Inside a computer: components and their jobs

Jordan's first hardware ticket comes from Prairie Paws Veterinary Clinic. The reception PC, BL-24817 in the ticket system, takes four minutes to reach the sign-in screen and another three before the practice-management app opens. The clinic manager wants to know whether to buy a new computer. Marisol hands Jordan a screwdriver and says that before anyone spends money, he should open the case and tell her what is inside it. "You can't recommend an upgrade," she says, "until you know what each part does and which one is holding the others back."

The parts of a computer system

Every computer, from a phone to a server, has the same basic jobs to do. It must take input, process it, keep data it is working on, store data for later, and produce output. In a desktop PC those jobs are split among separate parts that a technician can identify, test and replace. Knowing which part does which job is the foundation of hardware support, because a symptom usually points to the part responsible for that job.

The central processing unit, or CPU, executes the instructions that make up programs. It sits in a socket on the motherboard under a heat sink and fan or a liquid cooler, because it produces a great deal of heat. CPUs are described by their maker, model, number of cores and clock speed. More cores let the computer run more tasks at once; a higher clock speed lets each core work faster. For support work you rarely replace a CPU, but you often need to identify it, because software requirements, operating system upgrades and driver downloads depend on it.

Random access memory, or RAM, is the computer's working memory. Programs and the data they are using are loaded into RAM because it is far faster than storage. RAM is volatile: its contents disappear when power is removed. Desktop RAM comes as long modules called DIMMs, and laptop RAM as shorter SO-DIMMs; some thin laptops have memory soldered to the board, which cannot be upgraded. Modules are made to particular generations, such as DDR4 and DDR5, which are keyed differently and are not interchangeable. When a computer has too little RAM for what the user runs, the operating system moves data back and forth to storage, and everything slows down.

The motherboard and firmware

The motherboard is the main circuit board that connects everything else. It holds the CPU socket, the memory slots, expansion slots, storage connectors, the chipset that manages traffic among them, and the rear panel of ports. Motherboards come in standard sizes called form factors, such as ATX, micro-ATX and mini-ITX, which determine which cases they fit. A technician replacing a motherboard must match the form factor, the CPU socket and the memory generation, which is why a motherboard replacement often turns into a whole-computer replacement.

Firmware is the small program stored on the motherboard that starts the computer before the operating system loads. Modern computers use UEFI, the Unified Extensible Firmware Interface, which replaced the older BIOS; many people still say BIOS out of habit. Firmware runs the power-on self-test, finds a boot device and hands control to the operating system's boot loader. It also holds settings such as the boot order, the system clock, Secure Boot, virtualization support and whether certain devices are enabled. A small coin battery, usually called the CMOS battery, keeps the clock running when the computer is unplugged. A computer that loses its date and time every time it is unplugged usually needs a new battery.

Storage

Storage keeps data when the power is off. There are three common kinds. Hard disk drives (HDDs) store data on spinning magnetic platters read by a moving head; they are inexpensive per unit of capacity but slow and sensitive to shock. Solid-state drives (SSDs) store data in flash memory with no moving parts. SATA SSDs use the same connector and interface as hard disks and are several times faster. NVMe SSDs plug into an M.2 slot or a PCIe card and talk directly to the PCIe bus, making them faster again. For a user, the difference between a hard disk and any SSD is dramatic: start-up, sign-in and opening applications can take a fraction of the time.

Capacity is measured in gigabytes (GB) and terabytes (TB). Operating systems and drive makers count these differently, so a drive sold as 1 TB shows as about 931 GB in Windows; that is not a fault. Storage also wears out. Hard disks develop bad sectors and mechanical faults, often with clicking sounds; SSDs have a limited number of write cycles and can fail suddenly. Both report health data through a system called SMART, which diagnostic tools can read, and a drive reporting problems should be backed up and replaced before it fails.

Power, cooling and expansion

The power supply unit (PSU) converts mains power from the wall into the low direct-current voltages the components use. It is rated in watts, and it must supply enough power for the CPU, the graphics card and everything else, with some headroom. A failing power supply can cause random restarts, failure to start, or a computer that starts only after several tries. Power supplies store dangerous charge even when unplugged, so support technicians replace them as a whole unit and never open them.

Cooling keeps components within their temperature limits. Case fans move air through the case, the CPU cooler draws heat from the processor, and graphics cards have their own fans. Dust is the enemy of cooling: a clogged heat sink can make a computer slow down to protect itself, a behavior called thermal throttling, or shut off without warning. Cleaning dust with compressed air, with the computer off and the fans held still, is one of the cheapest fixes in hardware support.

Expansion cards plug into PCIe slots to add capabilities. The most common is a graphics card, needed for demanding visual work, gaming or multiple high-resolution displays; ordinary office computers use graphics built into the CPU. Other cards add network ports, Wi-Fi, extra USB ports or storage controllers. PCIe slots come in different lengths, labeled x1, x4, x8 and x16, which reflect how many data lanes they carry; a short card fits a longer slot, but not the other way round.

Identifying what is installed

Before opening a case, a technician can learn a great deal from the operating system. In Windows, the Settings app's About page shows the processor, installed RAM and Windows edition; System Information shows much more detail; and Task Manager's Performance tab shows the CPU, memory, disk and graphics, including the memory speed and how many slots are used. On a Mac, About This Mac gives the chip, memory and serial number. Command-line tools give the same information in a form that can be copied into a ticket.

C:\> systeminfo | findstr /C:"OS Name" /C:"Total Physical Memory"
OS Name:                   Microsoft Windows 11 Pro
Total Physical Memory:     8,034 MB

C:\> wmic diskdrive get model,mediatype
MediaType              Model
Fixed hard disk media  WDC WD10EZEX-08WN4A0

Every computer also has identifiers that matter for support: a serial number or service tag used for warranty and parts lookups, a model number, and often an asset tag applied by the organization. Brightline records all three in each client's asset list, so that when a user calls, the technician can see the computer's age, warranty status and history before asking a single question.

Laptops and all-in-ones

Laptops pack the same parts into a much smaller space, and that changes what a technician can do. Many laptops have one memory slot or none, a single M.2 slot for storage, a battery held in by screws rather than a latch, and a keyboard, touchpad and screen that are replaced as assemblies. The cooling system is a thin heat pipe and a small fan that clog with dust faster than a desktop's. All-in-one computers, which build the computer into the back of the monitor, are similar: they are tidy on a reception desk, but most repairs mean removing the screen. Before quoting an upgrade for either, look up the model's service manual or specifications to see what can actually be changed. When little can be changed, the right recommendation may be a replacement after all, and knowing that early saves the client a wasted visit.

Desktop towers, by contrast, are built to be opened: side panels come off with a thumbscrew or two, drive bays slide out, and most parts are standard sizes from many makers. That flexibility is why many small businesses keep desktops at reception and in back offices even when staff carry laptops elsewhere.

Jordan opens the case, finds the empty memory slots and an unused M.2 slot, photographs the labels on the existing memory modules and writes the part details into the ticket. Marisol checks them, approves the parts order and tells him the clinic will get its computer back faster than a new one could be delivered and set up. Lesson 3 covers the installation.