Inside the Computer: Hardware, Programs and the Operating System

A computer system brings hardware and software together. The hardware is the physical equipment. Software is the programs that tell that equipment what to do.

Parts and instructions

A keyboard, screen and processor are hardware. A drawing program is software. You can touch a disc that holds a game, but the instructions stored on it are software. The disc itself is hardware.

Neither side does the whole job alone. A laptop without an operating system can power on. It still needs software before it can open a browser or game. A program also needs hardware to carry out its instructions.

Follow the data

Four jobs help describe a computer system:

• Input: data enters, such as a tap or a temperature reading.
• Processing: the processor follows instructions to work on the data.
• Output: a result leaves, such as sound, an image or movement.
• Storage: data and programs are kept for later use.

These jobs connect, but they are not always four steps in a line. A running program may read stored data many times. It may produce output before saving anything.

Consider sending a voice message. The microphone picks up your voice. Audio circuits turn the sound into digital data. The processor runs instructions that compress the recording into a smaller file. Storage keeps the message in the chat history. The phone sends it wirelessly, and your friend's speaker plays it.

The touchscreen also supplies input when you tap "Send". One task can involve several inputs and outputs.

One machine, many jobs

Inside a digital computer, instructions and data use binary patterns. A bit is one binary digit, 0 or 1. Eight bits make a byte. Photos, sounds, text and program instructions can all be represented this way.

A general-purpose computer can load different programs. The same laptop can edit a picture, play music and run a game. A smartphone is general-purpose too. Its small size does not change how it works.

An embedded system sits inside another device and controls a particular job. A washing machine's computer follows a wash program. It takes in sensor readings and controls physical parts.

Devices around the computer

A peripheral adds input, output or storage to the main computer. A webcam supplies images, a printer produces pages, and an external drive holds files. Some peripherals are built into a laptop's case.

A separate monitor displays results. The CPU doing the processing is inside the computer case. An all-in-one desktop puts the computer's parts and display together in one enclosure.

An input device supplies data to a computer. An output device presents results or makes something happen. Judge the direction from the computer's point of view.

Getting data in

A keyboard supplies key presses. A mouse or trackpad supplies movement and clicks. Microphones capture sound; cameras capture images. Scanners turn paper pages into images. Barcode and QR code readers capture codes without typing each character.

A sensor measures a physical quantity automatically. A temperature sensor measures heat levels. A moisture sensor measures how wet something is. An accelerometer detects changes in motion and orientation, helping a phone rotate its display.

A graphics tablet captures pen movements for drawing. A 3D scanner captures an object's shape for a computer-aided design (CAD) project. Each supplies a different kind of data.

Sending results out

Screens and projectors show images. Speakers and headphones produce sound. Printers produce pages. A plotter draws designs, while a 3D printer or laser cutter produces a physical object from a digital design.

An actuator is an output device that creates a physical action. A motor turns, a pump moves water, and a valve opens or closes. In an automatic watering system, a moisture sensor supplies input. The program checks the reading and sends output to a pump.

Some devices combine both directions. A touchscreen displays buttons as output and detects your touch as input. A headset contains output speakers and an input microphone. Giving the whole device only one label misses half its job.

Match the task and the person

Start with what must go in and what must come out. Then consider speed, accuracy, cost and where the equipment will be used.

At a library checkout, a barcode scanner reads long book codes quickly. A card reader identifies the borrower. A touchscreen shows choices, and a small printer supplies a receipt. Typing every book code on a full keyboard would be slower and easier to get wrong.

The person matters too. Someone who cannot use a keyboard may use speech recognition and a microphone. Eye tracking or a switch can help someone with limited movement. A screen reader is software that sends text to speakers or a braille display. Large keys can make accurate typing easier.

Making the connection

Peripherals connect through ports or wirelessly. USB-A and USB-C are common USB port shapes. HDMI and DisplayPort connect screens. Bluetooth can connect headphones or a mouse; Wi-Fi can connect a printer. A device driver lets the operating system control a particular device.

