Memory and Storage
Memory
ROM
Masked
ROM
PROM
RAM
EROM
EEROM
Static Ram
Dynamic
Ram
Memory Hierarchy
Increasing performance
and
increasing cost
Slow and
inexpensive
Memory Hierarchy (contd)
Registers
top in the hierarchy
matched in speed to the CPU
but are large and consume significant power
number of registers in a processor are small
Secondary and off-line storage devices
at the bottom of the hierarchy
cost per stored bit small in terms of money and
power
but access time is very long compared to
registers
Random Access Memory(RAM)
The term random means any memory location
can be accessed in the same amount of time
regardless of its position in the memory.
Volatile memory
Types of RAM:
SRAM
DRAM
Static RAM
Static RAM (SRAM) based on flip flops
Contents of each location persist as long as
power is applied.
Fast but relatively large
Consume a lot of power
Used for memory applications that are small but
fast
Dynamic RAM
Dynamic RAM (DRAM) employ capacitors
Capacitor stores electric charge whose level
represents a 1 or 0
Capacitors dissipate with time and hence the
charge must be restored frequently
DRAMs
smaller, slower than SRAMs
support low cost, low power and high density and
hence used in main memory
Memory Chip organization
RAM chip
A0 Am-1 : Address lines from 0 to m-1
CS : Chip Select (CS = 0, chip selected)
WR : ReadWrite (WR = 0, write operation)
Read Only Memory(ROM)
Data stored in these chips is nonvolatile -- it is
not lost when power is turned off.
On turn on, the computer loads BIOS from ROM
Data stored in these chips is either
unchangeable or requires a special operation to
change
Applications: Videogames, calculators,
microwave oven, etc
Read Only Memory(ROM) (contd)
Lets see the circuit for a ROM that stores four
four-bit words(0101, 1011, 1110, 0000)
PROM burner allows contents of the ROM to be
overwritten only once
EPROMs could be written several times
EEPROMs allow the content to be rewritten
electrically
Flash Memory
A section of memory cells can be erased in a
single step, or 'flash', thus the name
Nonvolatile memory
Flash memory is used for easy and fast
information storage in such devices as digital
cameras, mobile, video game consoles
Did you know
Storage :
Provides capacity for all
the files and information
you need( e.g. capacity
of the computers hard
disk)
Memory :
Provides your working
space ( e.g. the amount
of RAM installed in the
computer)
Memory
In computing memory refers to the
physical devices used to store
programs or data on the temporary or
permanent basis for use in a computer
or other digital electronic device
Main memory is divided into two parts :
- Random Access memory ( RAM )
should be
better known as Read Write
Memory
- Read Only Memory (ROM)
Random Access Memory (RAM)
Structure :
A set of memory chips, each of them
is an integrated circuit (IC) made of
millions of transistors and capacitors
Volatile - their state is lost or reset
when power is removed from the
system.
Usage :
Holds data/application programs
from input devices or storages
Capacity : Determines the number
and size of the program can be run
at the same time as well as the
amount of data that can be
processed immediately.
Bus : processing speed
Types of RAM
Static Random Access
Memory (SRAM)
Dynamic Random Access
Memory (DRAM)
Semi conductor memory
Use flip-flop to store each bit of
memory so does not need to be
periodically refreshed
Faster and consumes low power
Expensive and have complex
structure (6 transistors) so not
use in high capacity applications
Store each bit of memory in
capacitor in an integrated circuit
Real capacitors leak charge so
capacitors need to be refreshed
periodically
Simple structure ( 1 transistor and
1 capacitor per bit) so it has very
high density
Read only memory (ROM)
Has contents which are fixed
when the chip is manufactured
Holds the bootstrap loader
part of the operating system
Retains its data when the
computer is switched off
Types of ROM
Programmable ROM (PROM)Empty of data when the chip is
manufactured, can be
programmed by the user. Once
programmed the data cannot be
erased.
Erasable PROM (EPROM)Like PROM only the chip can be
removed from the computer and
the program erased and another
stored in its place using
ultraviolet light.
