The Fetch-Execute Cycle: What's Your Computer Actually Doing?

Summary

This script explains the fundamental workings of a computer, likening it to an overgrown calculator. It simplifies the concept by detailing the 'Fetch-Decode-Execute' cycle of a CPU, using basic components like registers, RAM, and a clock. The script demonstrates a simple counting program to illustrate the process, then discusses the evolution from machine code to higher-level languages, emphasizing the importance of speed in modern computing. It concludes with a sponsored mention of Dashlane, a password manager, advocating for secure password practices.

Takeaways

• 💻 Every modern computer fundamentally operates on the same principle, executing instructions in a 'Fetch-Execute' cycle.
• 🕒 The CPU's operation is synchronized with a clock that dictates the pace of the 'Fetch-Decode-Execute' cycle.
• 📏 Registers in the CPU serve as fast storage for currently active data and instructions.
• 🔄 RAM (Random Access Memory) is where instructions and calculated values are stored, allowing random access to information.
• 🔢 A simple program can be created to demonstrate counting up by incrementing a value stored in RAM.
• 🔄 The process involves fetching instructions from memory, decoding them, and executing them in a continuous loop.
• 🔁 The ability to jump to different memory addresses allows for looping and recursive instructions, foundational to computer science.
• 🛠 Early programming involved working directly with binary or hexadecimal machine code, which was complex and error-prone.
• 🔝 Higher-level programming languages were developed to abstract away the complexities of memory management, focusing on what the computer should do.
• 🔧 A compiler translates high-level language code into machine code, making it executable and portable across different hardware.
• 🚀 Modern CPUs execute instructions at gigahertz speeds, performing billions of operations per second across multiple threads.
• 🔒 The video also promotes the use of a password manager like Dashlane for secure password storage and auto-fill, enhancing online security.

Q & A

What is the fundamental operation that all modern computers perform?

All modern computers fundamentally perform operations based on the 'Fetch-Execute' cycle, which involves fetching an instruction from memory, decoding it, and then executing it.

What is the role of the CPU in a computer?

The CPU, or Central Processing Unit, is the heart of a computer where the processing of instructions and data takes place. It is responsible for executing the instructions in the 'Fetch-Execute' cycle.

What are the three main steps in the CPU's operation cycle?

The three main steps in the CPU's operation cycle are 'Fetch', 'Decode', and 'Execute'. These steps are repeated in a loop to process instructions stored in memory.

What does RAM stand for and why is it important in a computer?

RAM stands for Random Access Memory. It is important because it serves as the computer's short-term memory, where instructions and data are stored temporarily while being processed by the CPU.

How does the CPU's clock influence the processing of instructions?

The CPU's clock acts as a metronome, dictating the pace at which the CPU fetches and processes instructions. Each tick of the clock corresponds to a step in the 'Fetch-Execute' cycle.

What is a 'Program Counter' and what is its function?

A 'Program Counter' is a register in the CPU that keeps track of the next memory address from which to fetch an instruction. It is incremented after each instruction is fetched, directing the flow of execution.

Why is the ability to 'jump' in a program important in computer science?

The ability to 'jump' in a program allows for loops and recursive instructions, which are foundational in computer science. It enables the CPU to execute a sequence of instructions repeatedly or to skip to a different part of the program based on conditions.

What is the difference between machine code and higher-level programming languages?

Machine code is the raw binary data that a CPU directly understands, while higher-level programming languages are more human-readable and abstracted from the hardware. Compilers or interpreters are used to convert higher-level languages into machine code.

How does a modern CPU differ from the simplified model described in the script?

A modern CPU differs from the simplified model by executing billions of instructions per second, often on multiple cores or threads, and includes advanced features like caching, pipelining, and error correction that are not present in the simplified model.

What is the significance of the 'ADD' and 'STORE' instructions in the script's example program?

The 'ADD' instruction is used to add a value to the accumulator, while the 'STORE' instruction saves the accumulator's value back into memory. These are fundamental operations that demonstrate how a computer can perform arithmetic and update memory.

Why is using a password manager like Dashlane recommended in the script?

Using a password manager like Dashlane is recommended because it securely generates, stores, and autofills unique, complex passwords for each site, reducing the risk of data breaches and simplifying the management of multiple passwords across different platforms.

