Exam practice

A computer is an electronic, programmable device that accepts data as input, processes it according to a set of instructions stored in its memory, gives out the result as meaningful information (output), and stores data and information for future use, all at very high speed.

IPOS is the cycle of four jobs every computer does:

• Input: data and instructions are entered through input devices (keyboard, mouse, microphone).
• Process: the CPU processes the data according to the program, turning it into information.
• Output: the information is shown through output devices (monitor, printer, speaker).
• Storage: data and information are saved on storage devices to use again; the CPU reads from storage and saves to it during the cycle.

For example, typed marks (input) are added and averaged by the CPU (process), the result is shown on the screen (output) and saved on the disk (storage).

GIGO stands for Garbage In, Garbage Out. A computer processes exactly the data it is given, so if the input is wrong, the output is wrong too, even though the computer worked correctly. For example, a mark typed as 9 instead of 90 gives a wrong result.

Any four of: speed (billions of instructions a second), accuracy (no mistakes of its own), diligence (works without tiring), versatility (does many kinds of work), storage (keeps huge amounts of data), automatic (works by itself once given the program).

Because it can do the same task again and again, for a very long time, without getting tired, bored or careless, and without losing speed or accuracy. The ten-thousandth calculation is as fast and as correct as the first, which is why computers are used for quality control and for controlling machines that run all day.

Any four, such as: education (online classes, results), banking (accounts, ATMs, mobile banking), medicine (CT scans, ECG, patient records), communication (email, video calls), entertainment (games, films), scientific research (weather forecasting, space), agriculture (precision farming, drones).

From smallest to largest: bit (a 0 or 1), nibble (4 bits), byte (8 bits), kilobyte (KB, 1024 bytes), megabyte (MB, 1024 KB), gigabyte (GB, 1024 MB), terabyte (TB, 1024 GB), petabyte (PB, 1024 TB), exabyte (EB, 1024 PB), zettabyte (ZB, 1024 EB) and yottabyte (YB, 1024 ZB).

A computer works on the IPOS cycle: it takes in data, processes it under the control of a stored program, gives out information and stores data for later use. Draw its block diagram (Figure 1.1): Input → Process (CPU) → Output, with Storage joined to the processor by arrows both ways.

• Input unit: input devices such as the keyboard, mouse, scanner and microphone take in data and instructions and turn them into the electrical signals (0s and 1s) the computer understands.
• Process (CPU): the central processing unit carries out the program’s instructions. Its control unit fetches each instruction and directs all the parts; its arithmetic logic unit does the calculations and comparisons; its registers hold the data being worked on.
• Output unit: output devices such as the monitor, printer and speaker turn the results back into a form people understand: text and pictures, a printed page, sound.
• Storage: primary memory (RAM) holds the program and data while they are being used; secondary storage (hard disk, SSD) keeps them permanently. The CPU reads from storage and writes results back.

So, for example, in working out a student’s result: marks typed (input) → added and graded by the CPU (process) → shown and printed (output) → saved on the disk (storage).

• Learning: students learn from websites, e-books, videos, interactive simulations and online courses, at their own pace, and search for information on any topic.
• Teaching: teachers prepare lessons, slides and worksheets and present them with multimedia (projectors, smart boards), which makes hard ideas easier to see.
• Online classes: video meetings and learning platforms let teaching go on at a distance, as during the COVID-19 lockdowns.
• Practice and tests: quizzes and practice questions that mark themselves and show what to study again.
• School administration: keeping students’ records and attendance, making fee bills and monthly statements, working out exam results and publishing them (SEE results are processed by computer and published online).
• Research and communication: students and teachers find up-to-date information and share work by email and cloud folders.

The CPU processes data: it fetches each instruction of a program from memory, decodes it, executes it (doing arithmetic and logical operations in its ALU, or moving data), and stores the result in memory or sends it to an output device. Its control unit also controls and coordinates all the other parts of the computer.

The motherboard is the main printed circuit board that connects every part of the computer. It holds the CPU and RAM in their sockets and slots, takes expansion cards and drives, and has the ports for outside devices; it carries power from the PSU to every part, and data between them along its buses; and its chipset manages the communication between the CPU, memory and the peripherals, so all the parts work together.

