How a CPU Works
Summary
TLDRThis script delves into the CPU's role as the computer's brain, using the 6502 model to illustrate its inner workings. It explains the clock's role in synchronization, contrasting it with modern CPUs' gigahertz speeds. The Scott CPU, a theoretical design, is explored for educational purposes, highlighting components like the ALU and control unit. The script further describes RAM's function, CPU-RAM interactions, and the CPU's instruction set, culminating in a simple guessing game program. It concludes with an exploration of the CPU's internal components, emphasizing the modern CPU's complexity and high-speed capabilities.
Takeaways
- 🧠 The CPU, or Central Processing Unit, acts as the brain of the computer, handling all the processing and information flow.
- 🔍 The 6502 CPU, used in classic computers like the Apple II and Commodore 64, is highlighted as an example to understand the inner workings of a CPU.
- 🔗 The clock within a CPU is crucial for synchronization, with modern CPUs operating at gigahertz speeds, enabling them to perform complex tasks quickly.
- 📚 The Scott CPU is a theoretical design from a book, used in this video with permission, to illustrate the fundamental components and operations of a CPU.
- 🔌 The CPU connects to the motherboard via pins, which allow for the transfer of information to and from other components.
- 💾 RAM, or Random Access Memory, is where the data being processed by the CPU is stored, and it can be accessed in a non-linear manner, hence 'random access'.
- 🔢 Data within RAM consists of instructions, numbers, addresses, and characters, all represented in binary form.
- 🛠️ The CPU's instruction set includes various operations like load, add, store, compare, jump, and I/O instructions, which dictate how the CPU processes data.
- 🔄 Inside the CPU, the Control Unit manages operations, the ALU performs mathematical and logical operations, and registers store and transfer data.
- 🏷 Flags within the CPU indicate the results of operations, such as comparison, and influence the flow of execution through instructions like 'jump if'.
- 🔗 The data bus and address bus facilitate the movement of data and instructions between the CPU, RAM, and other components.
Q & A
What is the role of the CPU in a computer?
-The CPU, or Central Processing Unit, is often referred to as the brain of the computer. It is responsible for processing instructions and managing the flow of data within the computer.
What is the significance of the clock in a CPU?
-The clock in a CPU turns on and off at a steady rate to keep everything in sync. Modern CPUs have clocks measured in gigahertz (GHz), meaning they turn on billions of times per second. This speed allows CPUs to perform complex tasks rapidly.
What is RAM and how does it interact with the CPU?
-RAM, or Random Access Memory, stores data that is currently being processed by the CPU. The CPU sends an address to RAM to retrieve or store data. The data in RAM includes instructions, numbers, addresses, and letters, all of which are used by the CPU to perform tasks.
What is the 'Scott CPU' mentioned in the script?
-The 'Scott CPU' is a fictional CPU design created by John Scott and described in his book 'But How Do It Know?'. It serves as an educational tool to explain how CPUs function without the complexity of modern processors.
How does the CPU use the instruction set to execute commands?
-The CPU uses an instruction set to perform tasks. Common instructions include 'load' (to load data from RAM), 'add' (to add numbers), 'store' (to save results), and 'compare' (to compare numbers). The CPU processes these instructions sequentially or jumps to different addresses based on the instructions.
What is the role of the Arithmetic Logic Unit (ALU) in the CPU?
-The Arithmetic Logic Unit (ALU) performs mathematical operations within the CPU, such as addition, subtraction, and comparison. The control unit directs the ALU to carry out these operations based on the instructions received from RAM.
What are flags in a CPU and how are they used?
-Flags are indicators used by the CPU to determine the result of operations, such as comparisons. For example, a flag might indicate whether one number is larger than another or if two numbers are equal. These flags help the CPU decide the next action, such as whether to jump to a different instruction.
How does the CPU store and retrieve data temporarily during operations?
-The CPU uses registers to store data temporarily during operations. Registers are faster than RAM and are used to hold data that is actively being processed by the CPU. The control unit manages data movement between registers and other components.