A printer receives data from the computer, so printing is output. Receiving something itself does not make the printer an input device: the direction is measured from the computer.

The parts inside a computer share work. The motherboard, its main circuit board, connects them. Smaller devices contain the same kinds of parts in a tighter space.

Processing, memory and storage

The central processing unit (CPU) carries out program instructions. On many desktops it fits into a socket on the motherboard. RAM holds the programs and data currently in use. Desktop RAM often comes on small boards called modules or sticks.

A storage drive keeps files and programs without power. An SSD uses chips and may fit directly into a motherboard slot. An HDD uses spinning disks and connects by cable. Some SSDs also connect by cable.

Copper tracks on the motherboard carry signals between parts. These communication paths form buses. Sockets, memory slots and drive connectors hold or connect the components.

Images and connections

The graphics processing unit (GPU) helps create images for the screen. Integrated graphics are built into the processor chip. A dedicated graphics card is separate and has its own memory.

A network adapter connects the computer to a network. It may provide an Ethernet connection or wireless connections such as Wi-Fi. It moves data between systems; it does not replace the CPU's instruction processing.

Power and cooling

A desktop's power supply unit (PSU) changes wall-socket AC electricity into low-voltage DC electricity. The internal parts use that DC power. A laptop has an external charger and an internal rechargeable battery.

Working chips give off heat. A heat sink spreads that heat into the air. Fans move warm air away, and some systems use liquid cooling. If a processor gets too hot, it reduces its speed. It can shut down if the temperature keeps rising. Clear air vents help cooling work.

Before opening a desktop, shut it down and unplug it. Have an adult supervise, and use anti-static protection to protect the chips. Never open a power supply unit or a damaged battery.

Follow one key press

Picture a desktop with a USB keyboard and an HDMI monitor:

1. The keyboard sends the key press through USB to the computer.
2. The CPU follows the word processor's instructions held in RAM.
3. The GPU prepares an image containing the new letter.
4. The image travels to the monitor through HDMI.
5. The monitor displays the letter.

The letter appearing does not mean it has been saved to the drive. Saving needs a separate storage action.

Inside a phone

A system on a chip (SoC) combines processor cores, graphics and other parts on one chip. This saves space in phones and tablets. In phones and many thin laptops, RAM is soldered onto the board. Those memory chips cannot simply be replaced with larger sticks.

The CPU follows instructions in small steps. It calculates, compares values, moves data and decides what comes next. Calling it a brain is a useful picture, but it follows instructions instead of thinking for itself.

Directing and calculating

The control unit (CU) gets an instruction and works out what it means. It sends control signals to carry out the instruction. The arithmetic logic unit (ALU) does arithmetic and logical comparisons. Adding two scores and checking whether a score beats a target are ALU tasks.

A program's large task is built from many small instructions. Displaying a video involves far more than one instruction, even though it looks like one action to the user.

Clock speed

A processor's clock provides regular electrical pulses. One pulse marks a clock cycle. Clock speed counts cycles per second, measured in hertz. One gigahertz, or GHz, means one billion cycles per second.

At 3.2 GHz, a processor has 3,200,000,000 cycles each second. One millisecond is one thousandth of a second:

That is 3,200,000 cycles in one millisecond. It is not necessarily that many completed instructions. An instruction may need several cycles, and a processor may finish several instructions in one cycle.

Compare clock speeds between similar processor designs. A newer 3.0 GHz design can outperform an older 3.5 GHz design because it does more useful work per cycle.

Cores share suitable work

A core is a processing unit inside the CPU. Four cores can work on four instruction streams at once. This helps when different programs need attention or one task can be divided.

For example, a video editor can split an export into eight parts. Eight cores can work on those parts together. A task written to use only one core does not spread itself across eight cores. Some steps also have to wait for earlier results. Doubling the core count therefore does not guarantee half the time.

Keep useful data nearby

Cache is small, very fast memory inside the processor. It holds copies of recently or often used instructions and data. Reading a cached value reduces the time spent waiting for RAM.

