Inside the PC: Core Components

Open up almost any desktop computer and you'll find the same handful of parts, give or take. Once you can name them, say what each one does, and — the part that actually trips people up — recognise when two parts won't work together, you've got the foundation the rest of the Hardware module is built on.

This lesson is a guided tour of those core components. At the end you'll put it to work in Build-a-PC: assemble a complete, compatible machine from a parts bin that's deliberately stocked with parts that don't fit. Reading about compatibility is one thing; catching a mismatched part before it costs you is another.

A PC is a system, not a pile of parts

It's tempting to learn the components as a vocabulary list — CPU, RAM, PSU, tick, tick, tick. But the thing the A+ exam (and real bench work) cares about is how they relate. No component does anything on its own. The processor can't run without memory to work in; memory is useless without power; none of it matters if the parts can't physically connect.

At the centre of it all sits the motherboard — the board everything else plugs into. Almost every compatibility question in this lesson comes back to it: will this part work with this motherboard? Keep that lens on as we go.

The case and form factor

The case (or chassis) is the shell that holds everything together. It's easy to dismiss as "just a box," but it introduces our first compatibility rule through something called form factor.

A form factor is a standardised size-and-layout spec that makes sure boards, cases, and power supplies from different manufacturers fit together. The three you need to know:

• ATX — the full-size standard. Most room, most expansion slots.
• microATX (mATX) — smaller, fewer slots, very common in budget and office builds.
• Mini-ITX (mITX) — the smallest mainstream size, for compact builds.

The rule: the case must support the motherboard's form factor. Cases are sized for the largest board they'll take, and bigger cases accept smaller boards — an ATX mid-tower happily fits an ATX, mATX, or ITX board. It doesn't work in reverse: an ATX motherboard will not fit inside a Mini-ITX case.

The motherboard

The motherboard is the backbone of the system — the printed circuit board that connects every other component and lets them talk to each other. When you look at one, the features that matter most for this module are:

• The CPU socket — where the processor mounts.
• The memory slots (DIMM slots) — where RAM installs.
• The expansion slots (PCIe) — where add-in cards like a graphics card go.
• The storage connectors — SATA ports and M.2 slots.
• Its form factor — ATX, mATX, or ITX, as above.

Here's the key idea that makes the motherboard the hub of compatibility: the board dictates what CPU and what memory you're allowed to use. Its socket determines which processors physically fit, and its chipset determines which generation of RAM it accepts. So a lot of questions that sound like they're about the CPU or the RAM are really questions about the motherboard.

The CPU

The CPU (Central Processing Unit), or processor, is the component that actually executes instructions — the closest thing the machine has to a brain. It mounts into the motherboard's socket, and that socket is where our next compatibility rule lives.

A socket is a specific physical and electrical interface. A processor built for one socket will not fit another — the pin layout is different, and it simply won't seat. Two current examples:

• AMD's AM5 socket, used by recent Ryzen processors.
• Intel's LGA 1700 socket, used by several recent Core generations.

The rule: the CPU's socket must match the motherboard's socket. An AM5 processor needs an AM5 board; drop it onto an LGA 1700 board and it won't physically go in. This is one of the first things to check when pairing a CPU and a motherboard.

One more thing to file away: CPUs generate heat when they work, and a hot CPU will slow itself down or shut off to protect itself. That's why every build needs cooling — we'll come back to it.

RAM (memory)

RAM (Random Access Memory) is the system's fast, short-term working memory. When you open an app or a file, it's loaded into RAM so the CPU can reach it quickly. RAM is volatile, which means it only holds data while powered — switch the machine off and whatever was in RAM is gone. (That's the difference between memory and storage, which is next.)

RAM installs into the motherboard's DIMM slots, and it comes in generations — the current ones being DDR4 and the newer, faster DDR5. This is where people get caught: the generations are not interchangeable. A DDR5 module has its connector notch in a different place than DDR4, so a DDR4 stick won't even seat in a DDR5 slot.

And remember the hub principle — the motherboard decides which generation it accepts. So "is this RAM compatible?" is really "does this RAM's generation match what the motherboard supports?"

Storage

Where RAM is fast but forgetful, storage is the opposite: it's non-volatile, so it keeps your operating system, programs, and files when the power is off. Two distinctions matter here.

First, the type of drive:

• A hard disk drive (HDD) stores data on spinning magnetic platters. It's mechanical, cheaper per gigabyte, and slower.
• A solid-state drive (SSD) uses flash memory with no moving parts. It's considerably faster and more durable.

Second, the interface — how the drive connects:

• SATA connects over a cable to a SATA port. It's the older, slower standard, used by HDDs and many 2.5-inch SSDs.
• NVMe, typically on an M.2 drive that plugs directly into a slot on the motherboard, is dramatically faster than SATA.

For the exam, the headline is the performance order: an NVMe M.2 SSD is faster than a SATA SSD, which is faster than a spinning HDD.

The power supply (PSU)

The power supply unit (PSU) takes the alternating current (AC) from the wall outlet and converts it into the lower-voltage direct current (DC) that the components actually run on. It's easy to overlook, but it carries two compatibility concerns of its own.

The first is wattage. Every component draws power, and the PSU has to supply enough for the whole build under load. Choose one that's too weak and the system becomes unstable or won't boot at all. So you add up roughly what the build will draw — the CPU and a dedicated graphics card are usually the big consumers — and pick a PSU comfortably above that.

The second is connectors. The PSU has to physically have the right cables for what's in the build: the large connector for the motherboard, a connector for the CPU, and — importantly — a PCIe power connector if you're running a dedicated graphics card. A PSU that doesn't provide the connector a component needs can't power it, no matter how many watts it has.

The GPU

The GPU (Graphics Processing Unit) handles rendering what you see on screen. It comes in two forms, and the distinction matters:

• Integrated graphics are built into the CPU. No separate card, lower performance — fine for office work and everyday use.
• A dedicated GPU is a separate expansion card for demanding tasks like gaming or video editing.

A dedicated graphics card is a good test of whether you've understood the whole system, because it depends on three other components at once. It needs a PCIe x16 slot on the motherboard to plug into, enough physical space in the case to fit, and enough power from the PSU — both sufficient wattage and the right PCIe power connector. Miss any one of those and the card can't go in the build.

Cooling

We flagged earlier that CPUs produce heat. Cooling is how that heat gets carried away so components stay in their safe operating range. Left unchecked, a hot CPU throttles (deliberately slows down) or shuts off entirely.

Two layers handle it: a CPU cooler sits directly on the processor — either an air cooler (a heatsink with a fan) or a liquid cooler — and case fans move air through the chassis, pulling cool air in and pushing warm air out. You won't assemble the cooling in this lesson's build, but knowing why every machine needs it is foundational.

The compatibility web

Step back and the picture is a web of dependencies, almost all of them anchored to the motherboard:

• The case must fit the motherboard's form factor.
• The CPU's socket must match the motherboard's socket.
• The RAM's generation must match what the motherboard supports.
• The PSU must supply enough wattage for everything and carry the right connectors.
• A dedicated GPU needs a PCIe slot on the board, space in the case, and power from the PSU.

That web is exactly what the exam probes when it asks whether a given set of parts will work together — and it's what you'll navigate now, hands-on.

Your turn: build a compatible PC

Below is a parts bin and an empty build. Drag a part into each slot to assemble a complete machine. Some parts in the bin won't fit — a CPU for the wrong socket, memory of the wrong generation, a power supply that's too weak. Use the rules from this lesson to build something that actually works. If a choice doesn't fit, you'll be told which rule it broke.

Once you've got a clean build, finish the room with the check questions to lock it in.