1 ( 20) + (1 ( 2-1) + (1 ( 2-3)

= 8 + 4 + 1 + 0.5 + 0.125 = 13.625

(572.6)8 = (5 ( 82) + (7 ( 81) + (2 ( 80) + (6 ( 8-1)

= 320 + 56+ 2 + 0.75 = 378.75

(2A.8)16 = (2 ( 161) + (10 ( 160) + (8 ( 16-1)

= 32 + 10 + 0.5 = 42.5

(341.24)5 = (3 ( 52) + (4 ( 51) + (1 ( 50) + (2 ( 5-1) + (4 ( 5-2)

= 75 + 20 + 1 + 0.4 + 0.16 = 96.56

2.4Decimal to Binary Conversion

One way to convert a decimal number to its binary equivalent is the sum-of-weights method. Here, we determine the set of weights (which are in powers of two) whose sum is the number in question.

Examples: Convert decimal numbers to their binary equivalent using sum-of-weights.

(9)10 = 8 + 1 = 23 + 20 = (1001)2

(18)10 = 16 + 2 = 24 + 21 = (10010)2

(58)10 = 32 + 16 + 8 + 2 = 25 + 24 + 23 + 21 = (111010)2

(0.625)10 = 0.5 + 0.125 = 2-1 + 2-3 = (0.101)2

We observe that multiplying a number x by 2n is equivalent to shifting left the binary representation of x by n positions, with zeroes appended on the right. For example, the value (5)10 is represented as (101)2, and (40)10 (which is 5 ( 23) is (101000)2. Similarly, dividing a number x by 2n is equivalent to shifting right its binary representation by n positions. This may help in quicker conversion.

The second method deals with the integral portion and the fractional portion separately, as follows:

Integral part: Repeated division-by-two

Fractional part: Repeated multiplication-by-two

Repeated Division-by-Two

To convert a decimal integer to binary, we divide the number successively by two until the quotient becomes zero. The remainders form the answer, with the first remainder serving as the least significant bit (LSB) and the last remainder the most significant bit (MSB).

The example below shows how (43)10 is converted into its binary equivalent.

2

43

2

21

rem 1

( LSB

2

10

rem 1

2

5

rem 0

2

2

rem 1

2

1