Computer Systems Salihu Ibrahim Dasuki (PhD) CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Aim ◦ Introduce the concepts of computer systems  Purpose ◦ Set the foundation for topics to come later, including Computer Networks, the Internet and the World Wide Web CSC102 INTRODUCTION TO COMPUTER SCIENCE

 A “system” comprises a set of components that work together to perform a task  A “computer system” comprises a processor, memory, external storage systems, interface devices and software to provide a tool that humans can use to solve problems CSC102 INTRODUCTION TO COMPUTER SCIENCE

 At the heart of a computer system is: ◦ The central processing unit (CPU), which is responsible for executing computer software ◦ Main Memory, which holds:  data to be processed by software/programs, and  the computer software /programs CSC102 INTRODUCTION TO COMPUTER SCIENCE

 The arithmetic logic unit (ALU) performs arithmetic and logic operations  Arithmetic operations include addition, subtraction, division, multiplication  Logical operations include AND, OR, and NOT ◦ A AND B is TRUE if A is true and B is true ◦ A OR B is TRUE if A is true or B is true ◦ NOT A is TRUE if A is False CSC102 INTRODUCTION TO COMPUTER SCIENCE

 The control unit is central to the operation of a computer. It processes computer instructions, which are of three types: ◦ Data Transfer – move data around ◦ Arithmetic/Logic – perform arithmetic or logical operations, for which it calls on the ALU ◦ Conditional – determine which program instruction to execute next CSC102 INTRODUCTION TO COMPUTER SCIENCE

 The control unit is central to the operation of a computer. It processes computer instructions, which are of three types: ◦ Data Transfer – move data around ◦ Arithmetic/Logic – perform arithmetic or logical operations, for which it calls on the ALU ◦ Conditional – determine which program instruction to execute next CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Here is a simple program to control the temperature in a room: 1.Get the temperature of the room 2.If the temperature is below 20oC, Go to step 5 3.TURN ON the air conditioning 4.Go To 1 5.TURN OFF the air conditioning 6.Go To 1  Statement 2 is a conditional instruction – the instruction executed after 2 is either 3 or 5 CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Everything in a computer system, including data and instructions, is a number, represented in the binary number system  Why  Because the underlying electronics – the hardware – is based on electronic switches, which can be in one of two states, On or Off  In the computer’s representation of the binary number system, On represents 1 (one) and Off represents 0 (zero) CSC102 INTRODUCTION TO COMPUTER SCIENCE

 We are familiar with the decimal number system – a number system in base 10 ◦ Each number is represented by: ◦ How many units it has ◦ How many tens it has ◦ How many hundreds it has ◦ How many thousands, ◦ –etc CSC102 INTRODUCTION TO COMPUTER SCIENCE

 For example, the number 241 has: ◦ Two hundreds ◦ Four tens ◦ One unit  From right to left we have units, tens, hundred columns: CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Another way of representing the columns  In the binary number system the columns are: 20, 21, 22, 23, and so forth  CSC102 INTRODUCTION TO COMPUTER SCIENCE

 In the Binary number system the representation of decimal 241 is :  Decimal has numerals 0,1,2,3,4,5,6,7,8,9  Binary has numerals 0 and 1 CSC102 INTRODUCTION TO COMPUTER SCIENCE

 We said: Everything in a computer system, including data and instructions, is a number  How a computer program uses the number determines what the number represents  Compare: ◦ If a stone is used to hammer a nail into wood, it is a hammer ◦ If the stone is used to hold a door open, it is a door stop ◦ If it is used to keep papers from CSC102 INTRODUCTION TO COMPUTER SCIENCE

 How we use the number: ◦ As an integer – add, subtract, divide, multiply ◦ As a character, e.g. ‘a’ – compare with other characters and form words, e.g. ‘account’ ◦ As an address in memory – use it to retrieve information from memory CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Representing everything as numbers has some benefits:  Question: How do you compare two characters?  Answer: Subtract their number representations ◦ If the result is 0 (zero) they are the same ◦ If the result is not 0, they are different CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Question: How would you compare two words? E.g. “this” and “that”  Answer: Count the number of characters in each word ◦ If the count is different, then the words are different ◦ If the count is the same, then compare the two words character by character. If one of the comparisons is not 0, the words are different CSC102 INTRODUCTION TO COMPUTER SCIENCE

 We have discussed the CPU and Main Memory, which comprise the heart of a Computer System  A computer system comprises other components to make it useful: ◦ Mass Storage – for retaining the contents of memory ◦ External Devices – including printers, DVD drives, keyboards, monitor CSC102 INTRODUCTION TO COMPUTER SCIENCE

 The contents of Main Memory are lost when the computer is switched off  Mass storage systems retain their content and offer large storage capacities  Examples include Magnetic Disks, CDs, DVDs, Flash drives  Disadvantage – they require significantly more time to store and retrieve data CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Flash memory is currently a popular system for off-line storage  Flash drives use flash memory technology and USB interfaces – they come in sizes of GigaBytes – 1, 2, 4, 8, 16,  SD (Secure Digital) Cards also use flash technology – used in electronic devices, including digital cameras, smartphones, music players, and more CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Communication with external devices is handled by controllers  The controller mediates between the computer and the external device – e.g., translating data into the correct format  Some controllers can handle a range of devices. For example, ◦ USB – Universal Serial Bus ◦ FireWire – developed by Apple CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Communication rates between the computer and its external devices is measured in bits per second (bps)  Kbps – Kilobits per second  Mbps – Megabits per second  Gbps – Gigabits per second  E.g. USB communicates in numbers of Mbps CSC102 INTRODUCTION TO COMPUTER SCIENCE

 Question: What is a ‘bit’?  Answer: It is one column of a binary number  For example, the binary representation of the decimal number 241 has 8 bits  Question: What is a ‘byte’?  Answer: A byte is 8 bits CSC102 INTRODUCTION TO COMPUTER SCIENCE

 The computer’s activities, notably its CPU, are coordinated by pulses generated by a clock  Clock speeds are measured in hertz (Hz)  1 Hz equals 1 cycle per second  Examples: ◦ Desktop computer speeds are now in the GHz range ◦ 1 GHz equals 1000 MHz  Early desktop computers operated in KHz CSC102 INTRODUCTION TO COMPUTER SCIENCE

 At the core of a computer system is the Central Processor Unit (CPU) and Main Memory  Mass Storage and External Devices provide additional system components to make a computer system useful  The binary number system is central to the operation of a computer system CSC102 INTRODUCTION TO COMPUTER SCIENCE