What happens to the data in RAM when the computer is turned off?
-When the computer is turned off, all data in RAM is lost. To store data permanently, it must be saved to a storage device like a hard drive, which uses magnetic disks to store data even when the power is off.
How do modern CPUs differ from the Scott CPU in terms of performance?
-Modern CPUs are much more complex than the Scott CPU and can process multiple instructions per clock tick. This allows them to execute tens or even hundreds of billions of instructions per second, enabling the rapid performance required by modern applications.
Outlines
🧠 CPU Basics and Its Role in Computing
This paragraph introduces the CPU (Central Processing Unit) as the brain of the computer, essential for understanding computer operations. It explains the CPU's internal structure, highlighting the 6502 CPU used in historic computers like the Apple II and the Commodore 64. The script mentions the clock as the timing mechanism of the CPU and contrasts its speed with modern CPUs measured in gigahertz. The Scott CPU, a theoretical design from 'But How Do It Know' by J. Scott, is introduced as an educational tool to understand CPU components. The paragraph also explains the CPU's interaction with RAM (Random Access Memory), the data it processes, and how instructions, numbers, and addresses within RAM guide the CPU's operations.
📝 Understanding CPU Instruction Set and Data Flow
The second paragraph delves into the CPU's instruction set, which defines the basic operations a CPU can perform, such as loading, adding, storing, comparing, and jumping between memory addresses. It describes how the CPU uses instructions in RAM for tasks like playing a guessing game, emphasizing the role of the 'jump if' instruction in altering the flow of execution based on conditions. The paragraph also provides an overview of the CPU's internal components, including the control unit, the arithmetic logic unit (ALU), and registers, explaining how they work together to process instructions and perform operations.
🔍 Inside the CPU: Control Unit and ALU Operations
This paragraph provides an in-depth look at the CPU's control unit and ALU, describing their functions within the CPU. The control unit is likened to a commander, translating instructions from RAM into operational commands for other components. The ALU is responsible for mathematical operations and comparisons, using inputs from registers and the bus. The explanation includes how the ALU uses flags to communicate comparison results to the control unit, which in turn affects the flow of execution through jump instructions. The paragraph also details the process of moving data between registers and the significance of the set and enable wires in this process.
🔄 CPU Data Processing and Instruction Execution
The fourth paragraph focuses on the CPU's data processing and instruction execution cycle. It explains how the CPU uses the instruction address register and the memory address register to fetch instructions from RAM. The paragraph clarifies the role of the instruction register in holding the current instruction and the use of flags and gates in conditional branching. It also touches on how the CPU interacts with external devices through input/output instructions, emphasizing the bus system's role in data movement within the CPU and the computer system as a whole.
💾 Completing the Picture: CPU, RAM, and Storage
The final paragraph wraps up the overview of the CPU's operation by discussing the computer's storage component, the hard drive. It contrasts the hard drive's slower speed with the CPU's rapid data processing capabilities and explains the necessity of moving data from the hard drive to RAM for processing. The paragraph also describes the physical connections between the CPU, RAM, and peripheral devices like monitors and keyboards through the motherboard. It concludes with a reference to the book 'But How Do It Know' for further information and acknowledges the differences between the book's content and the video presentation.
🙏 Closing Thoughts and Biblical Reference
In the closing paragraph, the script shifts from technical discussion to a philosophical and spiritual note. It includes a quote from the Bible, Colossians 3:23, encouraging viewers to work with dedication as if serving the Lord, rather than people. This paragraph serves as a reminder of the video's educational purpose and imparts a moral message to the audience.
Mindmap
Keywords
💡CPU (Central Processing Unit)
💡Clock
💡6502
💡Motherboard
💡RAM (Random Access Memory)
💡Instruction Set
💡ALU (Arithmetic Logic Unit)
💡Flags
💡Register
💡Bus
💡Hard Drive
Highlights
The CPU, as the brain of the computer, is fundamental to understanding computer operations.
