Chapter 1: Computer Organization
1.1) Introduction
1.2) Central Processing Unit (CPU)
1.3) Computer Memory System
1.4) Input/Output Unit
1.5) I/O Techniques
1.6) Peripherals (I/O Devices)
1.1) Introduction
Main Computer Components
A computer consists of 3 main computer components:
1. Central Processing Unit (CPU)
 Controls computer operations and performs data processing functions; often simply
referred to as processor
2. Main Memory (MM)
 Temporarily stores data and instructions that are being executed or will be executed
by the CPU
3. Input/Output Unit (I/O U)
- Connects the user and the computer through peripherals and controls the peripherals
These components communicate with each other to perform a task. The collection of
paths connecting the various modules is called the interconnection structure. The design
of this structure will depend on the exchanges that must be made between modules.
2
1.
Peripheral
s
Central
Processing
Unit
Computer
Systems
Interconnection
Input
Output
Communication
lines
3
Main
Memory
A line or pathway that connects the 3 main computer components is called BUS. There
are three types of bus that work together:
i
Data Bus

ii
Used to transfer data/instructions from one component to another
Address Bus

iii

Used to transfer the destination address of the data that is inside the Data Bus
Control Bus
Used to transfer signals that control data and address in Data Bus and Address
Bus
A bus consists of multiple communication pathways, or lines and each line is capable of
transmitting signals representing binary 1 and binary 0. For example, an 8-bit unit of data
can be transmitted over 8 data bus lines.
Generally, there are 4 ways to connect the 3 components with the I/O module acting as
the control point :
i
I/O to CPU
MM
CPU
I/O U
4
ii
I/O to MM
CPU
MM
I/O U
iii
I/O to Central Switch
CPU
Central
Switch
MM
I/O U
iv
I/O to Bus
CPU
MM
System Bus
5
I/O U
The most common interconnection structure is I/O to bus or known as bus
interconnection. The followings are three types of bus interconnection:
i. Bus Interconnection Scheme
ii. Traditional Bus Architecture
iii. High-Performance Architecture
6
7
The basic computer function is to execute programs, which includes:
o Data Processing
o Data Storage
o Data Movement
o Control

There are 3 main computer components, i.e. CPU, MM and I/O U that are
interdependent when performing their respective tasks.

CPU will perform the processing tasks based on the instructions given by programs

The instructions are stored in MM and the CPU will fetch and execute them one at a
time until all of them are completed.

Generally, computers use Stored Program Concept to perform their tasks. This
concept was introduced by John Von Newmann (1945)
Stored Program Concept
2. Data and instructions that will be executed are stored in memory
3. The content of the memory is addressed according to their position
Instructions are executed sequentially based on their position in the memory (except for
branch instructions)
8
1.2) Central Processing Unit (CPU)

Its function is to control the operation of the computer and performs its data
processing functions.

Can be divided into three main components:
i.
Control Unit (CU)

Controls the movement of data and instructions into and out of the CPU
and control the operation of the ALU and the computer as a whole.
ii.
Arithmetic Logic Unit (ALU)

iii.
Performs computer data processing functions
Registers

As a temporary storage in CPU

Also known as high speed memory

Its function is to temporarily hold data and instructions that are taken from
other components (example MM, I/O U, cache etc) to be processed in the
CPU.

The registers in the processor perform two roles:
o User-visible registers – enable the machine- or assembly language
programmer to minimize main memory references by optimizing
use of registers. (will be discussed in chapter 6 – 8)
o Control and status registers – used by the control unit to control the
operation of the processor and by privileged, operating system
program to control the execution of program. (eg: PC, MAR,
MBR and IR)
9
Structure – The CPU
CPU
Computer
I/O
Registers
System
Bus
Internal CPU
Interconnection
CPU
Memory
Control Unit
10
Arithmetic
and Logic
Unit
The Control Unit
CPU
ALU
Registers
Internal
CPU Bus
Control
Unit
CONTROL UNIT
Sequencing
Logic
Control unit
Registers and
Decoders
Control
Memory
11
12

The function of computers is to execute programs. Programs comprise a set of
instructions and data that is stored in the main memory. Every instruction will be
taken and executed by the CPU one by one until all of them are executed.

The process of executing a single instruction is called Instruction Cycle which can
be divided into:
i.
Fetch Cycle
- read the next instruction from MM into the CPU.
ii.
Execute Cycle
- interpret the opcode and perform the indicated operation.
13
Fetch Cycle

The process of fetching an instruction from MM into the CPU

4 registers are involved:
o PC (Program Counter)
 Contains the address of the instruction that has to be fetched and
executed
o MAR (Memory Address Register)
 A register that connected to the MM through the address bus
 The content in PC will be transferred into the MAR and then to the
address and to the location in MM that pointed by the address given.
o MBR (Memory Buffer Register)
 A register that connected to the MM through the data bus
 Used to hold instruction or data taken from the MM or data that will be
moved into the MM or other computer components after being
processed by the CPU.
o IR (Instruction Register)
 Used to hold instruction (operation code) before it is being decoded by
the Instruction Decoder inside the control unit.