Outlines

00:00

🖥️ The Basics of Computer Operation

This paragraph explains the fundamental workings of a computer, comparing it to an overgrown calculator. It delves into the CPU's 'Fetch-Execute' cycle, simplified with a clock that ticks with finger snaps, and three registers: the instruction cycle register, the memory instruction loader, and the Accumulator. The role of RAM as a storage for instructions and calculated values is also highlighted. The paragraph demonstrates a simple program that counts up, detailing the steps of fetching, decoding, and executing instructions in a loop. The process involves loading values into the accumulator, adding to it, and storing the result back into RAM, with the ability to jump to different memory addresses to create loops, a foundational concept in computer science.

05:01

🔢 From Machine Code to High-Level Programming

The second paragraph discusses the evolution from machine code, which is raw binary data, to more human-readable forms like hexadecimal and Assembly language, used in early games like 'Prince of Persia'. It emphasizes the complexity and potential for errors in low-level programming. The paragraph then introduces higher-level programming languages that abstract away the intricacies of memory management, allowing developers to focus on the computer's tasks. An example code is given, showcasing a simple loop that increments a variable, which is then compiled into machine code for execution. The capabilities of modern CPUs to execute billions of instructions per second on multiple threads are highlighted, underlining the speed at which complex tasks like video decompression and display are handled. The paragraph concludes with a discussion on password management, advocating the use of Dashlane for secure, automated password handling across various devices and browsers, and mentions a promotional offer for the product.

Keywords

💡Central Processing Unit (CPU)

The CPU, often referred to as the 'brain' of a computer, is responsible for executing instructions and performing calculations. It is integral to the video's theme as it illustrates the fundamental component that enables all computer operations. In the script, the CPU is described as going through a 'Fetch-Execute' cycle, highlighting its role in processing instructions from RAM.

💡Fetch-Execute Cycle

This cycle is the basic operation of a CPU where it fetches an instruction from memory, decodes it, and then executes it. The video script uses the fetch-execute cycle to demonstrate the simplicity of computer operations, showing how even complex tasks are broken down into a series of these fundamental steps.

💡Registers

Registers are small, fast storage locations within the CPU used to hold temporary data. The script mentions three types of registers: the instruction cycle register, the instruction register, and the accumulator. They are key to the CPU's operation, as they hold the values that the CPU is currently working on.

💡Random Access Memory (RAM)

RAM is a type of computer memory that can be read from and written to in any order, hence the term 'random access.' The video script describes RAM as the storage for instructions and calculated values, emphasizing its importance in holding both the program's logic and its outputs.

💡Instruction

In the context of the video, an instruction is a command that the CPU executes. The script explains that each instruction has two parts: the operation to be performed and a memory address where the relevant data can be found. Instructions are the building blocks of programs and are central to the video's demonstration of how computers process tasks.

💡Program Counter

The Program Counter is a register in the CPU that holds the memory address of the next instruction to be executed. The script uses the Program Counter to illustrate the sequential nature of instruction execution, showing how it is incremented after each instruction is fetched and executed.

💡Loop

A loop in programming is a sequence of instructions that is repeated until a certain condition is met. The video script describes a program that lacks a halt command, thus creating an infinite loop that continually increments a value, demonstrating the concept of looping in computer operations.

💡Machine Code

Machine code is the lowest-level programming language, consisting of binary instructions that a CPU can execute directly. The script contrasts the complexity of programming in machine code with higher-level languages, emphasizing the difficulty and potential for error in working directly with binary data.

💡Assembly Language

Assembly language is a low-level programming language that uses mnemonics to represent machine code instructions. The video script mentions assembly language as a step up from machine code, making it slightly more readable for humans but still very close to the hardware level.

💡Compiler

A compiler is a program that translates code written in a high-level programming language into machine code. The script explains that after writing code in a human-readable form, it is passed to a compiler, which then converts it into the machine code that the computer can execute.

💡Gigahertz

Gigahertz (GHz) is a unit of measurement for the clock speed of a CPU, indicating the number of cycles per second. The video script uses gigahertz to illustrate the speed at which modern CPUs operate, executing billions of instructions per second, which is a stark contrast to the manual clicking demonstration.

💡Password Manager

A password manager is a tool that stores and manages login credentials for various online accounts. The video script mentions Dashlane as an example of a password manager, emphasizing its utility in creating and autofilling unique, secure passwords for different sites, which is crucial for maintaining online security.

Highlights

Computers at their core function similarly to overgrown calculators, executing a 'Fetch-Execute' cycle.

A Central Processing Unit (CPU) is the heart of a computer, connected to other devices that enable its operation.

The CPU uses a clock to tick through its cycle, with each tick representing a step in the process.

Registers within the CPU serve as fast storage for values currently being processed.

Random Access Memory (RAM) is used for storing instructions and calculated values.

