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The CPU short for central processing unit is like the brain of the computer and once you understand how it works
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You'll understand the computer as well. Let's remove the cover of the CPU and zoom in to see what happens inside
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there are lots of different wires carrying information around the CPU this particular CPU is called the
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6502 and was used in computers like the Apple 2 and the Commodore 64 as well as in the original Nintendo Entertainment System
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This simulation of the 6502 can be found online at visual 6502 org in
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Every CPU there is a particular wire that turns on and off at a steady rate to help keep everything in sync
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That wire is called the clock and the clock in this simulation is turning on about twice a second
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Modern CPUs are measured in gigahertz
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giga meaning billion in hertz meaning times per second so the clock in modern cpus turns on several billion times per second
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That speed is what allows cpus to do very complicated things very quickly
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However, what the CPU does during each clock tick is actually very simple and something. We'll look at more in this video
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For now we'll zoom back out and put the cover back on
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The CPU and your computer might be manufactured by a company like Intel or AMD
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But the type of CPU we're going to look at today is called the Scott CPU
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The Scott CPU doesn't actually exist except as a design in a book called
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But how do it know by john scott the design of the scott cpu is copyrighted
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And it's being used in this video with john's permission
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The book is available at but how do it no com?
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This is a great book that goes through each of the components in the CPU very slowly without using a lot of overly technical
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Jargon, if you've been looking for a book that explains how a computer works. I would highly recommend this one
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So let's flip the CPU over and look underneath
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You'll see a lot of pins sticking out that allow the CPU to take in information and send it back out
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The CPU fits into what's known as the motherboard the motherboard allows all the components in the pewter to connect to each other
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So we'll flip the CPU back over and plug it into the motherboard on
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The right of the motherboard is a place for something called RAM RAM is short for random access memory
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And it just contains all the data that is being processed by the CPU
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Let's learn a little bit more about RAM by looking at how the CPU and RAM interact
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For now well remove the wires on the left and move the motherboard over to make room for the RAM chip
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Ram consists of a list of addresses and at each of those addresses is a piece of data
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the CPU normally requests and processes each piece of data from RAM in order one after the other
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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
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The data can be accessed randomly if it needs to be although normally. It's accessed in order
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When the computer first starts running a program it sends an address to RAM to begin retrieving that program
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The RAM address just consists of a series of ones and zeros representing on and off wires
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Ram doesn't do anything with that address though until the CPU also turns on the set or the enable wire
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If the enable wire is turned on Ram automatically sends whatever piece of data is at that address back to the CPU
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That data is then processed by the CPU accordingly
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Once the CPU is finished processing that piece of data it then sends another address to Ram
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Turns on the enable wire and gets the next piece of data from Ram this process happens over and over again inside the computer
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If the CPU needs to save data to Ram it outputs an address
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Output some data, and then turns on the set wire
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The RAM will then overwrite the data at that address with the new data
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But what is that data inside Ram because it just looks like a bunch of ones and zeros
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Well, it's made up of different things some of the most important pieces of data in RAM are the instructions
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Instructions just tell the CPU to do different things
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There are also numbers inside that data. These are numbers that you might want to add together compare or simply process in some way
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Another thing and this is kind of weird that is in the data is addresses at
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Particular memory addresses in RAM the data itself is also an address
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These addresses can be used for various things
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For instance if you want to output a number to an external device you have to know the address of that particular device
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Do you want to send data to the printer, or do you want to send it to the monitor for instance?
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There are also letters stored in RAM if you want to show some text on the screen
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You would actually store it as a bunch of ones and zeros and RAM
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Each letter is stored as a particular combination of ones and zeros according to a character code these character codes are arbitrary
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Someone just decided that this is a lowercase a and this is an uppercase G for instance
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So that's what's actually in the data inside Ram now
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Let's go back to seeing the data is just a bunch of ones and zeros and we'll now move the RAM chip
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into the RAM socket on the motherboard
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We'll then group the RAM addresses and data together
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Pull the CPU out of the motherboard and look at what's called the instruction set of the CPU
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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
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Understands so there might be a load instruction which loads a number from RAM into the CPU
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After a couple of these load instructions there might be an add instruction that adds these two numbers together
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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
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There might also be a compare instruction after some load instructions, which compares two numbers together to see which one is larger
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or if they are the same
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the compare instruction can be very useful when used in conjunction with what's called a jump if
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Instruction as we saw earlier the CPU generally requests each piece of data from RAM in order one after the other
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Sometimes though the programmer wants to jump to an out of order ram address to process some other instructions and memory
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The jump if instruction checks to see if a certain condition is true before it jumps
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It uses the results of the compare instruction to make this decision
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There is also a regular job instruction that jumps no matter what?
