COMPURT
ORGANAZTION
CHAPTER 5
Computer Organization
We can divide the parts that make up a computer into 3
subsystems :
1. Central processing unit (CPU).
2. Main Memory.
3. Input / output subsystem.
Computer System Component
Computer System
Central
processing
Unit (CPU)
Memory
Input\output
systems
5.1 CENTRAL
PROCESSING UNIT (CPU)
Central processing unit (cpu)
Central processing Unit (CPU)
Arithmetic Logic
Unit (ALU)
Logic Operations
NOT, AND, OR,
XOR
Shift Operations
Logical shift
operations,
arithmetic shift
operations
Arithmetic
Operations
Data Registers
R1,R2….Rn
Registers
Control Unit
Instruction
Registers
Program Counter
Registers

Fast stand –alone storage locations that holds data
temporarily.
 Data
registers
holds the input and the result of operations
 Computers use dozens of registers inside the CPU to speed
up their operations



Complex operations done using hardware instead of software
Data registers are named R1,R2,….Rn
 Instruction
 The
registers
CPU Responsible for fetching instructions one by one
from memory, storing them in the instruction registers,
decoding them and executing them
 Program
Counter
 Keep
track of the instructions currently being executed
 The counter is incremented to point to the address of the
next instruction in the memory, after the execution of the
instruction

The Control Unit
 Controls
the operations of each subsystem
 Controlling
subsystem
is achieved through signals sent from it to other
Computer System Component
Computer System
Central
processing
Unit (CPU)
Memory
Input\output
systems
5.2 MAIN MEMORY
Main Memory
Main memory Consists of collection of storage locations,
each with a unique identifier called an address
Main Memory

Data is transferred to and from memory in
groups of bits called words. A word can be a
group of 8 bits, 16 bits, 32 bits or 64 bits (and
growing). If the word is 8 bits, it is referred to
as a byte

16-bit word is referred to as a 2-byte word

32-bit word is referred to as a 4-byte word
Address space
each word is identified by an address.
 The total number of uniquely identifiable
locations in memory is called the address
space.

 For
example, a memory with 64 kilobytes and a word
size of 1 byte has an address space that ranges from 0
to 65,535.
Address space

Note: this is an approximation of number of bytes in
the power of 10, but the actual number of bytes is
in power of 2 to facilitate addressing.
Address space

Because computers store numbers as bit patterns,
memory address also represented as a bit pattern.
216= 65535
• Computer with 64
kilobytes (216) of
memory and a word
size of 1 byte
To define an
address
• We need a
bit pattern
of 16 bits
• First memory address:
Address 0: 0000000000000000
• Last memory address:
Address
65535:1111111111111111
N words of memory needs log2
bits for addressing
Memory addresses are defined using unsigned
binary integers.
Address space - Example 5.1
A computer has 32 MB (megabytes) of memory.
How many bits are needed to address any
single byte in memory?
Answer:
The memory address space is
, or 225 (25
× 220). This means that we need log2 225, or 25
bits, to address each byte.
Address space - Example 5.2
A computer has 128 MB of memory. Each word in
this computer is eight bytes. How many bits are
needed to address any single word in memory?
Answer:
The memory address space is
, which
means 227. However, each word is eight (23)
bytes, which means that we have 224 words. This
means that we need log2 224, or 24 bits, to
address each word.
Memory types
Memory types
RAM
SRAM
Static
RAM
ROM
DRAM
Dynamic
RAM
PROM
Program
mable
read-only
memory
EPROM
Erasable
program
mable
read-only
memory
EEPROM
Electricall
y
erasable
program
mable
read-only
memory

RAM
 Most
up of main memory
 Data item can access randomly
 Can be read from and written to
 Volatile
 Two categories:
 SRAM



Use traditional flip –flop gates to hold data
It holds their state (0,1)(means data stored as long as the power
is on and no need to refresh memory location
Fast and expensive
 DRAM
 Use
capacitors, that store energy for data storage
 Need to be refreshed periodically
 Slow and inexpensive

ROM
 Written
by the manufacturer
 CPU can read from but not write
 Nonvolatile
 Ex: boot program
 PROM
(programmable read only memory)
 Blank
when the computer is shipped
 The user with special equipment can store programs on it

Can not be overwritten
 EPROM
(Erasable programmable read only memory)
 Can
be programmed by the user and can be erased with
special device using ultraviolet

Require physical removal and reinstallation
 EEPROM
 Can
(Electronically EPROM)
be programmed and erased using elec
Memory hierarchy

Computer users need a lot of memory, especially
memory that is very fast and inexpensive. This demand
is not always possible to satisfy—very fast memory is
usually not cheap. A compromise needs to be made. The
solution is hierarchical levels of memory.
Cache memory

Cache memory is faster than main memory, but
slower than the CPU and its registers. Cache
memory, which is normally small in size, is
placed between the CPU and main memory.
Cache memory

It contains a copy of a portion of main memory. When
CPU needs to access a word in main memory, it follows
this procedure:
1
2
3
• The CPU checks the cache.
• If the word is there, it copies the word: if
not, the CPU accesses the main memory and
copies a block of memory starting with the
desired word. This block replaces the
previous content of cache memory.
• The CPU access the cache and copies the
word.
5.2 INPUT / OUTPUT
SUBSYSTEMS
Input / Output subsystems



This subsystem allows a computer to communicate
with the outside world and to store programs and
data even when the power is off. Input/output
devices can be divided into two broad categories:
Non-storage devices: they cannot store
information.
Storage devices: they can store large amounts of
information to be retrieved at a later time. They
are cheaper than main memory, and their contents
are nonvolatile (not erased when the power is
turned off).
Input / Output subsystems
Input / Output
subsystems
Non-storage devices
Keyboa
rd and
monitor
Printer
Storage devices
Optical
Magnetic
Magnetic
disk
Magnetic
tapes
CD ROM
CD-R
CD- RW
DVD
Magnetic Storage devices

Magnetic disk:
Magnetic Storage devices

Magnetic tape