A machine might have 12 MB of cache and 16 GB of RAM. Cache is much smaller, so it cannot replace RAM or hold a whole collection of files.

Give repeated calculations to a GPU

A GPU has many simpler cores. They suit large numbers of similar calculations, such as drawing a 3D scene, processing video or training an AI model. CPU cores handle varied, step-by-step work. The best performance depends on matching the work to the parts that can do it.

A computer needs working space and startup instructions. RAM provides the main working space. ROM keeps firmware available even while the power is off. These are forms of main memory; cache is separate memory inside the processor.

RAM holds current work

Random access memory (RAM) holds the operating system, open programs and current data. The processor can read and change its contents. "Random access" means it can reach locations directly, without reading through everything before them.

RAM is volatile: its contents disappear when power is lost. While you type an essay, new text is held in RAM. Saving writes a copy to secondary storage. If power fails, saved text remains, but changes made since the last save can be lost. Autosave reduces that gap by saving regularly.

ROM keeps firmware

Read-only memory (ROM) is non-volatile, so it keeps its contents without power. Startup firmware gives the processor its first instructions. On many computers this firmware is called BIOS or UEFI.

During ordinary use, programs do not rewrite this startup code. Modern computers often store it in flash memory. A special firmware update can replace that code. Non-volatile does not mean a chip can never be changed.

Booting a computer

The startup process is called booting:

1. Power comes on, and the processor begins running firmware.
2. The firmware checks key hardware, including memory.
3. It finds the operating system's startup code on the drive.
4. Startup software loads the operating system into RAM.
5. The operating system takes control and presents the sign-in screen.

Apps stored on the drive are also loaded into RAM when opened. Firmware can run directly from ROM or flash.

When working space runs short

Suppose a laptop has 8 GB of RAM. The operating system needs 3 GB, browser tabs need 3 GB, and a photo editor needs 2.5 GB.

That is 0.5 GB beyond the available RAM. The operating system can move some less-needed data onto the drive, then bring it back when needed. This use of drive space is part of virtual memory. Accessing it is slower than using RAM, so repeated movement can cause lag.

With 16 GB of RAM, this workload fits. Extra RAM helps when working space is the problem. If programs already fit comfortably, adding more does little for their speed. A phone's advertised 128 GB usually describes saved-file storage, not RAM.

Secondary storage keeps files and programs for later, including without power. It offers more space for the price than RAM but is slower. Programs on a drive are loaded into RAM to run.

Compare storage types

SSDs, USB flash drives and SD cards use solid-state storage. Magnetic tape holds very large backups in data centers. A single-layer CD holds about 700 MB, a DVD 4.7 GB, and a Blu-ray disc 25 GB.

Cloud storage is storage on remote servers reached over the internet. It helps with sharing and access from different devices. Files still sit on physical drives in data centers. Access depends on connectivity, the provider and account security; larger amounts often cost money.

Choose by capacity, read/write speed, portability, durability, reliability and cost per GB. A photographer might use a small SD card outdoors, an external HDD for a large collection, and cloud storage for a copy elsewhere.

Keep the units consistent

One byte contains 8 bits. Decimal storage units grow by 1,000:

• 1 KB is 1,000 bytes.
• 1 MB is 1,000 KB.
• 1 GB is 1,000 MB.
• 1 TB is 1,000 GB.

Binary units grow by 1,024. Their precise names are KiB, MiB, GiB and TiB. Some courses use KB, MB and GB for these steps too. Check the convention before calculating and use it throughout.

Work out how many files fit

A 128 GB phone uses 40 GB for its operating system and apps. Each additional photo uses about 4 MB. Using decimal units:

1. Find free space: $128-40=88\text{ GB}$.
2. Convert: $88\times \mathrm{1,000}=\mathrm{88,000}\text{ MB}$.
3. Divide by one file's size: $\mathrm{88,000}÷4=\mathrm{22,000}$ photos.

With a question using 1,024 per step, the numbers would give 90,112 MB and 22,528 photos. The unit labels then describe different byte amounts; changing labels does not create physical space.

