0.1.5 The CPU — the worker

In one line: the CPU is the only thing that actually does anything — every other part either feeds it instructions or stores its results.

The Intel 4004 was tiny and slow by modern standards, but it carried the central idea: one general-purpose chip, many possible instructions. Image: Wikimedia Commons, Intel 4004.jpg.

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A story — the chip that was meant for a calculator

In 1969 a Japanese company called Busicom asked Intel — then a tiny startup — to design twelve custom chips for a desktop calculator. Intel had a young engineer named Ted Hoff who looked at the spec, sighed, and said: “what if instead we build one general-purpose chip that runs calculator software?”

That chip — the Intel 4004 — shipped in 1971. It had 2,300 transistors, ran at 740 kHz, and could do roughly 92,000 instructions per second. It cost $200. Busicom got their calculator. Intel got the future.

Today’s Apple M3 Max has 92 billion transistors running at 4 GHz — about 40 million times faster than the 4004, and 40 million times more transistors. But the idea is identical. A CPU is a single chip that does one thing very well: it follows instructions, one after another, very fast.

What’s actually going on

The CPU (Central Processing Unit) is a tiny silicon square — usually 1–4 cm² — packed with billions of transistors arranged into circuits that do exactly four things:

1. Fetch an instruction from memory.
2. Decode what the instruction means.
3. Execute it (add, compare, jump, read, write…).
4. Store the result somewhere.

That’s it. That’s the whole job. The CPU does this loop billions of times per second.

A modern CPU has multiple cores — independent workers on the same chip — and each core can often run two threads at once (Intel calls this Hyper-Threading). A desktop CPU might be “8-core / 16-thread,” meaning sixteen instruction streams running in parallel.

The CPU also has caches built in (we’ll cover those in 0.1.7) — tiny, fast scratchpads of memory right next to the cores so they don’t have to wait for slow RAM every time they need a value.

Why a hacker cares

The CPU is a target and a weapon:

• Spectre and Meltdown (2018) — vulnerabilities in how CPUs do speculative execution that let untrusted code read memory it shouldn’t. Affected basically every Intel chip from the previous 20 years. Patched in microcode + OS.
• Microcode — the CPU’s own internal firmware. Intel and AMD ship microcode updates inside Windows / Linux updates. Compromise the microcode and you have a backdoor below the OS.
• Side-channel attacks — measure how long an instruction takes, or how much power it draws, and you can sometimes recover encryption keys from a CPU performing the encryption. Smart-card attacks have been doing this for 25 years.
• Cryptojacking — malware that quietly uses your CPU to mine cryptocurrency. Slow computer, hot lap, weird power bill.

The CPU is also the enforcer of security. Privileged execution levels (Ring 0 = kernel, Ring 3 = userland) are a CPU feature. Virtualisation extensions that let one machine run many isolated VMs are CPU features. TPM and secure-enclave chips are tiny CPUs whose only job is to keep secrets the main CPU can’t see.

In one sketch

   ┌─────────────── CPU chip ────────────────┐
   │                                         │
   │   ┌───────┐  ┌───────┐  ┌───────┐       │
   │   │ Core 1│  │ Core 2│  │ Core 3│  ...  │
   │   │ + L1$ │  │ + L1$ │  │ + L1$ │       │
   │   └───────┘  └───────┘  └───────┘       │
   │                                         │
   │   ┌─────────────────────────────────┐   │
   │   │       Shared L2 / L3 cache       │   │
   │   └─────────────────────────────────┘   │
   │                                         │
   │   Memory controller ▸  to RAM           │
   └─────────────────────────────────────────┘

Reference and image credit

• Intel_4004.jpg — LucaDetomi, GFDL / CC BY-SA 3.0.

Memory peg

Fetch, decode, execute, store. Repeat 4 billion times a second. That’s all a CPU does. Everything else is decoration.