Memory
Organisation
·
Memory is made up of
lots of storage locations.
·
Each location can hold
a word.
·
A word is the term given to the number of bits
that can be processed by the computer in a single operation.
·
So the correct data can
be found in main memory, each storage location has its own unique address.
Hence, the data wouldn’t be sent to the incorrect destination. This concept is
called addressability.
Memory
Hierarchy
Main
Memory
·
Main memory is a memory
unit that communicates directly to the CPU of the computer.
·
It is a combination
device of both RAM (random access memory) and ROM (read only memory).
·
Main memory is a part
of Central Processing Unit(CPU).
Characteristic of
Memory
·
Capacity, representing the global volume of information (in bits) that
the memory can store
·
Access time, corresponding to the time interval between the read/write
request and the availability of the data
·
Cycle time, representing the minimum time interval between two successive
accesses
·
Throughput, which defines the volume of information exchanged per unit of
time, expressed in bits per second
·
Non-volatility, which characterises the ability of a memory to store data when
it is not being supplied with electricity
Types
of Main Memories
·
Main Memory are
classified into two basic types:
i.
Random Access Memory
(RAM)
ii.
Read Only Memory (ROM)
What
is RAM?
·
Random Access Memory
(RAM) is a type of main memory which store the data temporarily when a program
is running.
·
RAM can be called as
primary memory/primary storage/main storage/internal memory/core memory/core
storage/internal storage
·
It is sometimes called semiconductor memory because storage cells are made
out of semi-conducting material.
·
It takes the form of integrated
circuits.
·
Hold the data and
instructions temporarily when the CPU is processing.
·
Examples of what is
stored in RAM:
§ the
operating system
§ applications
§ the
graphical user interface (GUI)
What
is ROM?
·
ROM is a type of main
memory ROM is computer memory which retains its data, even when the computer is
switched off.
·
ROM can be called as
auxiliary memory/physical memory/external memory
·
Hold data necessary for
starting up the computer permanently, whose contents cannot be changed by the
user.
Difference between RAM and ROM
Types
of RAM
There are two types
of RAM:
i.
Dynamic RAM (DRAM)
·
Analog device
·
Volatile in nature but
they need regenerator to retain the data
as long as they receive the power.
·
DRAM chips contain a
transistor that acts as a gate to a capacitor which purpose is to store the
bits as charge in it
·
The charge on the
capacitor indicates a ‘1’ bit and no charge indicates a ‘0’ bit. The charge on
the capacitor leaks away after a few milliseconds. Therefore, a dynamic RAM has
to be refreshed periodically after every two milliseconds.
·
DRAM uses its contents
in a very short time even though the power supply is on.
·
Consumes less power and
has higher packing density. It is cheaper than the static RAM.
·
DRAM’s be recommended
for large sized memories.(i.e. main memory)
DRAM
Structure
i.
Static RAM (S-RAM)
·
Digital device
·
Bits store as
flip-flops(on/off switches)
·
Volatile in nature but
they need no regenerator to retain the data as long as they receive the power.
·
Consumes more power and
is more expensive.
·
The structure of SRAM
chips are more complicated and hence require more space.
·
Static RAM’s are faster
than the dynamic RAM’s .Static RAM’s have an access time of approximately 85 to
90ns while the dynamic RAM’s may take 150 to 200ns to provide information.
·
SRAM’s are recommended
for medium sized memories.(i.e. cache memory)
SRAM structure
Differences between DRAM and SRAM
Types
of ROM
There are five
types of ROM:
i.
Programmable Read Only Memory (PROM):
·
A PROM program is used
to record information in the PROM chip.
·
The information once programmed
into the PROM chip is permanent and cannot be changed or erased.
·
The process of entering
the information into the PROM chip is known as “burning the PROM.”
·
It is need special
equipment to program.
·
The storing in ROMs
will be very expensive when only a small number is required. Thus, PROMs
provide a faster and less expensive approach for storing.
ii. Masked Read Only Memory (MROM)
·
The masking and
metallisation process permanently records the information.(ROM bits)
·
It is not easy to perform
this process as a large infrastructure is required, and therefore, it is
usually the manufacturers who perform this process.
iii. Erasable Programmable Read Only Memory (EPROM)
·
An erasable PROM.
·
It can be reprogrammed
using an EPROM programmer.
·
It is only one
transistor per bit.
·
The contents can be
erased by expose it to high intensity ultraviolet light with a wavelength of
2537A (angstrom) for 30 minutes (approximately).
·
The process of changing
the contents is not convenient, as the chip has to be removed from the board
for exposure to the ultra-violet light source.
·
If the user wants to
erase the contents of a single memory,the entire memory contents have to be
erased.
The EPROM chip is
cheap, reliable and widely available
iv. Electrically Erasable Programmable Read Only
Memory (EEPROM)
·
An electrically erasable PROM.
·
Using electrical
signals can alter the information and that is why the chip need not be removed
from the board.
·
EEPROM chip has over
the EPROM is that even single memory can be altered, i.e., the entire memory
need not be erased and reprogrammed unless required.
·
Changing the contents
does not require additional equipment.
·
The change in the
contents of the EEPROM chip is made in milliseconds, which is much less than
the erasing time for EPROM.
·
It is less dense than
EPROM.
v.
Non-Volatile RAM
·
Combination device of a
static RAM and EEPROM.
·
It operates as normal
RAM but in case the power supply fails, the entire contents of the RAM are
stored in EEPROM.
·
The data from EEPROM is
transferred back to the RAM when the power supply is restored.
References
Written by,
TAN SIN YEE
B031210297