Work out transfer time

At a steady 150 MB per second, reading 12 GB of video takes:

At 500 MB per second, the same amount takes 24 seconds.

An SSD and RAM both use chips, but they keep different things. SSD flash keeps saved files without power. RAM loses its working contents.

A program is a sequence of instructions. A stored-program computer holds instructions in memory as binary patterns, just as it holds data. The processor reads those instructions and carries them out.

Change the program, change the job

The hardware does not need rewiring whenever you choose a new task. Load a word processor to write a report. Load a game to control a character. The instructions change what the same hardware does.

This makes the computer general-purpose. Its programs can be replaced or updated. A software update changes stored instructions, giving the hardware a different sequence to follow.

An embedded system also holds a program. Its firmware is stored in ROM or flash and starts when power comes on. The device usually carries out a dedicated job, such as controlling a wash cycle.

Opening an app

Imagine opening a drawing app saved on an SSD:

1. The operating system locates the program file in secondary storage.
2. It loads the instructions and needed data into RAM.
3. The processor reads the instructions and starts carrying them out.
4. The running app can respond to input, such as a pen movement.

A large app needs time to load because data has to move from storage to working memory. Each open app also takes up RAM. The program file can remain on the SSD after you close the app.

Instructions and values use bits

A memory location holds a pattern of bits. That pattern does not carry a visible label saying "instruction" or "data". Its role depends on how the processor uses it.

When the processor fetches the next instruction, it treats those bits as a command. When an instruction asks for a stored value, the bits are used as data. In a von Neumann architecture, instructions and data share one memory.

The program is not permanently built into the CPU. The CPU holds the values and instructions it is working with, and its cache holds useful copies. Apps are kept in secondary storage and loaded into RAM. Firmware can run from ROM or flash.

How the idea developed

ENIAC, completed in 1945, originally used switches and cables to set up programs. Changing its task could take days. In 1945, John von Neumann described a design that held programs in memory. Alan Turing developed his ACE design in 1945–46.

On June 21, 1948, the Manchester "Baby" ran a program held in electronic memory. It demonstrated that instructions could be stored and read electronically along with data.

The ability to load instructions also explains a software risk. Harmful programs can ask the hardware to do harmful work. Operating-system controls help limit what software may run and what it can access.

A processor directly runs machine code: simple binary instructions. A large task becomes many small actions, such as loading a value, adding, storing or jumping.

Addresses locate instructions and data

Think of memory as numbered locations. A location's number is its address. The number at an address is its contents. Address 10 can hold 7; the address and the stored value are different numbers.

A processor follows its own instruction set, the commands its design supports. Different processor families can use different sets. For example, x86-64 and ARM machine code differ. Code for one needs a suitable translation system to run on the other.

Fetch, decode, execute

The processor repeats three broad stages:

1. Fetch: read the next instruction from memory.
2. Decode: the control unit works out the instruction's meaning.
3. Execute: carry out the action, such as adding or moving data.

The ALU performs calculations and comparisons. The clock keeps processor activity in time. A clock cycle is not the same as a complete fetch–decode–execute cycle.

Follow a small program

Use this made-up instruction set: LOAD takes a value from an address, ADD adds a value from an address, STORE writes the working result, and STOP ends the program.

First, the processor fetches LOAD 10. Decoding identifies a load action and address 10. Executing reads the 7 stored there. The working value is now 7.

Next, it fetches ADD 11. Address 11 holds 5, so the ALU adds 5 to the working value:

Then it fetches STORE 12. This writes the working value, 12, into memory address 12. The number after STORE names the destination. It is not a number to add.

Finally, it fetches STOP, which ends this program. Address 12 now contains the answer. Storing in RAM does not by itself save a file to a drive.

Change the next instruction

Normally, the next instruction follows the current one. A jump names a different next address. A conditional jump changes the route only when a test is met. Jumping back can repeat instructions, forming a loop.

