Memory units are devices that store binary information in groups of bits called words. They contain a number of words and number of bits per word. Memory units store programs and data that the CPU needs, as well as backup information not currently required by the CPU. There are two main types of memory - random access memory (RAM), where data can be accessed randomly in the same time, and sequential access memory, where access time varies. RAM can perform both read and write operations and allow random access to any location in the same time. Common types of RAM include SRAM and DRAM, while ROM is read-only memory used to permanently store programs.

1. MEMORY UNIT

2. Memory Unit
A memory unit is a device to which binary information is transferred
for storage and from which information is retrieved when needed
for processing.A memory unit stores binary information in groups of
bits called words.The internal structure of memory unit is specified
by the number of words it contains and the number of bits in each
word.The memory unit is an essential component in any digital
computer since it is needed for storing programs and data. Not all
accumulated information is needed by the CPU at the same
time.Therefore, it is more economical to use low-cost storage
devices to serve as a backup for storing the information that is not
currently used by CPU

3. Classification
In general the memory is classified in two types based on their mode of
access of a memory system.
1. Random Access Memory: The world of data reading or writing
from or to the memory requires same time. We can access the data
randomly.
Example: hard disk.
2.Sequential Access Memory: The information stored in
some medium is not immediately accessible but is available
as certain intervals of time. The access time is variable.
Example: magnetic tape.

4. Random Access Memory
RAM can perform both Write and Read operations.The
architecture of memory is such that information can be
selectively retrieved from any of its internal locations.
The time it takes to transfer information to or from any
desired random location is always the same – hence
the name random access memory.Integrated circuit
RAM units are available in two operating modes: static
and dynamic.RAMs are categorized into ROMs,Read
Write Memories(RWMs) and Read Mostly
Memories(RMMs).

5. RAM
The write input causes binary data to be
transferred into the memory, and the read input
causes binary data to be transferred out of
memory.

6. ROM
ROM stands for Read Only MemoryRead means
output data-bits corresponding to the address
bits set at the input. The bits are used by some
processor or device or circuit for further
operations.Data bits of different addresses store
at ROM and all bits remain stored in the memory
even after a power interruption or power switch
OFFROM is randomly accessed. An equal time
interval is taken for a read of data-bits from each
address. Address bits can be set randomly by an
external circuit.ROM has an address decoder (n
to 2n) decoder within it.

7. ROM
ROM is used for storing programs that are
PERMENTLY resident in the computer and for tables of
constants that do not change in value once the
production of the computer is completed.

8. Types Of ROM
1.Masked Programmed ROMs :- These
chips are integrated circuits that physically
encode the data to be stored, and thus it is
impossible to change their contents after
fabrication.The main advantage of mask ROM is
its cost. Per bit, mask ROM is more compact
than any other kind of semiconductor memory.
Since the cost of an integrated circuit strongly
depends on its size, mask ROM is significantly
cheaper than any other kind of semiconductor
memory. However, the one-time masking cost is

9. 2.User Programmed PROMs
Programmable Read-Only Memory (PROM) is
read-only memory (ROM) that can be modified
once by a user. A typical PROM comes with all
bits reading as "1". Burning a fuse bit during
programming causes the bit to read as "0"The
difference between a PROM and a ROM (readonly memory) is that a PROM is manufactured
as blank memory, whereas a ROM is
programmed during the manufacturing process.
To write data onto a PROM chip, you need a
special device called a PROM programmer or
PROM burner. The process of programming a

10. Types Of RWMs
–Static
RAM (SRAM)Dynamic RAM (DRAM)

11. Types Of RMMs
EPROM (Erasable Programmable Read-Only
Memory) is programmable read-only memory
(programmable ROM) that can be erased and reused.Once programmed, an EPROM can be
erased by exposing it to strong ultraviolet light
source.An EPROM differs from a PROM in that a
PROM can be written to only once and cannot
be erased.EPROMs are used widely in personal
computers because they enable the
manufacturer to change the contents of the
PROM.

12. 2.EEPROM
EEPROM stands for Electrically Erasable and
Programmable Read Only Memory.EEPROM is a type
of non-volatile memory used in computers and other
electronic devices to store small amounts of data that
must be saved when power is removed, e.g., calibration
tables or device configuration. Unlike EPROM chips,
EEPROMs do not need to be removed from the
computer to be modified.It also has a limited life i.e. the
number of times it can be reprogrammed is limited to
tens or hundreds of thousands of times.Applicationsstoring current date and time,storing port statusses.

13. Semiconductor Memory Types

14. Flash Memory
Flash Memory is a special form of EEPROM,
which uses normal PC voltages for erasure and
reprogramming.Flash memory is often used to
hold control code such as the basic input/output
system (BIOS) in a personal computer.When
BIOS needs to be changed (rewritten), the flash
memory can be written to in block (rather than
byte) sizes, making it easy to update.Examples
of flash memory device applications are storing
messages in a mobile phone, storing
photographs in a digital camera.