memory exhibits different types
of technique, organization, performance,
size and cost.
But, no technology is optimal in memory
requirements for a computer system.
As a result, a computer equipped with a
hierarchy of memory subsystems.
Some are internal to the system – RAM,
External memories are Hard disk, Optical
and Pen drive etc.,
cell unit consists of N cells each of
which can store 1 bit either 0 or 1.
Every location of RAM accessed
independently. So, the access and cycle
times can constant.
The address of memory location transferred
through MAR using address bus.
The address then proceed by the address
Which select required location in storage
contents of the selected location placed
in IR through MDR processor register.
The R/W control line specifies the type of
access to be performed.
Write operation, the word to be place in
MDR and the address specified in MAR
and then transferred to selected cell.
DDR RAM-Double Data Rate
synchronous dynamic random access
are written into a ROM when it is
ROM is mask programmed by the
manufacturer in the factory with the
contents ordered by the customers.
The contents are fixed by metal masks used
during chip fabrication.
Once programmed, the contents cannot be
Even a single bit wrongly programmed the
ROM chip is useless.
to store control programs such as
Character generation, code conversion, etc.,
is a field programmable device.
The customer buy a blank PROM and store
desired data using PROM programmer(burner).
Programmability achieved by inserting a fuse at
Before programmed, the memory contains all 0s.
The user can insert 1 by burning out the fuse in the
particular cell using high current pulse.
The PROM chip can be programmed only once
and its contents cannot be erased.
PROM are flexible , faster and less expensive
because they can be programmed directly by the
program in micro computer
EPROM-Erase programmable ROM
rewritable chip that holds its contents
Previous data can be erased and new data
can be inserted
EPROM chips are written on an external
programming device before being placed on
the circuit board.
Capable of retaining stored information for
a long time.
requires breakup the charges trapped
in the transistors of memory cell.[this is
done by break the chip to ultraviolet light].
This reason EPROM packaged with
Entire EPROM is erased as a whole and
selective erasing is not possible.
Should be removed from the chip for
EEPROM-Electrically Erased PROM
can be both programmed and erased
electrically( flashed back to Zero).
They do not need to removed when the chip
Also, erase selected content in the chip.
Erasing and programming dynamically
without removing the EEPROM from the
Different voltages are required for erasing,
reading and writing the data.
Critical system setup information in a nonvolatile way.
kind of EEPROM is flash memory.
Which is erased by ultraviolet light.
EEPROM is byte erasable whereas, flash
memory is a block erasable.
Like a EEPROM when erasing the contents
of flash memory no need to remove from
In flash it is possible to read the contents of
a single cell, but is only possible to write an
entire block of cells.
need single power supply and consume
Cell phones, hand held devices, digital
camera, washing machine, AC, Tablet etc.
NOTE: low power consumption of flash
memory makes it attractive for use in
portable equipment that is battery driven
flash chips on a small card.
That card simply plugged into a slot.
64 MB can store 1 hour songs.
Now a days, the device can equipped with
up to 32GB flash memory.
Speed, size and Cost
chip is very fast memory in the computer
system. but it is expensive because their basic
cells have 6 transistors.
Which impossible to packaging a very large
number of cells onto a single chip.
Cost wise also expensive.
Cache memory can be implemented based on
Alternative is to use DRAM, which can have same
basic cells and thus much less expensive. But
Still it is small in size compared to magnetic
Speed, size and Cost
memory can be build with DRAM.
Secondary storage- large memory space with
reasonable price but much slower than
semiconductor memory unit.
amount of cost-effective storage can
be provided by magnetic disk.
Main memory can be built with DRAM.
SRAM to be used in smaller unit where
speed is of the essence.(Cache)
access is to data held in processor
There are often two levels of cache, the
primary cache inside processor refer to as
L1. secondary cache placed outside
referred as LEVEL 2(L2) usually SRAM
Next level in the hierarchy is called main
The typical main memory is about ten
times slower than the access for the L1
bottom level in the hierarchy is called
the secondary memory such as magnetic
Disk provide a huge amount of
inexpensive storage. But they very slow.