Cache helps by keeping copies of useful instructions and data near the CPU. Several cores can each work through an instruction stream. Neither feature changes the need for instructions the processor can actually run. Python source code must be handled by software that connects it to machine instructions.

People usually write code in languages that are easier to read than binary. The processor still needs its own machine instructions. Translators bridge the gap.

High and low levels

A high-level language uses words and symbols suited to people. Python, JavaScript, Java and Scratch are examples. A line such as total = price * 3 describes a useful action without listing every hardware step.

A low-level language stays close to processor instructions. Machine code uses binary. Assembly language uses short command names, or mnemonics. An assembler translates assembly into machine code. One assembly instruction usually matches one machine instruction.

The high-level line above needs several small actions: load the price, multiply it by three, then store the total. High-level means further from hardware detail, not harder to learn.

Compile before running

A compiler translates a program before it runs. In the simple compiled model, it produces an executable machine-code file. That file can run again without repeating the compilation or needing the compiler on the user's computer.

A compiler reports errors it finds while translating. A program can compile successfully and still crash during use or calculate the wrong result because its logic is wrong.

Interpret during running

An interpreter carries out a program's statements during execution. It does not first have to produce a separate machine-code executable for the user. This makes testing small changes convenient.

Python commonly uses an interpreter that first turns source code into bytecode, an intermediate form. The interpreter then carries out the bytecode. Syntax errors can be found before execution; errors such as dividing by zero appear when that action is reached.

Compiled machine code is tied to its target processor and operating system. High-level source can be used across systems when suitable tools and features are available. For example, print("Hello") can run on an x86-64 laptop or an ARM Raspberry Pi with a suitable Python interpreter. Copying a Windows executable to an ARM phone does not give it the right environment.

Layers hide unnecessary detail

Abstraction hides details that a task does not need. A user chooses "Save" in an app. The app asks the operating system for a file service. The operating system works with the hardware. The user does not need to manage the drive's electrical signals.

The same idea lets a programmer write a multiplication without arranging circuits. Each layer provides a simpler way to use the layer below.

Software can be grouped by its job. Application software helps a person carry out a task. System software runs or maintains the computer and supports applications.

Sort by purpose

A word processor helps write a report. A browser helps use websites. A video editor helps make a film. These are applications, even when they come installed with the computer.

Operating systems, utilities, drivers and firmware are system software. The operating system manages the computer. A utility performs maintenance or protection. A driver lets the operating system control a device. Firmware supplies instructions built into a device, including startup code.

For example, Windows 11 is an operating system. An antivirus program is a utility. A printer driver supports a device. A calculator bundled with an operating system is still an application: its job is calculating for the user.

Different kinds of application

General-purpose applications support many everyday tasks. A spreadsheet can track a budget, calculate experiment results or make a chart. Word processors, browsers, email and presentation programs also belong here.

Specialist applications focus on a particular kind of work. CAD helps create designs. Video editors, music production software and accounting software each support a narrower field. A game is another specialist application.

This use of "general-purpose" describes the software's range of tasks. A general-purpose computer can run both broad and specialist applications.

Ready-made or custom

Off-the-shelf software is made for many customers. It is ready to obtain and use. Bespoke, or custom, software is written for one organization's needs, such as a hospital's appointment system.

Custom software takes time and usually costs more to develop. It can match a special process closely. A ready-made package is useful when its features already meet the need. These labels describe who software is made for, not whether it is system software or an application.

Where apps run

An application may be installed on a laptop, downloaded to a phone or opened inside a browser. A web app, such as an online document editor, runs through a browser. Its data may be stored on the device, on servers or in both places.

"App" is short for application. It applies to desktop and web programs too.

Follow a request through the layers

When you choose "Print" in a word processor, the application asks the operating system for printing services. The operating system uses the printer's driver to control that model. The physical printer then produces the page.

The application does not need separate instructions for every printer's electrical details. The system software supplies that shared service. The layers connect a person's task to the hardware that completes it.

The operating system (OS) manages hardware and supports running programs. Much of its work happens out of sight. Windows, macOS, Linux, Android, iOS, iPadOS and ChromeOS are examples used on different devices.

