Fetch Decode Execute Cycle in more detail
Summary
TLDRThe 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
🤖 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.
🔢 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.
Mindmap
Keywords
💡Central Processing Unit (CPU)
💡Fetch-Decode-Execute Cycle
💡High-Level Language
💡Assembly Language
💡Accumulator
💡Direct Addressing
💡Symbolic Addressing
💡Machine Code
💡Registers
💡Control Unit
💡Arithmetic and Logic Unit (ALU)
💡Adder Circuits
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
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