CPUs: sockets, cores, and the thermal limit that decides real speed

Why this matters

Processors are the component people are most confident about and most often wrong about. The confidence comes from the headline numbers — cores and gigahertz — and the wrongness comes from the fact that neither reliably predicts how a machine feels to use.

The exam asks about compatibility, about the physical installation, and about thermal behaviour. All three are decidable facts, and all three are places where a technician's advice is either useful or expensive. Recommending a processor that does not fit the board is the most avoidable mistake in this job.

The thermal material also sets up lesson 36, where a machine that works at idle and fails under load turns out to be a cooling problem rather than a component failure.

The lesson

Sockets and compatibility: why a processor fits one board and not another

A processor is designed for a specific socket, and the socket is a physical and electrical interface. The two mechanical styles are:

• LGA (land grid array), where the pins are on the motherboard socket and the processor has flat contacts. Intel's desktop parts use this, and recent AMD desktop parts do too.
• PGA (pin grid array), where the pins are on the processor and the socket has holes. Older AMD desktop parts and many mobile parts.

Which one you are holding decides where the damage happens if something goes wrong. Bent pins on an LGA board are a motherboard problem; bent pins on a PGA processor are a processor problem. Either can sometimes be straightened and neither should ever need to be.

Fitting the socket is necessary and not sufficient. Also required:

• Chipset support. The board's chipset must support that processor generation, even within one socket.
• Firmware support. A board may need a firmware update before it will start with a newer processor — and if the only machine that could apply the update is the one that will not start, that is a genuine trap. Some boards support updating without a processor fitted, which is worth checking before ordering.
• Power delivery. A board's voltage regulation may not support the higher-power parts in the same family.

The professional habit is to check the board manufacturer's published compatibility list, which names each supported processor and the minimum firmware version. It takes a minute and it is definitive.

Cores, threads and clock speed, and which of the three a user actually notices

A core is an independent execution unit. Simultaneous multithreading (Intel calls it Hyper-Threading) lets one core present two logical processors, so an eight-core part may show sixteen threads. That gives maybe a quarter to a third more throughput on parallel work, not double.

Clock speed is cycles per second, and it only compares meaningfully within a processor family. A 3 GHz part from this year is substantially faster than a 3 GHz part from eight years ago because it does more per cycle.

Modern processors also have two distinct clock figures — a base clock and a boost clock — and the boost is sustained only while power and temperature allow. Some designs add performance and efficiency cores in one package, where the counts are not comparable to each other at all.

What a user actually notices, roughly in order:

1. Storage type. A machine moved from a mechanical drive to solid state feels transformed regardless of processor. This is the single most effective upgrade in this exam's scope.
2. Sufficient memory. A machine that is swapping feels broken.
3. Single-thread speed, which governs how responsive ordinary applications are.
4. Core count, which matters for compilation, rendering, virtualisation and heavy multitasking, and very little otherwise.

Recommending more cores to a user whose complaint is slowness is one of the most common wrong answers in the field, and the exam tests it.

Integrated graphics, and the machines that will not display without a card

Many processors include a graphics processor. Many do not — particularly higher-core-count desktop parts and most server parts.

That produces a failure that looks dramatic and is not: a machine assembled with a processor that has no integrated graphics and no discrete card powers on, fans spin, and the display stays blank. It is working perfectly and has nothing to display with.

The related points:

• A discrete card usually takes priority when both are present, and the monitor must be plugged into the card rather than the board. A display cable in the motherboard port of a machine with a graphics card is a classic no-signal cause, covered again in lesson 38.
• Firmware settings can force one or the other, or enable both.
• Integrated graphics use system memory, which is subtracted from what the operating system reports. A machine with 8 GB reporting 7.8 GB is normal.

Thermal design power, throttling, and why a clean heatsink is a performance fix

Thermal design power is the heat the cooling system must remove, in watts. It is a design target rather than a maximum consumption figure, and it is the number to match when choosing a cooler.

Modern processors monitor their own temperature continuously and reduce clock speed when they approach the limit. That is throttling, and it is a protective feature working correctly. The machine gets slower instead of failing.

Which means the following is a real and common diagnosis: a machine that has become slow over two years, with no software cause, is frequently a machine whose heatsink is packed with dust. Cleaning it restores the performance. That is why "clean the cooling" belongs in the performance checklist and not only the maintenance one.

The full picture of what limits sustained speed:

• Cooler capacity against the processor's thermal design power.
• Contact quality, which is the mounting pressure and the thermal compound.
• Case airflow, covered in lesson 28.
• Ambient temperature, which is why a machine in a cupboard throttles.
• Dust, which degrades all of the above gradually enough that nobody notices the day it started.

At the extreme, a processor that reaches its critical temperature will shut the machine down without warning. A machine that powers off abruptly under load and restarts fine is describing a thermal fault, and lesson 36 covers how to confirm it.

Installing a processor: orientation, force, paste, and the mistakes that bend pins

The procedure, and the reasons behind each step:

1. Observe anti-static precautions, as lesson 2 set out.
2. Open the socket retention mechanism fully. On LGA that is a lever and a load plate; on PGA it is a lever that releases the socket's grip.
3. Align using the marked corner and the notches. There is a triangle on one corner of the processor and a matching mark on the socket, plus keying notches. Alignment is unambiguous if you look.
4. Lower it straight down. Do not slide, and do not press. A correctly aligned processor drops into place under its own weight. If it needs force, it is wrong, and forcing it is how pins bend.
5. Close the retention mechanism. On LGA this takes noticeable force and that is normal — the lever is clamping the load plate, not the processor.
6. Apply thermal compound if the cooler does not have a pre-applied pad. A small amount in the centre is enough; the mounting pressure spreads it. Too much is worse than too little because it insulates.
7. Fit the cooler evenly. Tighten mounting screws in a diagonal pattern, a few turns each, so the pressure stays even.
8. Connect the fan to the correct header, which is the one labelled for the processor fan. A fan on the wrong header may not spin up under load, or the board may refuse to start reporting a fan fault.

The mistakes that account for most failures: forcing an unaligned processor, forgetting the compound entirely, forgetting to connect the fan, and removing a cooler by pulling when it has bonded to the processor — which on PGA parts can lift the processor out of a closed socket and bend every pin at once. Warming the machine first and twisting gently before lifting avoids that.