Give programs processor time

A running task is a process. Many processes can need the CPU: a browser, music playback and a background print task, for example.

The OS scheduler decides which process gets processor time next. On one core, it switches rapidly among tasks. This is multitasking. Several cores can also execute work at the same time. The OS coordinates the shared resources in both cases.

A window does not show every running process. Some apps continue in the background after their window closes. Task Manager on Windows and Activity Monitor on macOS can show running processes.

Manage working memory

Each running program needs RAM. The OS assigns memory to programs and protects their separate working spaces. This helps stop an ordinary app from changing another app's data. When a program closes, its memory can be reused.

If RAM is under pressure, the OS can move less-needed data to virtual memory on the drive. Repeated movement between the drive and RAM slows the work. Memory management keeps track of these locations; it does not turn drive storage into equally fast RAM.

Control devices through drivers

A device driver handles the commands needed by a particular device. The application makes a general request, such as printing. The OS and driver turn it into instructions the device can use.

Plug and play helps the OS find or install a suitable driver for a new device. A device without the right driver may fail or offer only basic features. A cable connection alone does not supply the software needed to control it.

The OS also directs input to the right application. A key press goes to the window receiving typing. When a device needs attention, it can signal the system.

Keep printing and music moving

Suppose you print a report while listening to music:

1. The word processor requests printing from the OS.
2. The report enters a print queue, waiting its turn.
3. The OS sends the job through the printer driver.
4. The scheduler continues sharing CPU time with music and typing.
5. Memory management keeps track of each program's working data.

The printer can take time to produce the pages while you keep working. The queue holds waiting jobs in order.

Does every computer need a full OS?

Phones and laptops run many applications sharing hardware, so they use an operating system. A simple embedded controller can run one firmware program that controls its hardware directly. Its stored program still makes it a computer, even without a full desktop-style OS.

The operating system provides ways to control a computer and manage its files. It also controls access to data and devices. These services help different people use the same hardware.

Choose an interface

A graphical user interface (GUI) uses visual controls such as windows, icons, menus and buttons. A pointer can select them on a desktop. Touch interfaces use gestures such as tapping, swiping and pinching. Desktop controls are sometimes called WIMP: windows, icons, menus and pointer.

A command-line interface accepts typed commands. For example, dir on Windows or ls on macOS and Linux lists files. It can be quick for an experienced user but takes practice. A voice interface accepts spoken commands.

Accessibility settings include larger text, magnification, screen readers, high contrast, captions and dictation. Choosing suitable controls can make the computer usable for someone with different needs.

Find a file's place

Folders form a tree, with folders inside other folders. A path identifies a file's place in that tree. For example, /Users/sam/Documents/Science/report.docx identifies a document within Sam's Science folder.

The OS lets users create, rename, search, copy, move and delete files. Moving a file within the same drive usually changes its folder entry without making another full copy. Copying makes an additional file and takes more storage space.

Deleted desktop files often go to the Recycle Bin or Trash. They can usually be restored before the bin is emptied. This recovery area is not a separate backup.

A file extension indicates the file type and helps choose an app. Examples include .docx for documents, .jpg for photos, .mp3 for audio and .py for Python source. Renaming photo.jpg to photo.mp3 changes its name, not its photo data.

Control who can do what

A user account gives someone a sign-in and personal settings. Passwords, PINs and biometrics help check identity. Permissions decide what files a user may read or change, or what an app may access.

An administrator can make system-wide changes. A standard account has more limited power, reducing damage from mistakes or harmful programs. It can still change personal settings and, on some systems, install apps for itself.

Give apps access that fits their job. A flashlight app does not need contacts to light the screen. Install OS security updates promptly to fix weaknesses.

Sync and backup do different jobs

A cloud account can keep a folder synchronized across devices. When files change or are deleted, those changes can spread too. A backup is a separate recoverable copy. Sync helps keep current work aligned; backup helps recover earlier work after a mistake.

