RAM:
Random access memory.
Allows the information to be stored and retrieved by system.
Increase the speed of a system.
Temporary storage.

History of RAM:
Vacuum tubes used to store data.
Drum memory.
Core memory information stored by magnetic field.
Transistorized means vacuum tubes placed are very small.
RAM invented by Dennard in 1968.

Working of RAM:
CS indicates Chip select enable or disable RAM.
Address is for read from or write data.
RD/WR for reading and writing.
To read from RD/WR should be set to 1.
To write to RD/WR should be set to 0.

Types of RAM:
SRAM.
DRAM.

SRAM:
Static random access memory.
Bit of data stored is 6 transistor.
Expensive and faster.
No Refresh cycle.

Refresh cycle:
Reads and then rewrites the contents.

Types of SRAM:
Volatile.
Asynchronous.
Synchronous.
Special.

Volatile SRAM:
Lose data when power is lost.

Asynchronous SRAM:
Data is constant until power is supplied.
Used as main memory.

Synchronous SRAM:
Output of one data is connected as input of another.
Faster.

SRAM operations:
Stand by.
Reading.
Writing.

Stand by:
Word line is not asserted the access transistor disconnect the cells.

Reading:
If the cell is in state1, the signal on bit line is high and signal of bit negation is low.

Writing:
Places appropriate value on bit line b and its complement on negation of b and active the world line.

SRAM working:

Advantages:
Low power.
Faster.
Used in cache memory.

Disadvantages:
Reliability is less.
More transistors.
Expensive.

DRAM:
Dynamic.
Refresh cycle.
Bit of data on 1 transistor.
Capacitor is either charged or discharged.
Capacitor cannot be stored for a long time because a capacitor holds electrical charge for limited time.

DRAM working:

Advantages:
Less power .
Low cost.
High reliability.
Lesser area.

Disadvantages:
Refreshing it needs power.

DDR:
Double data rate.
Synchronous Dynamic RAM.
We have versions like DDR1, DDR2, DDR3.

Synchronous Dynamic RAM:
Increase the performance of processor.

How to know which one is DDR2 and DDR3?
Based on the clutch.
DDR2 clutch is towards left side.
 DDR3 clutch is towards right side.

ROM:
Read only memory.
Used to store firmware.
Modifying is impossible or difficult.
Located on motherboard at BIOS.
Non volatile.

When was ROM invented?
In 1980.
First used by Sony in 1982.

Types of ROM:
Mask ROM.
Programmable ROM.
Erasable Programmable ROM.
Electrically Erasable programmable ROM.
Electrically Altered ROM.
Flash ROM.

Mask ROM:
Contents are programmed by integrated circuit manufacturer.
Cost is effective.

PROM:
Can be written but only once.
Useful for companies that make their own ROM's from software they write.

EPROM:
Can be erased and reprogrammed.
Little glass window is installed at top of ROM package.
Ultraviolet light of specific frequency can be shined through window for specified period of time through will erase EPROM and allow it to be reprogrammed again.

EEPROM:
Erased under software control.
Refers to BIOS upgrade by flashing.

EAROM:
Can be modified bit at a time.

Flash ROM:
Used to detect, read, verify, erase or write BIOS.

Speed:
Reading.
Writing.

Reading:
Speed of reading data from ROM is slower than RAM.
While reading ROM is copied to RAM sometimes.

Writing:
Speed is lower than reading speed.

Advantages:
Permanent and secure data storage.
Small long life battery.

Disadvantages:
To improve speed, many systems load frequently used files into RAM and read from there.
Requires more time for reading.

Difference between RAM and ROM?

Features	RAM	ROM
Volatility	Volatile.	Non volatile.
Accessibility	Directly accessed by processor.	Can't be directly accessed by processor.
Storage	Temporary.	Permanent.
Hardware structure	Form of chip.	Optical drives made up of magnetic tape.
Cost	High.	Low.
Chip size	Large.	Small.
Speed	Fast.	Low.
Storage	16GB.	4MB.