Transcript System Analysis (Part 1)

The CPU
The Central Presentation Unit
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Main Memory and Addresses
Address bus and Address Space
Data Bus
Control Bus
The Instructions set
Mnemonics
Opcodes
Operands
Central Processing Unit
Arithmetic Logical Unit
Accumulator
Control Unit
Input
Unit
Program Counter
Instruction Register
Central Memory
ROM
RAM
Control Bus
Data Bus
Backing
Storage
Output
Unit
Main Memory

Main Memory is where most of the results
are temporarily stored;

Main memory is much faster than the hard
disk

Each byte is stored in location in the main
memory and each location has an address,
this way the CPU can store and retrieve
information very easily
Addresses

Since each location in Main Memory has an
address, the address has to be specified and saved

When data is required back, the CPU will read
this data by calling its address

When the CPU writes information to the Main
Memory two things have to be supplied
1.
2.

the address section – address bus
the data section. - data bus
However, these two still pass from the same bus,
they just pass from different sections along the
same bus.
Address Bus and Space

The address bus allows the CPU to address
different locations in Main Memory

The maximum number of available addresses is
determined by the width of the address bus.

An address bus is made up of tiny copper strips
from which the information passes. The more
strips there are the more addresses there will be.

For example if the address bus has 9 strips the
maximum number of addresses would be 29 =
512
Data Bus

The data bus structure is very similar to the address
bus

It is used to allow the transfer of data to and from the
CPU

The largest amount of data passed at a single time is
called the word length this specifies how much data can
pass at one go

The word length is measured in 8, 16, 32 and even 64
bits

The larger the word length the faster the CPU will
work
Control Bus

The control bus is a two way line that
controls the operations and the data flow
between the memory unit, the arithmetic
logic unit, and other computer
peripherals.
The Instruction Set

The instruction set is the complete
collection of instructions which are used by
the CPU

These instructions will be written in
machine-code or assembly language (we will
go into these later)

The instruction set is part of the design of a
CPU hence the machine code of different
types of computers is rarely compatible.
Mnemonics

If programming was to be carried out in binary, it
would be very difficult to write a complete
program using just 1s and 0s

Instead of remembering the binary code used for
each function, mnemonics are used.

Mnemonics is a word which can be easily
remembered as it resembles the English language

For example adding a number
0001
ADD
Example

Lets say we wish to add the number 9

Mnemonics:
ADD
5
Opcode Operand

Binary:
0001
1001
Opcodes

Opcode is short for operational code

An opcode is the mnemonic part of the
instruction stating what is to be done
Opcode
Operation
ADD
Addition
SUB
Subtraction
DIV
Divide
MUL
Multiply
Operands

Operand is the mathematical part of the
instruction

An operand is the data upon which the
opcode is to carry out the action
Opcode with Operand
Operation
ADD 7
Add by 7
SUB 9
Subtract by 9
DIV 2
Divide by 2
MUL 2
Multiply by 2
Machine Code

Machine code is a system of codes understood by the
computer.

Machine code is composed only of the two binary
digits 0 and , assemble language uses mnemonics

Every CPU model has its own machine language,
although

If CPU A understands the full language of CPU B it
is said that A is compatible with B, CPU B may not
be compatible with CPU A, as A may know a few
codes that B does not.
Machine code…

The "words" of a machine language are called
instructions;

Each instruction causes an action by the CPU, such as
reading from a memory location

A program is a long list of instructions that are
executed by a CPU

Instructions are patterns of bits, different patterns
correspond to different commands to the machine.
Humans use mnemonic codes to refer to the useful
bit-patterns.