Welcome to Systems Architecture!

Ever wondered what actually happens inside your computer when you click a button or open a game? In this chapter, we are going to explore the "brain" of the computer—the Central Processing Unit (CPU)—and how it works with memory to make everything happen. Don't worry if it sounds a bit technical; we’ll break it down piece by piece with plenty of examples!

1. The Central Processing Unit (CPU)

The CPU is the most important part of any computer system. Its job is to process data and follow instructions. Think of it like a very fast, very logical chef in a busy kitchen.

Major Components of the CPU

The CPU isn't just one single block; it’s made of several specialized parts that work together:

  • Arithmetic Logic Unit (ALU): This is the "calculator." It performs all the arithmetic (adding, subtracting) and logical (comparing if one number is bigger than another) operations.
  • Control Unit (CU): This is the "manager." it coordinates all the activities of the CPU. It tells the other components how to respond to instructions.
  • Clock: This sends out a pulse (like a metronome) to keep all the components in sync. Each pulse is one "cycle."
  • Register: These are tiny, super-fast storage locations inside the CPU. They hold small amounts of data or instructions that the CPU is working on right this second.
  • Bus: These are like the "highways" of the computer. A bus is a collection of wires used to send data and signals between different parts of the computer.

Quick Review: The ALU does the math, the CU manages the flow, the Clock keeps the beat, Registers hold the immediate data, and Buses move it all around!

Key Takeaway: The CPU is the hardware that executes programs. It relies on internal components like the ALU and CU to process information.

2. The Fetch-Execute Cycle

How does the CPU actually run a program? it follows a never-ending loop called the Fetch-Execute Cycle. It’s like a person reading a set of instructions: Get the instruction -> Understand it -> Do it.

The Three Stages:

  1. Fetch: The CPU fetches (collects) the next instruction from the main memory (RAM) and brings it into the CPU.
  2. Decode: The Control Unit inspects the instruction to work out what needs to be done. (Is it a math problem? Does it need to move data?)
  3. Execute: The CPU carries out the instruction. This might involve the ALU doing a calculation or the CPU moving data to or from memory.

Memory Aid: Just remember F-D-E (Fast Dogs Eat). Fetch, Decode, Execute!

3. Performance: What Makes a CPU Fast?

Why is a gaming PC faster than an old laptop? It usually comes down to these three things:

  • Clock Speed: This is how many Fetch-Execute cycles the CPU can do per second. It is measured in Hertz (Hz). A \(3.5 \text{ GHz}\) processor can do 3.5 billion cycles every single second!
  • Number of Processor Cores: A "core" is like a mini-CPU. If you have a Dual-core processor, it’s like having two chefs in the kitchen instead of one. They can handle different tasks at the same time.
  • Cache Size: Cache is a very fast type of memory located right on the CPU. It stores frequently used data so the CPU doesn't have to wait for the slower RAM. A larger cache means the CPU can work faster.

Common Mistake: Students often think doubling the cores doubles the speed. Not always! Some programs aren't designed to be split between multiple cores, so they might not run much faster.

4. Memory and Storage

Computers use different types of memory for different jobs. The main thing to understand is the difference between Volatile and Non-volatile memory.

Volatile vs. Non-Volatile

  • Volatile Memory: Temporary. If you turn off the power, the data is lost. (Example: RAM).
  • Non-Volatile Memory: Permanent. Even without power, the data stays saved. (Example: ROM, Hard Drives).

RAM vs. ROM

These two often get confused, so let's clear it up:

  • RAM (Random Access Memory): This is the "worktable." It stores the programs and data currently in use. It is volatile, fast, and can be read from and written to.
  • ROM (Read Only Memory): This is the "instruction manual." It contains the start-up instructions (BIOS) for the computer. It is non-volatile and usually cannot be changed.

Did you know? Your computer's Registers are the fastest memory, followed by Cache, then RAM, and finally Secondary Storage (like your SSD).

5. Secondary Storage

Because Main Memory (RAM) is volatile, we need Secondary Storage to keep our files and programs permanently. Without it, your computer would "forget" everything as soon as you turned it off!

Types of Secondary Storage

The syllabus requires you to know about Magnetic and Solid State storage:

  • Magnetic Storage (e.g., Hard Disk Drives - HDD): Uses spinning magnetic platters.
    Pros: Very cheap, huge capacity.
    Cons: Slower, can be damaged if dropped because of moving parts.
  • Solid State Storage (e.g., SSDs, USB Sticks): Uses electrical circuits (NAND gates) to store data.
    Pros: Extremely fast, very durable (no moving parts), silent.
    Cons: More expensive per gigabyte than magnetic.

Cloud Storage

Cloud storage is simply storing your data on remote servers owned by someone else (like Google or Apple), which you access via the internet.
Pros: You can access files from anywhere; easy to share files.
Cons: You need an internet connection; there are potential security/privacy risks.

Quick Review: Magnetic is for big/cheap storage; Solid State is for speed/durability; Cloud is for access-anywhere convenience.

6. Embedded Systems

Not every computer is a PC or a Laptop. An Embedded System is a small computer built inside a larger device to do one specific job.

  • Examples: A washing machine controller, a microwave timer, or an engine management system in a car.
  • Difference: A non-embedded system (like a PC) is general-purpose and can do many different tasks (gaming, writing, browsing). An embedded system is dedicated to just one or two functions.

Key Takeaway: Embedded systems are usually cheaper to make, more reliable, and require less power because they only have one specific job to do.


Well done! You've just covered the essentials of Systems Architecture. Don't worry if the names of the CPU parts feel new—just remember the analogies of the chef, the metronome, and the highway, and you'll be an expert in no time!