Fetch Decode Execute Cycle in more detail

Summary

The script delves into the CPU's fetch-decode-execute cycle, illustrating how a high-level program is translated into assembly and machine code. It explains the role of CPU components like the accumulator, ALU, and registers in executing a simple program involving arithmetic operations. The process is simplified for clarity, highlighting the CPU's function as an advanced adding machine, emphasizing the foundational role of memory addressing and data manipulation in computation.

Takeaways

• 💡 The central processing unit (CPU) operates through a fetch, decode, execute cycle when running a stored program.
• 🔍 High-level language code is translated into assembly language, which requires more commands and consideration of the accumulator.
• 📍 Assembly language can use direct or symbolic addressing to refer to memory locations.
• 🤖 Once assembly code is turned into machine code, it is stored in RAM, and the memory addresses are in binary.
• 🔢 The CPU contains several registers, including the accumulator, program counter, current instruction register, memory address register, and memory data register.
• 🔄 The program counter holds the address of the next instruction to be fetched and increments after each instruction is fetched.
• 📥 The memory address register is used to load the address of the instruction or data to be fetched from RAM.
• 📤 The memory data register is used to transfer data to and from the RAM.
• ⚙️ The control unit within the CPU decodes the instruction and directs the ALU to perform the necessary operations.
• 🧩 The arithmetic and logic unit (ALU) performs operations such as addition, using simple adder circuits.
• 🔚 The final step of the cycle is to store the result from the accumulator back into memory, completing the execution of the program.

Q & A

What is the fetch-decode-execute cycle in the context of a CPU?

The fetch-decode-execute cycle is the fundamental process that a CPU follows to execute a program. It involves three main steps: fetching the instruction from memory, decoding the instruction to understand what it means, and then executing the instruction by performing the required operation.

Why is a high-level language easier to understand than assembly language?

A high-level language is designed to be more human-readable and abstracts away the low-level details of the hardware. Assembly language, on the other hand, is closer to machine code and requires a deeper understanding of the CPU's architecture, making it less intuitive for humans.

What is direct addressing in assembly language?

Direct addressing is a method in assembly language where memory addresses are referred to by their numerical values. It is a straightforward way to access data stored at specific memory locations.

What is symbolic addressing and how does it differ from direct addressing?

Symbolic addressing is an alternative to direct addressing where memory addresses are referred to by symbolic names or labels instead of numerical values. This makes the code more readable and easier to maintain but requires the assembler to translate these symbols into actual memory addresses.

What is the purpose of the accumulator in a CPU?

The accumulator is a register in the CPU that is used to hold intermediate results during arithmetic and logical operations. It plays a central role in many CPU operations, such as storing the result of an addition before it is stored back into memory.

Can you explain the role of the control unit in the CPU during program execution?

The control unit manages the operation of the CPU, coordinating the flow of data and instructions between the various components of the CPU. It decodes the instructions and generates the necessary control signals to execute them, ensuring that the CPU operates in a coordinated and efficient manner.

What are the main components of the CPU that are involved in executing a program?

The main components involved in executing a program are the control unit, the accumulator (part of the ALU), the program counter, the current instruction register, the memory address register, and the memory data register.

How does the program counter (PC) function during the execution of a program?

The program counter holds the memory address of the next instruction to be executed. After an instruction is fetched, the PC is incremented to point to the next instruction. This ensures a sequential execution of instructions unless altered by control flow instructions like jumps or branches.

What is the role of the memory address register and memory data register during the fetch and execution of instructions?

The memory address register holds the address of the memory location from which data or instructions are to be fetched. The memory data register is used to transfer data between the memory and the CPU's internal registers, such as the accumulator, during the fetch, decode, and execute stages.

How does the ALU contribute to the execution of arithmetic operations?

The ALU (Arithmetic Logic Unit) performs arithmetic operations like addition, subtraction, multiplication, and division, as well as logical operations like AND, OR, and NOT. It is responsible for the computation part of the CPU's function, executing the arithmetic instructions decoded by the control unit.

What is the significance of the 'add' instruction in the context of the CPU's operation?

The 'add' instruction is a fundamental operation in the CPU that tells the ALU to add the contents of a specified memory location to the value currently in the accumulator. It is an example of how the CPU performs basic arithmetic operations, which are the building blocks of more complex computations.

Outlines

00:00

🤖 CPU's Fetch-Decode-Execute Cycle

This paragraph delves into the fundamental operation of a CPU when executing a stored program, focusing on the fetch-decode-execute cycle. It begins with a high-level language code example that performs a simple operation with variables X, Y, and Z, and then contrasts it with the equivalent assembly language commands, highlighting the necessity of considering memory locations and the accumulator. The explanation transitions into a discussion on machine code and the structure of a 16-bit machine, emphasizing the use of operation codes and operand addresses. The paragraph also introduces the various registers within the CPU, such as the control unit, accumulator, arithmetic and logic unit, program counter, current instruction register, memory address register, and memory data register, outlining their roles in running a program. The process is illustrated with a step-by-step walkthrough of how these components interact during the execution of a program, starting from the initial fetch of the first instruction to the increment of the program counter and the eventual execution of the instruction set.

