Ultra violet rays are used to erase the contents of EPROM. EPROM retains its contents until it is exposed to ultraviolet light. The ultraviolet light clears its contents making it possible to reprogram the memory.

A program counter (PC) is a CPU register in the computer processor which has the address of the next instruction to be executed from memory. It is a digital counter needed for faster execution of tasks as well as for tracking the current execution point.

It depends on
(a) Clock speed
(b) Instruction set
(c) Word size

Flash memory	EEPROM
It is faster	It is slower
Flash memory is read/write memory	EEPROM is read only memory
Flash memory can store 4GB to 2TB of data	EEPROM can store only less data
Flash memory is used in PDA'S, Digital camera to store data.	It is used to store critical programs.

PROM	EPROM
Can be Programmed only once.	It is reusable and can be programmed multiple times.
This has Permanent memory.	EPROMs memory can be erased using UV light.
Enclosed in plastic package so that not visible to UV light.	EPROMs are fused with quartz window.
Uses high Voltage.	Also use high Voltage but not enough to alter the semiconductor layer permanently.

Differences between computer organisation and Computer Architecture:

Computer Organisation	Computer Architecture
Often called micro architecture (low level)	Computer architecture (a bit higher level)
Transparent from programmer (ex. a programmer does not worry much how addition is implemented in hardware)	Programmer view (i.e. Programmer has to be aware of which instruction set used)
Physical components (Circuit design, Adders, Signals, Peripherals)	Logic (Instruction set, Addressing modes,Data types, Cache optimization)

Read-only memory (ROM):
1. Read only memory refers to special memory in a computer with pre-recorded data at manufacturing time which cannot be modified.
2. The stored programs that start the computer and perform diagnostics are available in ROMs. ROM stores critical programs such as the program that boots the computer.
3. Once the data has been written onto a ROM chip, it cannot be modified or removed and can only be read.
4. ROM retains its contents even when the computer is. turned off. So, ROM is called as a non-volatile memory.
Programmable Read-Only Memory (PROM):
1. Programmable read-only memory is also a nonvolatile memory on which data can be written only once. Once a program has been written onto a PROM, it remains there forever.
2. Unlike the main memory, PROMs retain their contents even when the computer is turned off.
3. The PROM differs from ROM. PROM is manufactured as a blank memory, whereas a ROM is programmed during the manufacturing process itself.
4. PROM programmer or a PROM burner is used to write data to a PROM chip. The process of programming a PROM is called burning the PROM.
Erasable Programmable Read-Only Memory (EPROM):
1. Erasable Programmable Read Only Memory is a special type of memory which serves as a PROM, but the content can be erased using ultraviolet rays.
2. EPROM retains its contents until it is exposed to ultraviolet light. The ultraviolet light clears its contents, making it possible to reprogram the memory.
3. An EPROM differs from a PROM, PROM can be written only once and cannot be erased.
4. EPROMs are used widely in personal computers because they enable the manufacturer to change the contents of the PROM to replace with updated versions or erase the contents before the computer is delivered.
Electrically Erasable Programmable Read Only Memory (EEPROM)
1. Electrically Erasable Programmable Read Only Memory is a special type of PROM that can be erased by exposing it to an electrical charge.
2. Like other types of PROM, EEPROM retains its contents even when the power is turned off.
3. Comparing with all other types of ROM, EEPROM is slower in performance.

A memory is just like a human brain. It is used to store data and instructions. Computer memory is the storage space in the computer, where data and instructions are stored. There are two types of accessing methods to access (read or write) the memory. They are sequential access and random access. In sequential access, the memory is accessed in an orderly manner from starting to end. But, in random access, any byte of memory can be accessed directly without navigating through previous bytes. Different memory devices are arranged according to the capacity.