6502 CPU was used in iconic computers like the Apple II and Commodore 64, as well as the original Nintendo Entertainment System.
The clock wire in a CPU is pivotal for synchronization, with modern CPUs operating in gigahertz.
The Scott CPU, a theoretical design from a book, serves as an educational tool for understanding CPU components.
The motherboard facilitates communication between the CPU and other components in a computer.
RAM is essential for holding data being processed by the CPU and operates on a random access basis.
The interaction between CPU and RAM involves addresses and data retrieval, controlled by set and enable wires.
Instructions, numbers, addresses, and letters are the primary data types stored in RAM.
The CPU's instruction set defines the operations it can perform, such as load, add, store, and compare.
Jump if and jump instructions allow for non-linear execution paths in a program.
The ALU within the CPU performs all mathematical operations and comparisons.
Flags within the CPU indicate the results of operations, influencing control unit decisions.
Registers within the CPU temporarily store numbers, managed by set and enable control wires.
The CPU bus facilitates the movement of data between CPU components, but only allows one number at a time.
The instruction address register and memory address register work in tandem to fetch instructions from RAM.
Modern CPUs process multiple instructions per clock tick, showcasing their incredible computational speed.
The hard drive serves as permanent storage, but data must be transferred to RAM for CPU processing.
The video provides a comprehensive overview of the CPU's role in data processing within a computer.
Transcripts
The CPU short for central processing unit is like the brain of the computer and once you understand how it works
You'll understand the computer as well. Let's remove the cover of the CPU and zoom in to see what happens inside
there are lots of different wires carrying information around the CPU this particular CPU is called the
6502 and was used in computers like the Apple 2 and the Commodore 64 as well as in the original Nintendo Entertainment System
This simulation of the 6502 can be found online at visual 6502 org in
Every CPU there is a particular wire that turns on and off at a steady rate to help keep everything in sync
That wire is called the clock and the clock in this simulation is turning on about twice a second
Modern CPUs are measured in gigahertz
giga meaning billion in hertz meaning times per second so the clock in modern cpus turns on several billion times per second
That speed is what allows cpus to do very complicated things very quickly
However, what the CPU does during each clock tick is actually very simple and something. We'll look at more in this video
For now we'll zoom back out and put the cover back on
The CPU and your computer might be manufactured by a company like Intel or AMD
But the type of CPU we're going to look at today is called the Scott CPU
The Scott CPU doesn't actually exist except as a design in a book called
But how do it know by john scott the design of the scott cpu is copyrighted
And it's being used in this video with john's permission
The book is available at but how do it no com?
This is a great book that goes through each of the components in the CPU very slowly without using a lot of overly technical
Jargon, if you've been looking for a book that explains how a computer works. I would highly recommend this one
So let's flip the CPU over and look underneath
You'll see a lot of pins sticking out that allow the CPU to take in information and send it back out
The CPU fits into what's known as the motherboard the motherboard allows all the components in the pewter to connect to each other
So we'll flip the CPU back over and plug it into the motherboard on
The right of the motherboard is a place for something called RAM RAM is short for random access memory
And it just contains all the data that is being processed by the CPU
Let's learn a little bit more about RAM by looking at how the CPU and RAM interact
For now well remove the wires on the left and move the motherboard over to make room for the RAM chip
Ram consists of a list of addresses and at each of those addresses is a piece of data
the CPU normally requests and processes each piece of data from RAM in order one after the other
However, if the CPU is instructed to pull data out of order it can do so that is why it's called random access memory
The data can be accessed randomly if it needs to be although normally. It's accessed in order
When the computer first starts running a program it sends an address to RAM to begin retrieving that program
The RAM address just consists of a series of ones and zeros representing on and off wires
Ram doesn't do anything with that address though until the CPU also turns on the set or the enable wire
If the enable wire is turned on Ram automatically sends whatever piece of data is at that address back to the CPU
That data is then processed by the CPU accordingly
Once the CPU is finished processing that piece of data it then sends another address to Ram
Turns on the enable wire and gets the next piece of data from Ram this process happens over and over again inside the computer
If the CPU needs to save data to Ram it outputs an address
Output some data, and then turns on the set wire
The RAM will then overwrite the data at that address with the new data
But what is that data inside Ram because it just looks like a bunch of ones and zeros
Well, it's made up of different things some of the most important pieces of data in RAM are the instructions
Instructions just tell the CPU to do different things
There are also numbers inside that data. These are numbers that you might want to add together compare or simply process in some way
Another thing and this is kind of weird that is in the data is addresses at
Particular memory addresses in RAM the data itself is also an address
These addresses can be used for various things
For instance if you want to output a number to an external device you have to know the address of that particular device
Do you want to send data to the printer, or do you want to send it to the monitor for instance?