A simple program in RAM can be executed by the CPU through a loop of fetch, decode, and execute steps.

Instructions in a program consist of an operation and a memory address, dictating what the CPU should do.

The CPU's Program Counter keeps track of the current instruction cycle.

The ability to jump and loop within a program is fundamental to computer science.

Programming at the binary level is complex and error-prone, leading to the development of higher-level languages.

Assembly language is a symbolic representation of machine code, making it more readable but still close to hardware.

Higher-level programming languages abstract away memory management, allowing developers to focus on functionality.

A compiler translates high-level language code into machine code for execution.

Modern CPUs execute billions of instructions per second on multiple threads, enabling complex tasks.

The speed of a computer's operation is crucial for performing complex tasks like video decompression and display.

Dashlane is a password manager that offers a secure way to handle and autofill passwords across devices.

Using a password manager like Dashlane can prevent the security risks associated with password reuse.

Dashlane provides additional features like credit card and address autofill, as well as a VPN for secure browsing.

The security architecture of Dashlane ensures that even the company cannot access users' passwords.

Transcripts

Every modern computer, when you get right down to the bare metal,

is doing basically the same sort of thing.

I’ve said before that computers are just overgrown calculators,

but how do you go from a simple calculator to playing video games,

sending stuff over the internet,

or even decompressing and displaying the millions of pixels in this video?

In short, what’s your computer actually doing?

Behind me is a scaled-up version of a computer,

but we’re going to go much, much simpler.

If you take apart your phone or PC,

somewhere in the heart of it will a Central Processing Unit, or CPU,

connected to all the other devices that make it work.

Now, to show a really basic example, we’re not going to use all those devices.

The first one we are going to use is the clock.

With every tick of the clock,

our CPU goes through a step in what’s called the “Fetch-Execute” cycle,

or “Fetch-Decode-Execute”.

This clock is slightly magic,

in that it ticks (click) every time (click) I click my fingers. (click)

(click)

In the CPU I’m going to have three registers.

These are bits of fast storage where the CPU holds values that it’s working on right now.

These are: a register that keeps track of our instruction cycle,

another that loads our instructions from memory,

and an Accumulator.

The final thing we need in our simplified computer is somewhere to keep the instructions

and any values that we end up calculating.

That is RAM, Random Access Memory.

We call it Random Access because it doesn't matter when or in what order

the information is read or written.

So: that is our computer.

Let’s run a simple program.

All it’s going to do is count up.

The processor has three steps: Fetch, Decode, Execute.

It will just repeat those on a loop,

that’s the one thing that’s actually built into it.

So we need some instructions, actually in memory,

so let’s load our program into RAM.

The RAM is also used to store our answers, our outputs.

In the real world, these would all be stored in binary,

but let’s not overcomplicate things right now, let's keep them human-readable.

An instruction has two parts.

The first part is the instruction itself.

And the second part is usually a memory address.

On each clock tick, the CPU will do one of three things:

It will fetch an instruction from a memory address.

It will decode that instruction.

And it will execute the instruction.

Round and round in a loop. So it’s going to count up.

We're going to begin with a number,

and add one to it, over and over again.

(click) Fetch.

One clock tick. The Program Counter is set to 0,

so the CPU fetches the instruction at address 0 in the memory

and puts it into the instruction register.

(click) Decode.

The CPU decodes the instruction.

The first part is the instruction,

and the second part is a location.

In our case, the instruction is LOAD and the address is 6.

So we will be loading the value in address 6 into the accumulator.

(click) Execute.

The CPU executes this instruction.

It takes the value at address 6,

and loads it into the accumulator.

In this case the value is 1.

(click) Fetch.

The program counter is incremented,

and the CPU fetches the next instruction in the next bit of the memory.

(click) Decode.

The CPU decodes the instruction.

This time, it’s ADD, and the address is 7.

So we’ll be adding what’s at address 7 into what is already in the accumulator.

(click) Execute.

The CPU executes the instruction. We add the value at address 7.

In this case, it's the value 1.

1 + 1 is 2.

(click) Fetch.

From the next memory location, number 2.

(click) Decode.

An instruction to STORE the value in the accumulator into RAM, at address 6.

(click) Execute.

Now, notice that we are overwriting what’s already there,

so address 6 now has 2 in it, instead of 1.

(click) Fetch.

A new instruction: JUMP.

With a jump,

the next address we fetch from is the one in this instruction.

(click) Decode.

So we’re going to jump to address number 1.

(click) Execute.

The Program Counter is now back at 1.

