9 Digital computation and intelligent devices

 9.1.2 Computer operation

As has already been mentioned, the fundamental role of a computer is the manipulation of data. Numbers are used both in quantifying items of data and also in the form of codes that define the computational operations that are to be executed. All numbers that are used for these two purposes must be stored within the computer memory and also transported along the communication buses. A detailed consideration of the conventions used for representing numbers within the computer is therefore required.

Number systems

The decimal system is the best known number system, but it is not very suitable for use by digital computers. It uses a base of ten, such that each digit in a number can have any one of ten values within the range 0–9. Items of electronic equipment such as the digital counter, which are often used as computer peripherals, have liquid crystal display elements that can each display any of the ten decimal digits, and therefore a four element display can directly represent decimal numbers in the range 0–9999. The decimal system is therefore perfectly suitable for use with such output devices.

The fundamental unit of data storage within a digital computer is a memory element known as a bit. This holds information by switching between one of two possible states. Each storage unit can therefore only represent two possible values and all data to be entered into memory must be organized into a format that recognizes this restriction. This means that numbers must be entered in binary format, where each digit in the number can only have one of two values, 0 or 1. The binary representation is particularly convenient for computers because bits can be represented very simply electronically as either zero or non-zero voltages. However, the conversion is tedious for humans. Starting from the right-hand side of a binary number, where the first digit represents 2⁰ (i.e. 1), each successive binary digit represents progressively higher powers of 2. For example, in the binary number 1111, the first digit (starting from the right-hand side) represents 1, the next 2, the next 4 and the final, leftmost digit represents 8, Thus the decimal equivalent is 1 + 2 + 4 + 8 = 15.

Example 9.1

Convert the following 8-bit binary number to its decimal equivalent: 10110011

Solution

Starting at the right-hand side, we have:

(1 × 2⁰) + (1 × 2¹) + (0 × 2²) + (0 × 2³) + (1 × 2⁴) + (1 × 2⁵) 

                 + (0 × 2⁶) + (1 × 2⁷)

               = 1 + 2 + 0 + 0 + 16 + 32 + 0 + 128 = 179

For data storage purposes, memory elements are combined into larger units known as bytes, which are usually considered to consist of 8 bits each. Each bit holds one binary digit, and therefore a memory unit consisting of 8 bits can store 8-digit binary numbers in the range 00000000 to 11111111 (equivalent to decimal numbers in the range 0 to 255). A binary number in this system of 10010011 for instance would correspond with the decimal number 147.

This range is clearly inadequate for most purposes, including measurement systems, because even if all data could be conveniently scaled the maximum resolution obtainable is only 1 part in 128. Numbers are therefore normally stored in units of either 2 or 4 bytes, which allow the storage of integer (whole) numbers in the range 0–65 535 or 0–4 294 967 296.

No means have been suggested so far for expressing the sign of numbers, which is clearly necessary in the real world where negative as well as positive numbers occur. A simple way to do this is to reserve the most significant (left-hand bit) in a storage unit to define the sign of a number, with ‘0’ representing a positive number and ‘1’ a negative number. This alters the ranges of numbers representable in a 1- byte storage unit to -127 to +127, as only 7 bits are left to express the magnitude of the number, and also means that there are two representations of the value 0. In this system the binary number 10010011 translates to the decimal number -19 and 00010011 translates to +19. For reasons dictated by the mode of operation of the CPU, however, most computers use an alternative representation known as the two’s complement form.

The two’s complement of a number is most easily formed by going via an intermediate stage of the one’s complement. The one’s complement of a number is formed by reversing all digits in the binary representation of the magnitude of a number, changing 1s to 0s and 0s to 1s, and then changing the left-hand bit to a 1 if the original number was negative. The two’s complement is then formed by adding 1 at the least significant (right-hand) end of the one’s complement. As before for a 1-byte storage unit, only 7 bits are available for representing the magnitude of a number, but, because there is now only one representation of zero, the decimal range representable is  -128 to +127.

Example 9.2

Find the one’s and two’s complement 8-bit binary representation of the following decimal numbers: 56 -56 73 119 27 -47

Method of Solution

Take first the decimal value of 56

Form 7-bit binary representation: 0111000

Reverse digits in this: 1000111

Add sign bit to left-hand end to form one’s complement: 01000111

Form two’s complement by adding one to one’s complement: 01000111C1D01001000

Take next the decimal value of -56

Form 7-bit binary representation: 0111000

Reverse digits in this: 1000111

Add sign bit to left-hand end to form one’s complement: 11000111

Form two’s complement by adding one to one’s complement: 11000111C1D11001000

We have therefore established the binary code in which the computer stores positive and negative integers (whole numbers). However, it is frequently necessary also to handle real numbers (those with fractional parts). These are most commonly stored using the floating-point representation.

The floating-point representation divides each memory storage unit (notionally, not physically) into three fields, known as the sign field, the exponent field and the mantissa field. The sign field is always 1 bit wide but there is no formal definition for the relative sizes of the other fields. However, a common subdivision of a 32-bit (4-byte) storage unit is to have a 7-bit exponent field and a 24-bit mantissa field, as shown in Figure 9.3.

The value contained in the storage unit is evaluated by multiplying the number in the mantissa field by 2 raised to the power of the number in the exponent field. Negative as well as positive exponents are obtained by biasing the exponent field by 64 (for a 7-bit field), such that a value of 64 is interpreted as an exponent of 0, a value of 65 as an exponent of 1, a value of 63 as an exponent of -1 etc. Suppose therefore that the sign bit field has a zero, the exponent field has a value of 0111110 (decimal 62) and the mantissa field has a value of 000000000000000001110111 (decimal 119), i.e. the contents of the storage unit are 00111110000000000000000001110111. The number stored is +119 × 2^-2. Changing the first (sign) bit to a 1 would change the number stored to -119 × 2^-2.

However, if a human being were asked to enter numbers in these binary forms, the procedure would be both highly tedious and also very prone to error. In consequence, simpler ways of entering binary numbers have been developed. Two such ways are to use octal and hexadecimal numbers, which are translated to binary numbers at the input–output interface to the computer.

Octal numbers use a base of 8 and consist of decimal digits in the range 0–7 that each represent 3 binary digits. Thus 8 octal digits represent a 24-bit binary number.

Hexadecimal numbers have a base of 16 and are used much more commonly than octal numbers. They use decimal digits in the range 0–9 and letters in the range A–F that each represent 4 binary digits. The decimal digits 0–9 translate directly to the decimal values 0–9 and the letters A–F translate respectively to the decimal values 10–15. A 24-bit binary number requires 6 hexadecimal digits to represent it. The following table shows the octal, hexadecimal and binary equivalents of decimal numbers in the range 0–15.