There are also letters stored in RAM if you want to show some text on the screen
You would actually store it as a bunch of ones and zeros and RAM
Each letter is stored as a particular combination of ones and zeros according to a character code these character codes are arbitrary
Someone just decided that this is a lowercase a and this is an uppercase G for instance
So that's what's actually in the data inside Ram now
Let's go back to seeing the data is just a bunch of ones and zeros and we'll now move the RAM chip
into the RAM socket on the motherboard
We'll then group the RAM addresses and data together
Pull the CPU out of the motherboard and look at what's called the instruction set of the CPU
As we saw earlier some of the pieces of data in RAM are instructions and each CPU has its own set of instructions that it
Understands so there might be a load instruction which loads a number from RAM into the CPU
After a couple of these load instructions there might be an add instruction that adds these two numbers together
After an add instruction might be a store instruction, which saves the result of that addition back out to RAM to be used later on
There might also be a compare instruction after some load instructions, which compares two numbers together to see which one is larger
or if they are the same
the compare instruction can be very useful when used in conjunction with what's called a jump if
Instruction as we saw earlier the CPU generally requests each piece of data from RAM in order one after the other
Sometimes though the programmer wants to jump to an out of order ram address to process some other instructions and memory
The jump if instruction checks to see if a certain condition is true before it jumps
It uses the results of the compare instruction to make this decision
There is also a regular job instruction that jumps no matter what?
finally there is an out instruction and an in instruction these will output data to an external device like a monitor or
Input data from an external device like a keyboard
These two instructions are often used in conjunction with an address like we talked about earlier
There are some other instructions in the CPUs instruction set that these are some of the more commonly seen ones
So as we saw the data in RAM consists of things like instructions
numbers addresses and letters
So let's go through a program. That would use this instruction set to play a guessing game
So it would load a number like nine into the CPU
Let's say that the programmer decided that that was the right answer
So he went ahead and put that number into RAM and then comes an end instruction to retrieve the user's guess after
The in instruction is the address of the keyboard so we can know where we're getting the data from
Next comes a compare
Instruction that checks to see if those two numbers the one saved by the programmer and the one entered by the user are the same
Following the compare instruction is a jump if equal instruction which will jump to another address in RAM if those two numbers
We just talked about are the same
the jump if equal instruction is immediately followed by a new RAM address if
The two numbers are the same
The CPU jumps to that new address to begin processing its next set of instructions from there if the two numbers are not the same
Then the computer ignores the jump if equal instruction and the corresponding address and just keeps going
Following the jump to address comes an out instruction with the address for the monitor, then the letter capital G
And then below that would be the letters U e
SS. Space again, so guess again
So if the user guesses the wrong number the program would tell him to guess again
And then jump back up to the in instruction to retrieve that new guess and then process these instructions all over again
By the way the in and instructions used here have been simplified somewhat, but you'll find them covered in more detail in the book
So now let's briefly take a look inside the CPU itself to see how it would process an instruction
as we saw earlier
This is the inside of the 6502 CPU
Let's take away the 6502 wiring and see what's inside the Scot CPU
The first component is the control unit which is kind of like a captain in the army
it receives its orders from RAM in the form of an
instruction and then breaks that instruction down into specific commands for the other components
One of the most important components under the command of the control unit is the arithmetic logic unit or ALU for short?