Generally the process in the Fetch Cycle can be depicted by:
MAR ← PC
MBA ← M (MAR)
PC ← PC + 1
IR ← (MBR)
14
Execute Cycle

The process of executing the instruction that has been fetched earlier

The followings are steps that are involve in execute cycle:
i.
Instruction decoding (to determine the operation that has to be performed by
the CPU)

ii.
Operand (data) fetching
iii.
Data Processing
iv.
Store Result
The sequences of operations that happen in execution cycle differ according to the
type of instruction.
2
1
X
ALU
3
(Data
)
(Data)
MM
Example:
R1
CPU
ADD R1, X
This instruction adds the content in the R1 register with the content in the MM at
location X and the result is stored in the R1 register, i.e.: R1 ← R1 + M (X)
(Observe the diagram above)

Before the addition operation is performed in the ALU, the required data is taken and
moved into a register in the CPU

The data in R1 is already in the CPU (R1 is one of the registers in CPU)

The addition operation is performed in ALU and the result is stored in the R1 register

The process which takes place in the Execution Cycle for instruction ADD R1,X can
be depicted by:
MAR ← IR (address X)
MBR ← M (MAR)
R1 ← R1 + MBR
15
16
1.3) Computer Memory System
Generally, computer memory system can be divided into 2 categories:
i.
Internal memory
ii.
External/Secondary Memory
17
Internal Memory

There are 3 types of internal memory:
a) Registers (in the CPU)
b) Cache
c) Main Memory (RAM & ROM)

Information stored in internal memory can be accessed with greater speed by the CPU
compared to external memory.

Internal memories are volatile, only with exception to ROM
External Memory

Memories that are separated from the main computer components

Information contained in external memory can be accessed by the CPU through the
I/O Unit.

Used to store information or program that has already or has yet to be executed by the
CPU

Instances of external memory are hard disks, diskettes, CD-ROM, etc

All external memory are non-volatile
Main Memory

Central Storage of the computer.

Used to store instructions and data that are being executed (run)

They are built based on Semiconductor Integrated Circuits

Basically, they are divided into RAM and ROM
RAM

Acronym for Random Access Memory

Information in RAM can be read and modified (write)

Used to store programs and user data that is being executed (run)

A kind of volatile memory, i.e. the content in RAM will be wiped out if electricity
supply is interrupted.
18

There are 2 types of RAM:
1. Static RAM (SRAM)

Made from flip-flop

Disadvantages : expensive, small storage capacity in a chip

Advantages : easier to use and faster access time (read and write cycle)
2. Dynamic RAM (DRAM)

Made from capacitors

Disadvantages : slower access time, harder to use because capacitors need to
be charge for a certain amount of time

Advantages: cheaper, bigger storage capacity in a chip

A few improvements has been done on DRAM to improve its performance
(access time) with different method and names such as EDRAM, CDRAM,
SDRAM, RDRAM etc

Following is a block diagram for a unit (chip) RAM that has 2k word and the
length of each word is n bit
n input line (from data
bus)
k address line (from
bus address)
RAM
(2k x n)
From control
bus
read
write
n bit
n output line (to data bus)
19
2k word
ROM

Acronym for Read Only Memory

Information in ROM can only be read (cannot be written or modified)

Used to store permanent programs like bootstrap loader, i.e. a program used to start
the operation of a computer when the computer is turned on

There are various kinds of ROM, among them are PROM, EPROM and EEPROM.
The difference is the ability to change data in them using specific methods and
equipments

The following is a block diagram of a ROM with the size of 2k x n (2k word and the
length of each word is n bit)
k address line (from
address bus)
n data line (from
data bus)
2k word
n bit
Note:
Why don’t ROM chips have read-write pins?
20
Cache Memory

A small and fast memory that situated between the CPU and the MM (conceptually,
not physically).

Reduces the difference of time of the memory cycle and the CPU processing time.

The memory cycle time is the time used to fetch an instruction or data from the main
memory until it is usable.

With the technology available today, the processing time of the CPU is faster than the
memory cycle time by far. Hence, a memory, which was called Cache, was
introduced and it was situated between the CPU and the MM.

The method used for cache manufacturing rendered its memory cycle time far faster
than that of the main memory but its cost was exorbitant.

We would like the size of the cache to be small enough so that the overall average
cost per bit is close to that of main memory alone and large enough so that the overall
average access time is close to that of the cache alone.

What is the ideal size for the cache??????

How does cache work?
CPU
CACHE: divided into a few slots
MM: divided into a few blocks or lines
(block size = slot size)

When the CPU needs certain data or instruction, it will refer to the cache memory
first. If it is in cache, then data access will be fast. Otherwise, it will have to be
fetched from the MM and the data block that contains the required data or instruction
21
will be moved into the Cache Memory with the hope that the next instruction/data
required by the CPU is contained in the block.