The ability to jump, to loop, and to build instructions recursively is one of the foundations

of computer science. So: we're back up there.

(click) Fetch from location 1.

(click) Decode.

It's the ADD instruction again.

(click) Execute.

Our accumulator still contains the values from before,

so: 2 + 1 = 3.

(click) Fetch.

(click) Decode.

STORE again.

(click) Execute.

Storing it into location number 6.

(click) Fetch.

(click) Decode.

(click) Execute.

And we jump again.

(click) Fetch. (click) Decode. (click) Execute. (click) Fetch. (click) Decode. (click) Execute.

(continues clicking) We're in a loop, and we’re counting upwards

by one on every sixth clock cycle. (stops clicking)

Our program, with these simple instructions, doesn’t have a halt command,

or any way to interrupt it,

so it will just keep incrementing that value by one (many fast clicks)

until the number becomes so large it can no longer be held by the memory address.

How it breaks then… well, that’s a whole other video. (stops clicking)

And my fingers are tired.

This is a very fiddly way to program a computer.

In theory, it can be but at this level,

these instructions are just encoded in raw binary data,

which is basically unreadable for humans.

So we can convert that base 2 binary to base 16, hexadecimal,

at that level we call it machine code.

The next step up from that is a symbolic language called Assembly, which is a bit more readable,

but it’s still close to working at that bare metal.

The original "Prince of Persia" game was completely programmed in assembly.

That is almost unbelievable to me:

painstakingly figuring out each pixel of animation and encoding it into something that the computer

almost understands directly.

Programming like that is complex, and hard,

and prone to the sort of human error that introduces massive security problems.

It is difficult to code and difficult to debug.

So rather than dealing with the messiness,

or, well, the pristine logic of machine code,

higher-level languages were developed as an intermediary step.

Those languages handle all of that memory reading and writing for us,

so all we need to focus on is what we want the computer to do.

So, here’s my code:

just the same instructions, phrased a little bit differently,

phrased for humans to be able to read.

I specify a variable, X. I then write a function that loops forever,

and each passing through that loop I increment X by 1.

Once I've written that code, I then pass it to a compiler,

which turns it into that original machine code for me.

So when I run the program,

it’s loaded into the computer's memory, and executed.

If I want to run it on a completely different type of computer, a Mac instead of a PC,

I can compile it for that CPU instead.

But this still doesn’t answer the question of how the computer is

doing something as complex as decompressing and displaying this video.

The answer to that is:

speed.

At the speed I was clicking, at the end there,

we were executing one instruction every couple of seconds on one thread, one bit of the system.

A modern CPU executes billions of instructions per second – gigahertz – on multiple threads.

But at the heart of your PC, or your phone,

there is still just a ticking clock and a fetch-execute cycle.

I’ve used a password manager for years,

and if you’re techie enough to reach the end of this video, you should too.

And I’m not just saying that because this video is sponsored by Dashlane, a password manager,

and if you go to dashlane.com/tomscott, you can get a free 30-day trial of their premium version.

Why should you use Dashlane? Well, first, reusing passwords is a terrible idea.

If you’re like me a few years back,

then you’re reusing very similar passwords that have a few letters changed,

or have the site’s name stuck somewhere in them.

It’s not great.

One data breach at one of those sites, and it’d be time to start worrying.

These days, Dashlane sits in my browser, and when I want to log in somewhere,

I unlock Dashlane with the single password that I have to remember,

and it autofills it for me.

Dashlane also stores and autofills credit cards and address information across your devices,

and it’s got a VPN to encrypt your traffic on public wifi networks if you want to.

If you are the kind of person who can remember

20-character unique symbol-filled passwords for dozens of different sites, congratulations.

But if you’re not superhuman, then your choice is basically:

use insecure passwords or use a password manager.

Now, you could use post-it notes on your monitor as a password manager…

or you could use Dashlane,

which generates, stores, and autofills long, secure, different passwords for every site.

I don’t have to try and type in a 20-character password

filled with symbols on my phone any more.

I pull up Dashlane instead.

Everything just works, synchronised on all my devices,

across Windows, Mac, iPhone, Android, all automatically.

It plugs into Chrome, Firefox, Edge, Safari, Opera, even Internet Explorer.

And despite the fact that it’s all synchronised in the cloud,

Dashlane themselves don’t know what those passwords are

and based on their security architecture, can never find them out.

I’ll explain that next time.

So: dashlane.com/tomscott for a 30-day free trial of Dashlane Premium,

which includes unlimited password storage and sync.

And if you like it, you can use the code “tomscott” for 10% off.