A utility program performs a maintenance or protection job. Some utilities come with the operating system. Others are installed separately. Choose each utility by the problem it solves.

Detect harmful software

Anti-malware, also called antivirus software, checks files and running programs. It looks for known malware patterns and suspicious behavior. It can quarantine suspicious files or remove threats. Regular updates help it recognize new threats.

No detector catches every threat. Keep it alongside updates, careful downloading, strong passwords and backups.

Keep recoverable copies

A full backup copies all selected data. An incremental backup copies changes since the previous backup. It needs less time and space when only a little changes.

A family has 200 GB of files. About 2 GB changes each week. Four full weekly backups use:

One full backup plus three incremental backups uses:

To restore the latest state from that incremental sequence, keep the full backup and the needed later increments. A folder on the same drive is a weak backup: drive failure can destroy both copies.

The 3-2-1 rule suggests three copies, on two kinds of storage, with one copy elsewhere. Include the working copy when counting the three. An off-site copy helps after theft or fire.

Make files smaller

Compression reduces file size. Lossless compression, such as ZIP, lets you recover the exact original. Lossy compression, used in formats such as JPEG and MP3, removes some detail permanently.

A 50 MB text folder zipped to 30 MB saves 20 MB:

The compressed folder is 40% smaller. Already compressed photos often shrink very little in a ZIP file.

Clean up and maintain drives

Disk cleanup removes unneeded files, such as temporary files, to free storage. Check downloads before deleting them: they may contain work you want.

On an HDD, pieces of a file can become scattered across the disk. Defragmentation brings them together so the read head travels less. An SSD has no moving read head and does not need this treatment. Modern operating systems choose suitable drive maintenance automatically.

Protect access and repair weaknesses

Encryption scrambles data so a key is needed to read it. It helps protect files on a stolen device when the encrypted data is locked. Someone who already has access to an unlocked account may still read its files.

Updates fix bugs and security weaknesses. A device that no longer receives security fixes becomes harder to keep protected. These tools solve different problems: encryption limits reading, while backup restores lost or damaged data.

Computers range from devices worn on a wrist to machines filling a data center. Size and appearance do not decide whether something is a computer. Look for processing, memory and a stored program.

Personal devices

A desktop stays in one place and often offers useful upgrade options. A laptop brings a keyboard, screen and battery together for portable work. Tablets use touchscreens and are light to carry. Smartphones add pocket-size access, cameras and sensors. Wearables, such as smartwatches, can measure steps and heart rate.

A smartphone runs an operating system and chosen applications. It is a general-purpose computer, even without a separate keyboard or mouse. Price, power and battery life vary across device designs.

Computers serving other computers

A server provides services over a network. It may store shared files, deliver websites or run an online app. Servers commonly operate continuously in data centers.

A mainframe handles very large numbers of transactions, such as bank records. A supercomputer combines large numbers of processors for demanding calculations, such as weather forecasts. They meet different needs: processing many transactions is not the same workload as modeling the atmosphere.

Embedded control

An embedded system is built into a larger product to control a dedicated task. It often uses a microcontroller, a chip combining a processor, memory and input/output connections. Firmware starts when the device powers on.

Consider a washing machine:

• Buttons select a wash program.
• Door, temperature and water-level sensors supply readings.
• The microcontroller checks readings and follows the stored program.
• Valves, a heater, motor and pump carry out physical actions.
• The display shows time or status.

The machine does not need to offer a browser or let users install games. Its dedicated program still takes input and produces output.

Computers in industry

Medical scanners collect data that computers turn into body images. Farm systems use moisture readings to control irrigation and GPS to guide tractors. Factory robots combine sensors with motors to assemble products. Cars contain many electronic controllers, each helping manage a task.

Connected devices and the cloud

Internet of Things (IoT) devices connect everyday objects to online services. A smart thermostat can send readings and accept remote settings. This enables automation, but also creates privacy and security questions. Default passwords and missing updates leave devices more exposed. Some features can stop if the provider closes its service.