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finally there is an out instruction and an in instruction these will output data to an external device like a monitor or
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Input data from an external device like a keyboard
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These two instructions are often used in conjunction with an address like we talked about earlier
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There are some other instructions in the CPUs instruction set that these are some of the more commonly seen ones
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So as we saw the data in RAM consists of things like instructions
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numbers addresses and letters
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So let's go through a program. That would use this instruction set to play a guessing game
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So it would load a number like nine into the CPU
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Let's say that the programmer decided that that was the right answer
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So he went ahead and put that number into RAM and then comes an end instruction to retrieve the user's guess after
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The in instruction is the address of the keyboard so we can know where we're getting the data from
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Next comes a compare
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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
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Following the compare instruction is a jump if equal instruction which will jump to another address in RAM if those two numbers
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We just talked about are the same
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the jump if equal instruction is immediately followed by a new RAM address if
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The two numbers are the same
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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
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Then the computer ignores the jump if equal instruction and the corresponding address and just keeps going
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Following the jump to address comes an out instruction with the address for the monitor, then the letter capital G
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And then below that would be the letters U e
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SS. Space again, so guess again
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So if the user guesses the wrong number the program would tell him to guess again
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And then jump back up to the in instruction to retrieve that new guess and then process these instructions all over again
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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
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So now let's briefly take a look inside the CPU itself to see how it would process an instruction
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as we saw earlier
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This is the inside of the 6502 CPU
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Let's take away the 6502 wiring and see what's inside the Scot CPU
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The first component is the control unit which is kind of like a captain in the army
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it receives its orders from RAM in the form of an
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instruction and then breaks that instruction down into specific commands for the other components
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One of the most important components under the command of the control unit is the arithmetic logic unit or ALU for short?
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The ALU is what performs all the mathematical operations inside the CPU such as addition subtraction
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Or even comparison like we saw earlier
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The arithmetic logic unit has two inputs will label them input a and input B and assume
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They are two numbers from some previous load instructions
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Now we might want to add those two numbers together
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The control unit receives that instruction from RAM and then tells the ALU what type of operation to perform
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The ALU performs the operation and then outputs the answer
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Sometimes though depending upon the type of instruction the output from the ALU can actually be ignored
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For instance if you have a compare instruction the ALU doesn't need to output an answer instead
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It just needs to tell the control unit how the two numbers compare to each other
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for this the ALU uses
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What are called flags and they help the control unit decide what to do when it receives the next?
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Instruction like jump if which we'll see later
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For now though. Let's say that we are working with an instruction that does produce an output. Where does that output actually go?
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well the eight wires coming out of the ALU would actually run to what is called a register a
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Register is a very simple component whose only job is to store a number temporarily
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registers act just like RAM except
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They are inside the CPU making them faster and more useful for storing a number temporarily Wow instructions being processed
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When the ALU sends the output to the register it won't actually be saved until
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the control unit turns on the registers set wire
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The set wire is just like the one we saw earlier for RAM
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When the set wire is turned on the register saves whatever number is on its input wires
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Once we have the output saved in the register though. How do we get it back out?
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Well when we are ready to move a number out of the register we need another control wire called the enable wire
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That also runs from the control unit to the register as soon as the control unit turns the enable wire on
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The register will output whatever number is saved inside
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the output wires of the register then connect to what is called the CPU bus a
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Bus as we saw earlier on the motherboard is simply a group of wires that connect multiple components inside a computer on
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The bus are some other registers with their own set and enable wires
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These may have numbers from previous instructions already saved inside
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so the control unit will then turn on the set wire of the
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Particular register that it wants to save that number to and that number will be saved in that register
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Afterward the control unit will then turn off the enable wire from the first register and clear the bus
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The four registers at the top are just used for storing numbers between operations
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So they have output wires that go directly back onto the bus
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So now we have moved a number from one register to another just by turning some wires on and off
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That's the advantage of the bus easily moving numbers between components
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The disadvantage of the bus is that you can only have one number on it at a time
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Because of this limitation the arithmetic logic unit uses a temporary register for input B
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When the control unit is processing an instruction involving the ALU it will move one of the inputs to the temporary register
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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
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The other input to the ALU comes directly from the bus
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The control unit will enable another register and that number will become input a to the ALU that
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number stays on the bus until the ALU is finished processing the instruction and
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So now there are two inputs to the ALU and we're ready for the ALU to perform an operation
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As we saw earlier the control unit knows what operation to tell the ALU to perform
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Because of the instruction it receives from rim