Because there are fewer cache lines than main memory blocks, an algorithm is needed
for mapping main memory blocks into cache lines/slots. Three techniques can be
used: direct, associative and set-associative.

Once the cache has been filled, when a new block is brought into the cache, one of
the existing blocks must be replaced. A replacement algorithm is needed. What types
of algorithm???????
22
Word Transfer
CPU
Block Transfer
Main Memory
Cache
Cache and Main Memory
Address
Buffer
Processor
Control
Cache
Control
Data
Buffe
r
Data
Typical Cache Organization
23
System Bus
Address
START
Access the block
that contains RA in
the MM
Receive address
RA from the CPU
Prepare a Cache slot
for this MM block
Block contains RA in
Cache?
Nope
Yup
Send word RA to the
CPU
Access word RA
and send to the
CPU
Move the MM block
into the Cache slot
STOP
Read Cache Operation
External Memory

Also known as secondary memory

Used to store information (program and data) that has not or has already been
processed by the computer system

Consists of secondary storage devices such as discs and tapes where information
contained in them are accessed by the CPU through the I/O unit.

The most commonly used today: disk
24
Disk
The 2 most popular types:
1. Magnetic Disk
2. Optical Disk
Magnetic Disk

There are 2 kinds, i.e. hard disk and floppy disk.

A round platter made from metal or plastic and shrouded with magnetic coating
Sector
Track
Intersector gap

Consists of tracks and sectors

Track: 500 – 2000 tracks/surface

Sector: 10 – 100 sector/track (fixed or varied)

Data are moved into/taken out from the disk in block size (sector/track)

The amount of data in every track is same => data density varies

There are 2 types of read/write heads:


Fixed head: One head per track

Movable head: One head for all tracks
Disks are inserted into the disk drive. There are 2 types of disk:


Movable disk: can be inserted and taken out from the disk drive, such as the
floppy disk.
Fixed disk: cannot be taken out from the disk drive, for instance hard disk.
25
1.4) Input/Output Unit

Input and output operations are very important in a computer system. They enable the
user to communicate with the computer.

These operations are performed using peripherals such disk drive, keyboard, mouse,
printer, monitor etc

Every peripheral has different operation

The data format and word length in the peripheral also differs with the internals
computer system.

The data transfer rate of peripherals is far slower compared to that of the main
memory or the CPU.

I/O Unit is needed to coordinate the difference stated above.
Address Line
System Bus
Data Line
Control Line
I/O Module
Connects to
peripheral devices
General Module of I/O
26
Interface to
System Bus
Data
Line
Interface to
External Device
External
Device
Interface Logic
Data Register
Status/Control
Register
.
.
.
Address
Line
Control
Line
I/O
Logic
External
Device
Interface Logic
.
.
.
Block Diagram for I/O Module
Control from I/O
Module
Data
Status
Control
Data (bits) to and
from I/O Module
Status to I/O
Module
Control
Logic
Buffer
Transducer
Data to and from the
environment
Block Diagram for External Devices
27
Data
Status
Control
1.5) I/O Techniques

I/O Operations – the operation of inserting or extracting data to/from the computer
system

I/O Techniques refers to how I/O operations are executed.

There are 3 ways to perform the I/O operations (I/O Techniques):
1. Programmed I/O
2. Interrupt-Driven I/O
3. Direct Memory Access (DMA)
Forward the read instruction
to the I/O Module
CPU
Read the I/O Module status
Not
ready
Check
Status
I/O
I/O
CPU
Error Situation
Ready
Read word from I/O
Module
I/O
CPU
Write word to memory
CPU
No
Done ?
Yes
Next Instruction
1. Programmed Input/Output
28
Memory
Forward the read instruction
to the I/O Module
CPU
I/O
Perform other
Read I/O Module status
I/O
Check
Status
Interrupt
CPU
Error Situation
Ready
Read word from I/O
Module
I/O
CPU
Write word into memory
CPU
No
Done ?
Yes
Next Instruction
2. Interrupt Driven Input/Output
29
Memory
tasks
Forward the read instruction
to the I/O Module
CPU
DMA
Perform other tasks
Interrupt
Read DMA module status
DMA
CPU
Next Instruction
3. Direct Memory Access
Diagram 1, 2 and 3 above are flow charts that depicts the execution of the 3 stated I/O
techniques
30
1.6) Peripherals OR External Devices

Connected to the computer system through I/O unit

Can broadly classify into three categories:
1. Human readable

Suitable for communicating with the computer user

Example: VDT, printer etc.
2. Machine readable

Suitable for communicating with equipment

Example: disk drive, tape drive etc.
3. Communication


Suitable for communicating with remote device

Example: modem etc.
Based on usage, peripherals can be divided into:
1. Input Device

Devices used to accept input only

Example: keyboard, mouse etc
2. Output Device

Devices used only to provide output

Example: printer, VDT etc
3. Input and Output Devices

Can be used to accept input and provide output

Example: touch screen, disk drive etc
31