• Most of the main memory in a general purpose
computer is made up of RAM integrated circuits
chips, but a portion of the memory may be
constructed with ROM chips
• RAM– Random Access memory
– Integrated RAM are available in two possible operating
modes, Static and Dynamic
• ROM– Read Only memory
Random-Access Memory (RAM)
• A RAM is a read/write memory in which data
can be written into or read from any selected
address in any sequence.
• When a data is written into a given address,
data unit previously stored at that address is
replaced by new data unit.
Random-Access Memory (RAM)
• When a data unit is read from, data unit
remains stored and is not erased.
• A RAM is typically used for short term data
storage because it cannot retain stored data
when power is turned off.
Types of Random-Access Memory(RAM)
SRAM with burst
• All static RAMs are characterized by latch
• As long as DC power is applied to a static
memory cell, it can retain a 1 or 0 state
• If power is removed, stored data bit is lost.
Asynchronous Static RAM
Operation is not synchronized
with system clock.
Synchronous SRAM with burst feature
• A synchronous SRAM is synchronized with
• The address, the read/write input, the chip
enable, and input data are all latched into
their respective registers on an active clock
• Address burst feature allows memory to read
or write up to four sequential locations using a
Applications of SRAM
• One of the major applications of SRAMs is in
cache memory in computers.
• Cache memory is a relatively small, high speed
memory that stores the most recently used
instructions or data from the larger but slower
• Dynamic memory cells store data bits in small
capacitors rather than in a latch.
• DRAM requires a frequent refresh operation
to preserve stored data bit resulting in
• DRAMs use a technique called address
multiplexing to reduce number of address
• Advantage: Allows very large memory arrays to be
constructed on a chip at lower cost per bit.
• Disadvantage: Storage capacitor cannot hold its charge
over an extended period of time and will lose the
stored bit unless it is charge is refreshed periodically.
To refresh requires additional memory circuitry and
complicates the operation of DRAM.
• Applications: The major application of DRAM is in main
memory of computers. The DRAM consists of transistor
and capacitor allowing much greater densities and
results in greater bit capacities for given chip
area,although much slower access time.
Types of DRAM
• Fast Page Mode DRAM(FPM DRAM):
i. Idea is based on probability that next several memory
address to be accessed are in same row.
ii. Saves time over pure random accessing.
• Extended Data Out DRAM(EDO DRAM):
i. Also called hyper page mode.
ii. Next column address can be accessed before external
system accepts current valid data.
iii. Idea is to speed up access time.
Types of DRAM
• Burst Extended Data Out DRAM(BEDO DRAM):
i. EDO DRAM with address burst capability.
ii. Burst feature allows upto four address to be
internally generated from single external
address which saves some access time.
• Synchronous DRAM:
i. Synchronized with system clock.
ii. Allows processor to handle other tasks while
memory read/write operations are in progress.
• Contains permanently stored data, which can
be read from the memory but cannot be
• ROM stores data that are used repeatedly in
system applications, such as programmed
instructions for system operation.
• ROMs retain stored data when power is
turned off and are therefore nonvolatile
Types of ROM
• Referred to simply as a ROM.
• Permanently programmed during
manufacturing process to provide widely used
standard functions, such as to provide user
• Once the memory is programmed, it cannot
• PROMs come from manufacturer unprogrammed, and
are custom programmed in field to meet user’s need.
• Basically same as mask ROM once they are
programmed, that is, it cannot be changed.
• Can be reprogrammed.
• Uses an NMOSFET array with an isolated gate
• Erasure of data bit is a process which removes gate
• The isolated gate in FET of UVEPROM is
floating within an oxide insulating material.
• The programming process causes electrons to
be removed from floating gate.
• Exposure is done by exposure of memory chip
to high intensity UV radiation through UV
window on top of package.
Electrically Erasable PROM
• Can be both erased and programmed with
• Can be rapidly programmed and erased
• Types: Floating gate MOS and Metal nitride
• Application of voltage on control gate in
floating gate structure permits storage and
removal of charge from floating gate.