05:01

🔢 Arithmetic Operations and Memory Interaction

The second paragraph continues the exploration of the CPU's inner workings by detailing the execution of arithmetic operations and the interaction with memory. It explains the process of decoding and executing an 'add' instruction, which involves the control unit, arithmetic and logic unit (ALU), and the memory address and data registers. The paragraph clarifies the function of the ALU's adder circuits, emphasizing that at its core, a computer is an advanced adding machine. The summary includes a step-by-step account of how the CPU fetches, decodes, and executes each instruction, from loading values into the accumulator to adding them and storing the result back into memory. The paragraph concludes with the final steps of the program, where the result is stored in a specified memory location, marking the completion of the program execution cycle.

Keywords

💡Central Processing Unit (CPU)

The CPU is the primary component of a computer that performs the basic arithmetic, logic, and input/output (I/O) operations specified by the instructions of the computer program. In the context of the video, the CPU is described as having various registers that work together to execute a program, emphasizing its role in the fetch, decode, and execute cycle.

💡Fetch-Decode-Execute Cycle

This is the fundamental operation cycle of a CPU where it fetches an instruction from memory, decodes what operation it specifies, and then executes that operation. The video script describes this cycle in detail, showing how a single line of code is processed through these stages in the CPU.

💡High-Level Language

A high-level programming language is a language that is easier for humans to understand and use than low-level languages like machine code or assembly language. The script mentions a line of code written in a high-level language, which simplifies the process of coding compared to assembly language.

💡Assembly Language

Assembly language is a low-level programming language that is specific to a particular computer architecture. It is converted into machine code by an assembler. The video script contrasts assembly language with high-level language, indicating that more commands are needed in assembly to perform the same task.

💡Accumulator

The accumulator is a type of register in a CPU used for storing intermediate results of arithmetic and logic operations. The script refers to the accumulator in the context of an assembly language example, where it is used to hold and add values from memory locations.

💡Direct Addressing

Direct addressing is a method of referring to memory locations by their numerical addresses in assembly language. The script uses direct addressing as an example when explaining how memory locations are specified in assembly language instructions.

💡Symbolic Addressing

Symbolic addressing is a method of using symbolic names instead of numerical addresses to refer to memory locations in assembly language. The script contrasts this with direct addressing, indicating that it depends on the language and can make code more readable.

💡Machine Code

Machine code is the low-level binary code that is executed directly by a computer's CPU. The script explains that the assembly language source code is turned into machine code, which is what the CPU actually runs.

💡Registers

Registers are small, fast storage locations within a CPU used for rapid data access. The video script describes various registers such as the program counter, instruction register, memory address register, and memory data register, and how they interact during program execution.

💡Control Unit

The control unit is a part of the CPU that directs the operation of the processor by decoding instruction fetches and by providing timing and control signals to coordinate activities among the various components of the system. The script describes the role of the control unit in the fetch-decode-execute cycle.

💡Arithmetic and Logic Unit (ALU)

The ALU is a part of the CPU that performs arithmetic operations such as addition, subtraction, multiplication, and division, as well as bitwise logic operations. The script explains how the ALU is used in the execution of an add instruction, emphasizing its role in processing data.

💡Adder Circuits

Adder circuits are the fundamental building blocks within the ALU that perform addition operations. The script mentions adder circuits to illustrate the simplicity of the underlying hardware that enables complex computer operations.

Highlights

The explanation of the fetch, decode, execute cycle within the CPU when a stored program runs.

Conversion of a high-level language code to assembly language, emphasizing the need for more commands and the use of the accumulator.

Introduction to direct and symbolic addressing in assembly languages.

Transformation of assembly language source code into machine code through the assembly process.

Description of CPU registers including the accumulator, control unit, program counter, and memory registers.

The role of the program counter in storing the address of the next instruction to be fetched.

Process of fetching an instruction from RAM using the memory address register and memory data register.

Incrementing the program counter to always point to the next instruction after fetching the current one.

Decoding of instructions by the control unit and its role in opening various pathways for execution.

Execution of the 'load' instruction, detailing the process of moving data from memory into the accumulator.

Explanation of the arithmetic and logic unit (ALU) and its function in performing addition.

The concept of adder circuits within the ALU and the fundamental nature of a computer as an advanced adding machine.

Fetching and decoding subsequent instructions to continue the program execution.

The final instruction to store the result from the accumulator back into memory, completing the program.

Clarification of common misconceptions about program storage and memory addressing in real-world scenarios.

The practical application of the CPU's registers in running a program, from fetching to storing results.