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The instruction itself is an another register called the instruction register
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By the way the input wires from the bus won't affect this register since the instruction was already saved in a previous step
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This register like the temporary register, also has no need for an enable wire since it just outputs to the control unit
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Based upon this instruction the control unit then tells the ALU what type of operation to perform
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So let's say the instruction. We're processing is a compare instruction with the compare instruction
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We're not interested in the number that is output by the ALU
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We only want to know how the two inputs compare to each other
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For that we use the flags that we talked about earlier
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Each flag is just a wire that turns on or off depending upon whether or not a certain condition is true
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Inside the Scott CPU there are four flags, and we'll look at two of them now
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The a is larger flag will turn on if input a is larger than input B
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if the inputs are the same
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Then the equal flag turns on and if both of these flags are off that means input B is larger
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But in this case the equal flag is on so that means both inputs are the same
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Once the compare instruction is over. We still need to use the flags for the next instruction
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So we'll save them to a register that only has 4 inputs and four outputs one for each flag
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Once the flags are set into the flags register the CPU is finished with the compare instruction and it can then request the next
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instruction from REM
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generally the next instruction after a compare instruction is a jump if
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instruction this combination of a compare and a jump if instruction by the way is very common in
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programming
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Anytime there is more than one possible path through a program the computer is using these instructions to tell it which way to go
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So now that we're finished with the compare instruction and our flags are saved in the flags register
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We need to tell RAM that we're ready for the next piece of data in this case the next instruction
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So inside the CPU another register that is very important is what's called the instruction address register
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The CPU uses this register to know where the next instruction should come from in RAM
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When the CPU is ready for the next instruction it enables the instruction address register onto the bus
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eventually the instruction address will flow to RAM
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But it doesn't get there directly there is an intermediary register called the memory address register
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Whose only job is to tell Ram what memory address the CPU wants next since it won't always be an instruction
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Once the instruction address is set into the memory address register it has automatically sent to RAM since the memory address register
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Doesn't have an enable wire the control unit then turns on the enable Ram wire and RAM
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Automatically sends back the data at that address which in this case is an instruction
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That instruction is then saved in the instruction register, and the control unit begins processing it in
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This case it's a jump if equal instruction, which checks to see if the equal flag is on
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It does that by running one of its wires and the equal flag wire into a NAND gate
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If both inputs to the and gate or on then the output wire turns on as well
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This output wire will then trigger the jump that
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jump eventually retrieves the next piece of data from RAM
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Which happens to be an address and move it into the instruction address register
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when the jump if equal instruction is over the CPU then processes the instruction at that new address at
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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
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So we'll add the final four wires to our CPU which are used to control the external devices like the monitor and keyboard
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We now have a nearly complete picture of what the Scotts CPU looks like
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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
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Each instruction that we have seen can be processed by the Scott CPU in about 6 clock ticks
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Modern CPUs can process multiple instructions per clock tick meaning that the computer you're using to watch this video is likely
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processing tens or even hundreds of billions of instructions per second
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that phenomenal speed is accomplished by using more than one of each component and
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Making sure that all the components are active as much as possible
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This makes modern CPUs much more complicated than the Scott CPU
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But they are still fundamentally doing the same things as the Scott CPU
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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
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The right are the set RAM and enable RAM wires on
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The top are the RAM address wires on the bottom are the data wires that run to both RAM and the external devices
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And on the left are the input/output control wires
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So let's zoom back out to see the rest of the chip
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And we'll put the cover back on the CPU and put it back in the motherboard
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Using the ports on the Left we can now plug in the cables that connect our monitor and our keyboard
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Each of these ports has an address and that port address is what the CPU uses with an in or an out instruction
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That port address by the way is sent using the data bus since the address bus in this computer is only used for RAM
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So we'll zoom out to see how the motherboard fits inside the computer case in the computer case is the last component
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we'll look at which is the hard drive as
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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
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Store it more permanently for that we use a hard drive
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Inside the hard drive is a spinning disk covered in tiny magnets with a small metal arm floating above it
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The arm moves around to the different parts of the disk where a different data can be stored and retrieved
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The disk and the arm generally move very very quickly, but nowhere near as fast as the CPU can process data
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For this reason all the data from the hard drive must first be moved to RAM before it can be processed
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So we'll put the hard drive back inside the computer and zoom out here. We can see the program
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We just ran and the message telling the user that he guessed correctly
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So now you've seen the very basics of how a computer processes information
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You'll find much more about the Scot CPU in the book at the website, but how do it know comm?
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Also, there are a few small differences between the book and the video
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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.
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Whatever you do, work at it with all your heart, as working for the Lord, not for human masters -Colossians 3:23