Cache memory is a small, very fast memory (SRAM) between the CPU and RAM, built into the processor. It keeps copies of the data and instructions the CPU uses most often. The CPU looks in the cache first; if what it needs is there, it gets it at once, without the much longer wait for RAM. Since programs use the same data and instructions again and again, most requests are found in the cache, so the CPU spends far less time waiting and the computer runs faster.

A bus is a set of parallel wires, or electrical pathways, that carries signals between the parts of a computer: between the CPU, memory and the input, output and storage devices. There are three kinds: the data bus (carries the data), the address bus (carries where it is going or coming from) and the control bus (carries control signals such as read and write).

An input device is used to enter raw data and instructions into the computer, turning them into the electrical signals the computer processes. Without input, a computer has nothing to work on. Examples: keyboard, mouse, microphone, scanner, webcam, joystick, touchscreen, touchpad.

A peripheral device is a hardware device connected to a computer from outside, through a port or wirelessly, such as a printer, scanner, webcam, external hard disk or speakers. Peripherals extend what the computer can do: they give it new ways to take in data (a scanner), give out results (a printer), store more (an external disk) and connect to networks (a Wi-Fi adapter), so one computer can do many more jobs.

A computer system is built from four kinds of unit, joined by buses on the motherboard. Draw it as in Figure 1.7: the CPU (control unit, ALU, registers) in the middle, joined to memory and to the input and output devices by the data, address and control buses.

• Input devices (keyboard, mouse, scanner, microphone) take in data and instructions and turn them into electrical signals.
• The CPU processes them. Its control unit fetches and decodes each instruction and directs every other part; its ALU does arithmetic and logical comparisons; its registers hold the data and results being worked on.
• Memory (primary): RAM holds the programs and data being used now (fast, volatile); ROM holds the BIOS that starts the computer (non-volatile); cache keeps the most used data close to the CPU, to make it faster.
• Storage (secondary): the hard disk or SSD keeps programs and files permanently; they are loaded into RAM when needed.
• Output devices (monitor, printer, speakers) turn the results into a form people understand.
• The buses (data, address and control) carry the data, the addresses and the control signals between these parts.

So data flows from input, through memory, to the CPU, and its results go back to memory, out to output devices and into storage: the IPOS cycle.

The operating system (OS) is the system software that manages all the computer’s hardware and software and lets the user work with it. It is loaded first when the computer starts, and every other program runs on top of it; without it the computer can’t be used. It is the coordinator between the user, the programs and the hardware. Its functions:

• Process management: it starts and stops programs and shares the CPU’s time between them, so music plays while you type and download.
• Memory management: it gives each program its space in RAM, keeps programs from overwriting each other, and frees the space when a program closes.
• File management: it organizes files in folders on the disks, and lets you create, open, copy, move and delete them (File Explorer in Windows).
• Device management: through device drivers, it controls the keyboard, printer, disks and screen, sending a document to the printer when you press Print.
• User interface: it gives the user a way to work with the computer: a GUI of windows, icons and menus (Windows, Android), or a CUI of typed commands (DOS, the Linux terminal).
• Security: user accounts, passwords and permissions keep each person’s files and the system safe.
• Error handling: it notices problems, such as a program that stops responding, and deals with them.

Examples: Windows, macOS, Linux (Ubuntu, Fedora), Android and iOS.

• Cache memory: a few megabytes of very fast SRAM inside the processor, holding copies of the most used data and instructions. Because the CPU finds most of what it needs there, it waits less for RAM. More cache (and a good hit rate) makes the CPU faster.
• RAM: the main working memory, a few gigabytes, holding the operating system and every open program and file. With enough RAM, many programs stay open and switch instantly; with too little, the operating system must keep moving data out to the much slower disk and back, and the computer slows down badly.
• Storage devices (HDD, SSD): keep everything permanently, and are where programs and files are loaded from. Their speed decides how fast the computer starts, programs open and files copy: an SSD (up to 3,500 MB/s and more) makes a computer feel many times quicker than a hard disk (about 150 MB/s). Their size decides how much can be kept.

Together they form a hierarchy: the fastest memory is the smallest and costliest, so each level holds what the one above it is likely to need. The CPU works at full speed only when the data arrives in time, so performance depends on all three, not just the processor.

