The processor (CPU)¶

Exercise Practical 1.3 PDF

Open any shop's website and look at a laptop. The first line of the spec sheet is almost always the processor: "8 cores, 16 threads, up to 4.4 GHz, 32 MB cache". What do those numbers actually mean? And which of them should you care about?

You already know what the CPU does: it runs the fetch → decode → execute loop from the Computer architecture page, billions of times per second. This page opens the lid and reads that spec sheet line by line — so next time someone tries to sell you "more GHz", you know what you are paying for.

From a room to a fingernail¶

A microprocessor is a whole CPU — Control Unit, ALU and registers — built on a single chip of silicon.

ENIAC filled a room. In 1971 Intel released the 4004, the first commercial microprocessor: the entire CPU on one chip, with about 2,300 transistors, running at 740 kHz. It was designed for a Japanese calculator (the Busicom 141-PF).

A transistor is a tiny electronic switch: on or off, 1 or 0 — the physical home of a bit. Today's processors pack billions of them on a chip the size of a fingernail.

That photo is an AMD Ryzen 7 3700X (2019). We will use its spec sheet as our running example:

Let's decode every line.

Cores: more chefs in the kitchen¶

A core is a complete, independent processing unit — its own Control Unit, ALU and registers — inside the CPU chip.

Remember the kitchen: the CPU is the chef. Until the mid-2000s, a home computer's processor had exactly one chef. Making that chef faster got harder and hotter every year, so manufacturers did something else: they put several chefs in the same kitchen. Each core runs its own fetch → decode → execute loop, at the same time as the others.

flowchart TB
    subgraph CHIP ["CPU chip (the kitchen)"]
        direction LR
        C1["<b>Core 1</b><br>chef 1:<br>CU + ALU + registers"]
        C2["<b>Core 2</b><br>chef 2:<br>CU + ALU + registers"]
        C3["<b>Core 3</b><br>chef 3"]
        C4["<b>Core 4</b><br>chef 4"]
    end
    CHIP <-->|"all chefs share<br>the same corridor"| RAM["<b>RAM</b><br>the countertop"]

The Ryzen above has 8 cores: eight chefs.

Threads: one chef, two dishes¶

A thread is one sequence of instructions the CPU is running. A core that supports SMT (Simultaneous Multithreading) can keep two threads going at once.

A chef waiting for water to boil does not stand still — they start chopping for a second dish. In the same way, a core often waits for data to arrive from memory. With SMT, the core keeps a second thread ready and works on it during those gaps. Intel calls its version Hyper-Threading and introduced it in 2002.

So 8 cores / 16 threads means 8 real chefs, each juggling 2 dishes. Careful: a thread is not a second chef. Two threads share one core's ALU and hands; the gain is usually well below 2×.

Clock speed: the beat of the kitchen¶

The clock is an electronic signal that ticks at a fixed rate. Each tick lets the CPU advance one step. Its speed is measured in hertz (Hz): ticks per second.

A 3.6 GHz CPU ticks 3.6 billion times per second. The Intel 4004 did 740,000. Same idea, about 5,000 times faster — per core.

Modern CPUs do not tick at one fixed speed. They have a base clock (the speed they can hold forever) and a boost clock (a sprint they can hold while they stay cool enough). The Ryzen: 3.6 GHz base, 4.4 GHz boost. When you do nothing, the clock drops even lower to save energy.

Cache: the tray at the chef's elbow¶

The architecture page left a promise open: the CPU is much faster than RAM, so it spends its life waiting at the bus — the Von Neumann bottleneck. Here is how it is fought.

Cache is a small, very fast memory built inside the CPU chip. It keeps copies of the data and instructions the CPU used recently, so most of the time it does not have to go to RAM at all.

The trick works because programs are predictable: what you used a moment ago, you will probably use again soon, and what sits next to it too. So the CPU keeps a copy close by.