Electrically EPROM (EEPROM)
Like EPROM but electricity is
used to erase and reprogram
selected contents.
Storage
Storage any devices that are capable of
holding information, even when the power is off
(non-volatile).
Ex: Floppy disk drive, hard disk drive,
USB flash disk,...
Offline storage type of storage that can be
removed and can be accessed from any
computers when connected.
Types of storage device
Optical storage devices
Magnetic storage devices
Flash memory devices
Optical storage devices
Compact Disc (CD)
Capacity: 700 MB
Available from 1982
DVD
Capacity: 4.7 GB
Available from 1995
Blu-ray Disc
Capacity: 25 GB
Available from 2006
Optical storage devices
Advantages:
Small and portable
Cheap to produce
Large market of music, movies and games
Most desktop computers or laptops are equipped with
CD/DVD/Blu-ray drive
Disadvantages:
Fragile, easy to get scratched
x
Slower data transferring rate
x
Required specified drives to write data to the discs
x
Limited number of writes
x
Magnetic storage devices
Floppy disk
First available in 1971
Capacity: 1.44 MB (3 12-inch floppy disk)
Discontinued.
Since 2010, no motherboard is still
manufactured with the support of floppy disk
drive.
Magnetic storage devices
Hard disk drive (HDD)
First introduced in 1956
Became dominant in the 1960s
Capacity keeps increasing
Max. capacity: 4 TB
(as of 2013)
Continuously improved.
Maintained an important position
of modern servers and PCs.
Magnetic storage devices
Advantages (HDD):
Large and increasing capacity
High transferring speed
No data loss when power is off (non-volatile)
Cheap price compared to other storage media (cost per MB)
Disadvantages (HDD):
Possible damages and data loss due to head crash
x
Because of working on mechanism basis, it will be eventually
broken
x
In case of internal hard drive, it is not easy to be transferred to
another computer
x
High power consumption
x
Flash memory devices
USB Flash drive
Commercially available in 2000
Max. capacity: 1 TB (as of 2013)
Small size
Ideal solution for storing small data
Flash memory devices
Solid state drive (SSD)
First introduced in 1995
Max. capacity: 1 TB (as of 2013)
Very high transferring speed in comparison to
normal physical hard disk
Flash memory devices
Advantages:
Large and increasing capacity
High transferring speed
Small size, portability
Low power consumption
Work more quietly than physical hard drive
Disadvantages:
High price compared to hard disks (cost per MB)
x
Limited number of reads/writes (write endurance rating)
x
Easy to lose because of small size
x
Memory cycle timing waveforms
access time
cycle time
the time required to complete a write cycle
access time cycle time
the time required to select a word and read it
equal to a fixed number of CPU clock
See Fig. 7-4
Memory Decoding
Decoder
select the memory word specified by the input
address
2-dimensional coincident decoding is a
more efficient decoding scheme for large
memories
Memory cell
One bit memory cell
4X4 RAM
Coincident Decoding
two-dimensional selection scheme
Decoder with k input and 2k output requires 2k
AND gates with k input
k input decoder can be implemented by two k/2
input decoders with one for column and another
for row
e.g., 101024 decoder can be implemented by
two 532 decoders
Example for two-dimensional
decoder
Address multiplexing
64K-word memory
Internal logic of 32X8 ROM
Programmomg the ROM according to
Taable 7-3
denote a connection in place of a dot used for
permanent connection
Example 7-1
Design a combinational circuit with 3-input using a ROM.
Output = square(input)
A2
A1
A0
B5
B4
B3
B2
B1
B0
16
25
36
49
ROM implementation of Example
7-1
Types of ROMs
The required path in a ROM may be
programmed in four different ways.
mask programming (mask ROM)
Mask is done by Fab. company during the last
fabrication
Customer must fill out the truth table
High cost
programmable read-only memory(PROM)
allows users to program in Lab.
the program is irreversible
Types of ROMs
Erasable PROM(EPROM)
by ultraviolet light
electrically-erasable PROM(EEPROM or EPROM)
by electrical signal
can be erased without removing it from tis socket