The ALU is what performs all the mathematical operations inside the CPU such as addition subtraction
Or even comparison like we saw earlier
The arithmetic logic unit has two inputs will label them input a and input B and assume
They are two numbers from some previous load instructions
Now we might want to add those two numbers together
The control unit receives that instruction from RAM and then tells the ALU what type of operation to perform
The ALU performs the operation and then outputs the answer
Sometimes though depending upon the type of instruction the output from the ALU can actually be ignored
For instance if you have a compare instruction the ALU doesn't need to output an answer instead
It just needs to tell the control unit how the two numbers compare to each other
for this the ALU uses
What are called flags and they help the control unit decide what to do when it receives the next?
Instruction like jump if which we'll see later
For now though. Let's say that we are working with an instruction that does produce an output. Where does that output actually go?
well the eight wires coming out of the ALU would actually run to what is called a register a
Register is a very simple component whose only job is to store a number temporarily
registers act just like RAM except
They are inside the CPU making them faster and more useful for storing a number temporarily Wow instructions being processed
When the ALU sends the output to the register it won't actually be saved until
the control unit turns on the registers set wire
The set wire is just like the one we saw earlier for RAM
When the set wire is turned on the register saves whatever number is on its input wires
Once we have the output saved in the register though. How do we get it back out?
Well when we are ready to move a number out of the register we need another control wire called the enable wire
That also runs from the control unit to the register as soon as the control unit turns the enable wire on
The register will output whatever number is saved inside
the output wires of the register then connect to what is called the CPU bus a
Bus as we saw earlier on the motherboard is simply a group of wires that connect multiple components inside a computer on
The bus are some other registers with their own set and enable wires
These may have numbers from previous instructions already saved inside
so the control unit will then turn on the set wire of the
Particular register that it wants to save that number to and that number will be saved in that register
Afterward the control unit will then turn off the enable wire from the first register and clear the bus
The four registers at the top are just used for storing numbers between operations
So they have output wires that go directly back onto the bus
So now we have moved a number from one register to another just by turning some wires on and off
That's the advantage of the bus easily moving numbers between components
The disadvantage of the bus is that you can only have one number on it at a time
Because of this limitation the arithmetic logic unit uses a temporary register for input B
When the control unit is processing an instruction involving the ALU it will move one of the inputs to the temporary register
The temporary register has no need for an enable wire since it only outputs to the ALU and doesn't conflict with any other registers
The other input to the ALU comes directly from the bus
The control unit will enable another register and that number will become input a to the ALU that
number stays on the bus until the ALU is finished processing the instruction and
So now there are two inputs to the ALU and we're ready for the ALU to perform an operation
As we saw earlier the control unit knows what operation to tell the ALU to perform
Because of the instruction it receives from rim
The instruction itself is an another register called the instruction register
By the way the input wires from the bus won't affect this register since the instruction was already saved in a previous step
This register like the temporary register, also has no need for an enable wire since it just outputs to the control unit
Based upon this instruction the control unit then tells the ALU what type of operation to perform
So let's say the instruction. We're processing is a compare instruction with the compare instruction
We're not interested in the number that is output by the ALU
We only want to know how the two inputs compare to each other
For that we use the flags that we talked about earlier
Each flag is just a wire that turns on or off depending upon whether or not a certain condition is true
Inside the Scott CPU there are four flags, and we'll look at two of them now
The a is larger flag will turn on if input a is larger than input B
if the inputs are the same
Then the equal flag turns on and if both of these flags are off that means input B is larger
But in this case the equal flag is on so that means both inputs are the same
Once the compare instruction is over. We still need to use the flags for the next instruction
So we'll save them to a register that only has 4 inputs and four outputs one for each flag
Once the flags are set into the flags register the CPU is finished with the compare instruction and it can then request the next