Keep the kitchen: the registers are what the chef holds in their hands; the RAM is the big countertop across the room. The cache is in between — trays and shelves right next to the chef:

flowchart LR
    REG["<b>Registers</b><br>in the chef's hands<br><i>~1 tick</i>"] <--> L1["<b>L1 cache</b><br>tray at the elbow<br>tens of KB per core<br><i>~4–5 ticks</i>"]
    L1 <--> L2["<b>L2 cache</b><br>shelf behind the chef<br>~0.5–2 MB per core<br><i>~12 ticks</i>"]
    L2 <--> L3["<b>L3 cache</b><br>shared shelf for all chefs<br>several to tens of MB<br><i>~40 ticks</i>"]
    L3 <-->|"the corridor<br>(the bus)"| RAM["<b>RAM</b><br>the countertop<br>GBs<br><i>hundreds of ticks</i>"]

• L1 — the smallest and fastest, one per core. It is split in two: L1i for instructions and L1d for data (the "Harvard inside" from the architecture page).
• L2 — bigger, a bit slower, usually one per core.
• L3 — the biggest, shared by all the cores. The Ryzen's "32 MB cache" is this one.

When the CPU finds what it needs in the cache, that is a cache hit — fast. When it does not, it is a cache miss, and the core waits for RAM. A bigger cache means fewer misses. The numbers above are typical, not exact: they change with every CPU model.

Instruction set: the language the chef reads¶

The instruction set architecture (ISA) is the list of instructions a CPU understands, and how each one is written in bits.

LOAD, ADD, STORE... the instructions in our example program are not universal. Each CPU family has its own "language". A program is compiled into one ISA, and a CPU only runs programs written in its own.

Two families rule the world today:

This is why an Android app built for ARM does not simply run on a Windows x86-64 PC, and why Apple had to add a translator (Rosetta 2) when it moved its Macs to ARM in 2020. Same chef, different language.

Socket, heat and cooling¶

The socket¶

The socket is the connector on the motherboard where the CPU is installed.

Each socket fits only its own CPU family: the Ryzen in the photo needs an AM4 socket (look at the label printed on the board around it). A CPU and a motherboard must match — the first check when you build or upgrade a PC. Many laptop and phone CPUs have no socket at all: they are soldered to the board and cannot be replaced.

Heat and TDP¶

Every tick, billions of transistors switch — and each switch turns a little electricity into heat. A busy kitchen gets hot.

The TDP (Thermal Design Power) is the amount of heat, in watts, that the cooling system must be able to remove when the CPU works under a heavy, sustained load.

The Ryzen's TDP is 65 W. Careful: TDP is a guide for choosing the cooler, not an exact measurement of how much electricity the chip uses — each manufacturer defines it in its own way.

The cooler¶

If the heat is not removed, the CPU protects itself: it lowers its clock (this is called thermal throttling) and, in the worst case, the computer shuts down. The cooler is the kitchen's extractor hood:

When your laptop's fan roars during a game, you are hearing the TDP at work.

Read your own spec sheet¶

On Linux, lscpu prints your CPU's spec sheet. Try it in class:

LC_ALL=C lscpu | grep -E 'Architecture|Model name|^CPU\(s\)|Thread|Core\(s\)|MHz'

And the cache levels, with their sizes:

lscpu --caches

On Windows, open the Task Manager (Ctrl+Shift+Esc) → Performance → CPU: cores, logical processors (threads), speed and cache are all there.

Key takeaways¶

• A microprocessor is a whole CPU on one chip; the first one was the Intel 4004 (1971, ~2,300 transistors). Today: billions.
• A core is a complete chef; threads (SMT) let one core juggle two dishes. More cores only help when the work can be split.
• The clock (GHz) is the CPU's beat, with a base and a boost speed — but a modern core can finish several instructions per tick, so GHz alone does not decide speed.
• Cache (L1, L2, L3) keeps recent data next to the cores and fights the Von Neumann bottleneck: small, fast and expensive.
• The ISA is the CPU's language: x86-64 (most PCs) and ARM (phones, Apple M) cannot run each other's programs directly.
• The CPU sits in a socket that must match it, and its heat (TDP) must be removed by a cooler, or it throttles.

Quick check¶

The CPU is only as useful as what it is connected to. Next: the board that connects everything — the motherboard and its buses.