Main memory is made up of RAM and ROM chips. RAM is read-write memory that can be accessed randomly; data is lost when power is off. There are static and dynamic RAM types. Static RAM retains data indefinitely if powered, dynamic RAM must be periodically refreshed. ROM is read-only and permanently stores data. There are mask, PROM, EPROM and EEPROM ROM types that can be programmed at different stages. Cache memory uses fast static RAM. Main memory often uses dynamic RAM for its ability to store large amounts of data at lower cost despite slower access.

1. Memory Hierarchy

2. Main Memory
• 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

3. 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.

4. 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.

5. Types of Random-Access Memory(RAM)
RAM
Static
RAM(SRAM)
Asynchronous
SRAM(ASRAM)
Synchronous
SRAM with burst
feature(SBSRAM)
Dynamic
RAM(DRAM)
Fast Page
Mode
DRAM(FPM
DRAM)
Extended Data
Out
DRAM(EDO
DRAM)
Burst EDO
DRAM(BEDO
DRAM)
Synchronous
DRAM(SDRAM)

6. Static RAM
• All static RAMs are characterized by latch
memory cells.
• As long as DC power is applied to a static
memory cell, it can retain a 1 or 0 state
indefinitely.
• If power is removed, stored data bit is lost.

7. Asynchronous Static RAM
Operation is not synchronized
with system clock.

8. Synchronous SRAM with burst feature
• A synchronous SRAM is synchronized with
system clock.
• The address, the read/write input, the chip
enable, and input data are all latched into
their respective registers on an active clock
pulse.
• Address burst feature allows memory to read
or write up to four sequential locations using a
single address.

9. Synchronous SRAM with burst feature

10. 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
main memory.

11. Dynamic RAM
• 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
complex circuitry.
• DRAMs use a technique called address
multiplexing to reduce number of address
lines.

12. DRAM
• 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.

13. 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.

14. 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.

15. Read-Only Memory(ROM)
• Contains permanently stored data, which can
be read from the memory but cannot be
changed.
• 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
memories.

16. Internal ROM Organization

17. Types of ROM
ROM
Mask ROM
Programmable
ROM(PROM)
Erasable PROM
Ultraviolet
EPROM
Electrically
Erasable PROM

18. Mask ROM
• Referred to simply as a ROM.
• Permanently programmed during
manufacturing process to provide widely used
standard functions, such as to provide user
specified functions.
• Once the memory is programmed, it cannot
be changed.

19. Programmable ROM
• 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.
Erasable PROM
• Can be reprogrammed.
• Uses an NMOSFET array with an isolated gate
structure.
• Erasure of data bit is a process which removes gate
charge.

20. Ultraviolet EPROM
• 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.

21. Electrically Erasable PROM
• Can be both erased and programmed with
electrical pulses.
• Can be rapidly programmed and erased
• Types: Floating gate MOS and Metal nitride
oxide silicone(MNOS).
• Application of voltage on control gate in
floating gate structure permits storage and
removal of charge from floating gate.