instruction from REM
generally the next instruction after a compare instruction is a jump if
instruction this combination of a compare and a jump if instruction by the way is very common in
programming
Anytime there is more than one possible path through a program the computer is using these instructions to tell it which way to go
So now that we're finished with the compare instruction and our flags are saved in the flags register
We need to tell RAM that we're ready for the next piece of data in this case the next instruction
So inside the CPU another register that is very important is what's called the instruction address register
The CPU uses this register to know where the next instruction should come from in RAM
When the CPU is ready for the next instruction it enables the instruction address register onto the bus
eventually the instruction address will flow to RAM
But it doesn't get there directly there is an intermediary register called the memory address register
Whose only job is to tell Ram what memory address the CPU wants next since it won't always be an instruction
Once the instruction address is set into the memory address register it has automatically sent to RAM since the memory address register
Doesn't have an enable wire the control unit then turns on the enable Ram wire and RAM
Automatically sends back the data at that address which in this case is an instruction
That instruction is then saved in the instruction register, and the control unit begins processing it in
This case it's a jump if equal instruction, which checks to see if the equal flag is on
It does that by running one of its wires and the equal flag wire into a NAND gate
If both inputs to the and gate or on then the output wire turns on as well
This output wire will then trigger the jump that
jump eventually retrieves the next piece of data from RAM
Which happens to be an address and move it into the instruction address register
when the jump if equal instruction is over the CPU then processes the instruction at that new address at
That new address, maybe some instructions that output the text you guessed correctly onto the screen because now we know that the user guessed correctly
So we'll add the final four wires to our CPU which are used to control the external devices like the monitor and keyboard
We now have a nearly complete picture of what the Scotts CPU looks like
Data moves around inside the CPU using the bus and is stored in each register according to how that data is going to be used
Each instruction that we have seen can be processed by the Scott CPU in about 6 clock ticks
Modern CPUs can process multiple instructions per clock tick meaning that the computer you're using to watch this video is likely
processing tens or even hundreds of billions of instructions per second
that phenomenal speed is accomplished by using more than one of each component and
Making sure that all the components are active as much as possible
This makes modern CPUs much more complicated than the Scott CPU
But they are still fundamentally doing the same things as the Scott CPU
So now let's zoom back out and we can see all the wires that run back out to the pins on the chip on
The right are the set RAM and enable RAM wires on
The top are the RAM address wires on the bottom are the data wires that run to both RAM and the external devices
And on the left are the input/output control wires
So let's zoom back out to see the rest of the chip
And we'll put the cover back on the CPU and put it back in the motherboard
Using the ports on the Left we can now plug in the cables that connect our monitor and our keyboard
Each of these ports has an address and that port address is what the CPU uses with an in or an out instruction
That port address by the way is sent using the data bus since the address bus in this computer is only used for RAM
So we'll zoom out to see how the motherboard fits inside the computer case in the computer case is the last component
we'll look at which is the hard drive as
Soon as the power to the computer is turned off all the data and RAM is lost so you have to have a way to
Store it more permanently for that we use a hard drive
Inside the hard drive is a spinning disk covered in tiny magnets with a small metal arm floating above it
The arm moves around to the different parts of the disk where a different data can be stored and retrieved
The disk and the arm generally move very very quickly, but nowhere near as fast as the CPU can process data
For this reason all the data from the hard drive must first be moved to RAM before it can be processed
So we'll put the hard drive back inside the computer and zoom out here. We can see the program
We just ran and the message telling the user that he guessed correctly
So now you've seen the very basics of how a computer processes information
You'll find much more about the Scot CPU in the book at the website, but how do it know comm?
Also, there are a few small differences between the book and the video
But those shouldn't detract from your understanding of either you can find a list of these differences in the video description. Thanks for watching.
Whatever you do, work at it with all your heart, as working for the Lord, not for human masters -Colossians 3:23
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