The significance of the memory address and data registers in moving data to and from RAM during program execution.

Transcripts

let's consider what's going on inside

the central processing unit when a

stored program runs namely the fetch

decode execute cycle first let's

consider this single line of code which

has been written in a high-level

language you should be able to get your

head around that take the contents of

variable X add Y and store the result in

Z if I was going to write something

similar in an assembly language I would

need more commands

I need to think about the accumulator so

what we're saying here is load the

contents of memory location 10 into the

accumulator add the contents of memory

location 11 to whatever's in the

accumulator and then store

whatever's in the accumulator back into

location 12 referring to memory

addresses by numbers is called direct

addressing some assembly languages will

allow you to use what's called symbolic

addresses so for example I might say

load X add Y store Zed that depends on

the language once my assembly language

source code has been turned into machine

code specifically once it's been

assembled it might look something more

like this you can see here we're using 6

bits for the operation code and 10 bits

for the operand address you might find

something like this in a very old 16-bit

machine now let's think about the

registers inside the CPU the blue box

shows the CPU and we can see inside

there we've got a control unit we have

the accumulator and the arithmetic and

logic unit we've also got some other

registers the program counter the

current instruction register the memory

address register and the memory data

register and we're going to take a look

at how these different registers

cooperate to run the program you can see

on the right hand side I've got the

memory and the program has already been

stored in memory

now I think we need to take a reality

check at this stage first of all the

accumulator is actually a register which

is part of the arithmetic and logic unit

but for the purposes of this

demonstration we've separated them

secondly the program wouldn't be stored

as assembly code as shown here it would

be the machine code that's inside the

RAM the memory addresses themselves

would also be in binary and to tell you

the truth

we'd be numbering from the bottom up but

for the sake of this presentation we've

distorted the reality a little bit so

what happens when a program runs to

begin with the program counter stores

the address of the next instruction to

be fetched in this case the first

instruction which you can see is in

memory location 100 before we can fetch

the instruction from the RAM that memory

address has to be placed into the memory

address register once this has happened

the contents of that memory address can

then be loaded into the memory data

register so we're fetching the first

instruction now next that instruction

needs to be placed inside the current

instruction register once it's been

fetched into the current instruction

register the program counter will

increment by one so the program counter

is always pointing to the memory address

of the next instruction to be fetched

before we've even executed this one now

we can decode the instruction and

without going into detail the control

unit comes into play and it opens

various pathways the next stage in the

process is to execute that instruction

the instruction is load 10 in other

words load the contents of memory

address 10 so we're going to fetch

something from memory address 10 so

memory address location 10 now goes into

the memory address register then we can

take the contents of memory location 10

and place that into the memory data

register overwriting its previous

contents and then the contents of that

memory address can be placed into the

accumulator that's the first instruction

executed now we're going to fetch the

next instruction the program counter is

already pointing to the memory address

of the next instruction so that memory

address can be copied from the program

counter into the memory address register

we can then take the contents of that

memory location and put it into the

memory data register now it will move

into the current instruction register

ready to be decoded but before we do

that we increment the program counter so

it's pointing to the next instruction

okay so it's time to decode this

instruction so add 11 is passed to the

control unit various pathways are opened

and the control unit makes sense of the

instruction so this instruction says add

the contents of memory location 11 to

whatever's in the accumulator so this is

where the arithmetic and logic unit

really comes into play an add

instruction is passed to the ALU the

contents of the accumulator are moved to

another place ready to be worked with

now we can get the contents of memory

location 11 and because we're going to

fetch something from the memory that

memory address has to be placed into the

memory address register once that's

happened we can get the contents of that

memory location and place it into the

memory data register you're probably

starting to see what the memory address

register and the memory data register do

whenever we want to get something from

the RAM or move something into the RAM

as we'll see in a moment

we must put its memory address into the

memory address register whenever data or

a program instruction moves to or from

the RAM it goes via the memory data

register so we have our number 3

that gets placed into the accumulator

and then the arithmetic and logic unit

will add that to the two that we

originally had and place the result back

into the accumulator so we're keeping a

running total it's accumulating the

result it might be worth mentioning at

this stage that the circuits inside the

arithmetic and logic unit are called

adder circuits they're actually very

simple the truth is a computer is just a

very sophisticated adding machine okay

so now we're ready to get the next

instruction which is in location 102 so

the memory location 102 is copied into

the memory address register the contents

of that memory location in this case the

last instruction are copied into the

memory data register this instruction

moves into the current instruction

register you can probably see what's

going to happen next we increment the

program counter we would probably have a

stop or a halt instruction in that

position now we can decode that

instruction which basically says store

the result in the accumulator back into

location 12 we're going to put something

into location 12 so that memory address

must be put into the memory address

register and the contents of the

accumulator move via the memory data

register into that memory location and

that's our program complete