The CPU, a single chip called a microprocessor in today’s computers, has three main parts, joined inside it by internal buses and kept in step by a clock (billions of ticks a second, measured in GHz):

• Control Unit (CU): it controls the whole computer. It fetches each instruction from memory, decodes it to find what must be done, and sends control signals to the ALU, the registers, memory and the input and output devices so that each does its part at the right moment. It manages the movement of data inside and outside the CPU.
• Arithmetic Logic Unit (ALU): it executes the calculations: arithmetic (add, subtract, multiply, divide) and logical operations (greater than, less than, equal to; AND, OR, NOT), which programs use to make decisions.
• Registers (memory unit): tiny, very fast memory places inside the CPU that hold the current instruction, its address, the data being worked on and the results. With the CPU’s cache, they let the CPU work without waiting for RAM.

The CPU is joined to memory and the devices by three buses: the data bus (data, both ways), the address bus (where, out from the CPU) and the control bus (signals such as read and write). It repeats the machine cycle, fetch, decode, execute and store, for every instruction, and handles interrupts from devices that need attention.

Open source means the software’s source code is public: anyone may see it, study it, change it, use it and share it, under its license. It is usually free, and is often made by a community of volunteers and companies together. Examples: Linux, LibreOffice, Firefox, Python.

System software is the software that runs and manages the computer itself. It controls the hardware, manages memory, files and devices, and gives a platform on which application software runs, acting as the bridge between the user, the applications and the hardware. Without it the computer can’t work. Examples: operating systems (Windows, Linux), language processors, utility software, device drivers and the BIOS.

Application software is software made to do particular tasks for the user, running on top of system software. It is either packaged (ready-made for everyone) or customized (made for one organization). Examples: word processors (Microsoft Word), spreadsheets (Excel), presentation programs (PowerPoint), web browsers (Chrome, Firefox), media players (VLC), video meetings (Zoom), photo editors (Photoshop), and school or hospital management software.

Utility software maintains the computer and its data, and keeps it running smoothly and safely: it finds and removes viruses, backs up and recovers files, compresses files, manages and cleans disks. Examples: antivirus (Avira, Windows Security), backup software, file compression (WinZip, WinRAR, 7-Zip), disk defragmenter, disk cleanup and CCleaner.

Software is of two types:

1. System software runs and manages the computer and is the platform for other software.

• Operating system: manages the hardware, memory, files, devices and running programs, and gives the user an interface (Windows, Linux, Android).
• Language processors: translate programs into machine language: assembler (assembly language), compiler (a whole program at once) and interpreter (a line at a time).
• Utility software: maintains the computer: antivirus, backup, file compression, disk defragmenter.
• Device drivers: let the operating system control a particular device, such as a printer or a sound card.
• Firmware (BIOS, UEFI): starts the computer and checks its hardware.

2. Application software does the user’s own tasks.

• Packaged (general purpose): ready-made for everyone: word processors, spreadsheets, browsers, media players, photo editors.
• Customized (tailored): made for the particular needs of one organization: SEE result processing, hospital management, banking, airline reservations.

System software manages the computer itself; application software does the user’s tasks on top of it.

Their roles: system software is the foundation that makes the hardware usable and gives programs a place to run; application software is what makes the computer useful to the user. A word processor (application) asks the operating system (system) to save a file, and the operating system uses the disk’s driver to write it.

Utility software is system software that does particular jobs to maintain, protect and tune the computer and its data, rather than the user’s own work. Its roles:

• Protection: antivirus software finds and removes viruses and other malware, keeping data safe and the computer working properly.
• Backup and recovery: backup software copies files to another disk or the cloud, and recovery tools bring back lost or deleted files.
• Speed: a disk defragmenter gathers the scattered pieces of files on a hard disk so they load faster; cleanup tools (such as CCleaner) remove temporary files and the browsing history, freeing space.
• Space and sharing: file compression (WinZip, WinRAR, 7-Zip) packs files smaller, to save space and send them faster.
• Disk management: tools to format, divide (partition) and check disks for errors.

So utility software keeps the computer fast, tidy, safe and reliable.