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Computer Architecture - Registers and The Fetch Execute Cycle

A presentation on Registers within the CPU and the Fetch Execute Cycle
by Ben Thain on 2 October 2013

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Transcript of Computer Architecture - Registers and The Fetch Execute Cycle

Computer Architecture - Registers & The Fetch Execute Cycle
15.01.2013
Ben Thain
Lesson Objectives
All Students will:
Some Students will:
Few Students will:
Understand what the registers do in a computer system and the characteristics of the Fetch Execute Cycle
Complete the Register identification work sheet
Represent different memory registers in a diagram and annotate their role and responsibility.
These students will also be able to identify whether the register falls into the special register or the general register category.
Illustrate the Fetch execute cycle with annotation free from tuition including registers and flow representation.
So, What do we remember about a processor (CPU) from before the holidays?
Control
Unit

The Control Unit controls all the components within the computer system, it reads through all program instructions, interprets them and turns them into control signals. The Control unit also decodes the instruction and directs the data to be moved from memory to the ALU
ALU
Or the Arithmetic/logic unit is the part of the CPU that does the adding, subtracting, multiplying and dividing of the information that it has been fed.
But that cant be ALL
there is to it!
NO, There is a lot, lot more to it behind the scenes!...Lets take a look!
Before we go any further we need to consider Registers.

Registers are the first piece of the puzzle to understanding how a processor works and how it helps move information round the CPU

Registers
What do we need to know
about Registers?
Registers in a CPU are small areas of memory
Registers, because they are attached directly to the CPU are super fast memory
There are two types of Register. General Purpose and Special Purpose
We need registers because information can ONLY be processed in the CPU if it comes from a memory source.
And what memory source could be faster!
Special Purpose Registers
General Purpose Register
These registers are used along with other registers to perform arithmetic & logical operations. These registers are also used for data movement purposes inside the computer. These registers are called EAX. EBX. ECX and EDX.

AX (Accumulator Register): It is used for arithmetic and other data operations.

BX (Base Register): It is used to-store memory addresses of data stored in main memory during arithmetic and data movement operations.

CX (Counter Register): It is used for counting purpose. It acts as counter for looping.

DX (Data Register): It is used to hold data during division and multiplication operations.
This diagram shows how the General Purpose registers are broken down. The Yellow chunks are 8-bit segments to the register. Two of these are obviously 16-bit registers and with the 'E' Prefix, it is then displaying a 32-bit register.
Instruction Register (IR): The instruction register holds the instruction currently being executed.

Memory Data Register (MDR): The memory data register (also known as the memory buffer register or data buffer) holds the piece of data that has been fetched from memory.

Memory Address Register (MAR): The memory address register holds the address of the next piece of memory to be fetched.

Program Counter (PC): The program counter holds the location of the next instruction to be fetched from memory. It is automatically incremented between supplying the address of the next instruction and the instruction being executed.

Accumulator: The accumulator is an internal CPU register used as the default location to store any calculations performed by the arithmetic and logic unit.
As we know, there are many General Purpose Registers for general data handling. A special Purpose Register Has a specific control or data handling task it needs to preform. Lets take a look at these SPR's...
What have we learnt?
There are multiple registers in a computer processor
They are essentially fast acting memory
There are some for general use
There are some that have a single, special purpose
How does this relate to the fetch execute cycle?
Registers are
IMPERATIVE
to the Fetch Execute Cycle, Nay, making a computer work!
So we know that there are General purpose Registers. We know there are Special purpose registers. We know the CPU contains a Control Unit and an ALU

We know there is a Bus system throughout the CPU and motherboard and we know there is main memory (what we would refer to as RAM or the HDD)

The Fetch Execute Cycle brings all these individual elements together to make a computer system function...Lets take a look
Stop Right There!
Lets take a look at the register task sheet. You are to complete this to the best of your ability with the information that you have been given so far.

This will be peer marked by a class mate and the papers retained by your lecturer for evidence of assessment.
Lets take a closer look at the Fetch Execute cycle
Also known as the Fetch Decode Execute Cycle this is how all instructions sent through the CPU are processed.
The following are the stages the instruction data takes through the CPU
The Control Unit Fetches the Instruction from the memory
The control unit decodes the instruction and directs that the necessary data be moved from memory to the arithmetic/logic unit. These first two steps together are called instruction time, or I-time.
The decoding is completed within the Control Unit. You may see the label 'Decode Unit' be banded around but it is yet another task the Control Unit takes care of.
The arithmetic/logic unit executes the arithmetic or logical instruction. That is, the ALU is given control and performs the actual operation on the data.
The arithmetic / logic unit stores the result of this operation in memory or in a register. Steps 3 and 4 together are called execution time, or E-time
SIMPLES!
But how does this tie into the registers?
Now that we have gone over the basic operation of the fetch execute cycle, lets see a more technical break down...
Any Questions?
Happy Days!
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