The cloud uses remote servers for processing and storage. A browser editor splits work between those servers and the user's device. The device still processes input and displays results. A low-cost laptop can use powerful online services, but the work it can do offline depends on the apps and locally available files.

A specification lists a computer's features. Useful choices connect those features to a user's tasks and setting. The largest number or highest price does not decide the answer.

Turn tasks into needs

First, list the work and where it happens. Many open programs need enough RAM. Large media collections need storage capacity. Video editing and some 3D or AI work benefit from a suitable GPU and several CPU cores. Travel makes weight, battery life and screen size important.

Set a budget, then compare the complete system. Check ports, wireless connections, the operating system and whether parts can be upgraded. A fast CPU cannot remove every other limit.

Compare two example laptops

For essays, research and bus travel, Laptop A fits the lighter workload. It costs $850 less and weighs 2.4 lb (1.1 kg) less. Its stated battery life is longer. Check likely battery life under the user's real workload.

For high-resolution video editing at a desk, Laptop B offers more RAM, cores and storage, plus a dedicated GPU. Those features help the heavier work. Explain the choice using these needs, not just "B is better."

Check file space and display detail

If one minute of raw footage needs 375 MB, three hours is 180 minutes:

That is a substantial amount before finished videos, apps and other files. Compare it with free space, not only the drive's advertised capacity.

Screen resolution counts pixels across and down:

The second screen has almost twice as many pixels. It can show finer detail but also gives graphics hardware more work.

Find the bottleneck

A bottleneck is the part limiting a particular task. Too little RAM can cause heavy virtual-memory use and pauses when switching apps. A slow drive can delay loading, while a busy CPU can limit calculations.

Match an upgrade to evidence. Extra RAM helps a workload running out of working space; it does not make every task faster. Clock speed also needs context: different designs do different work per cycle. Battery claims depend on test conditions, screen brightness and workload.

Good software must do the required work on the available device. It must also fit the people using it. Check features, compatibility, total cost, accessibility, privacy and continued support.

Match the task and the requirements

Use a word processor for reports, a spreadsheet for calculations and charts, and a database for many searchable records. Graphics editors make posters; presentation software supports talks. CAD software creates designs for making objects.

Check the file formats other people need. Check offline use if internet access is unreliable. A web app that works well at school may be unsuitable on a journey without service.

System requirements list what software needs. Minimum requirements describe a supported starting point, often at lower settings. Recommended requirements aim for a better experience.

Suppose a game requires 8 GB RAM, four CPU cores, 50 GB free storage and a dedicated GPU with 4 GB memory. A laptop with 8 GB RAM, six cores and integrated graphics meets the RAM and core counts. It still fails the dedicated-graphics requirement. A 256 GB drive also does not tell you whether 50 GB remains free.

Understand the license and cost

A license sets the rules for using software. A proprietary license restricts changing or sharing it. The source code is usually private. The software may cost money once, require a subscription or be free to download.

An open-source license allows access to the source code and permits studying, changing and sharing it under its terms. LibreOffice, Firefox and Linux are examples. A free download is not enough to identify open-source software.

Free apps can have costs through ads, data collection or paid features. Compare the features people actually need over the time they will use the software.

Make the choice usable for everyone

Test screen-reader support, keyboard control, captions and adjustable text where needed. Separate accounts help people sharing a device keep their settings and files apart. Offline apps can help when internet data is expensive or unavailable.

The digital divide is unequal access to devices, connectivity and useful digital services. Requiring an expensive app or a particular device can exclude people who cannot obtain it. A choice that works for one well-equipped user may fail for a shared household phone.

Keep devices useful longer

Making computers uses energy, water and mined materials. Data centers also use electricity. Discarded electronics, or e-waste, can contain harmful substances as well as valuable metals.

Repairing a device or upgrading suitable parts can extend its useful life. Refurbished devices can meet needs without buying a newly made one. Use power-saving settings and take old electronics to an appropriate collection point. Continued security updates matter too: keeping an unsupported device needs a different decision from keeping one that remains maintained.