Optical Disk

The optical disk is a storage media from which data is read and to which it is written on by lasers. Optical disks can store upto 6 gigabytes (6 billion bytes). That figure is uch higher that that for portable magnetic media, such as floppies. there are four basic types of optical disks:

CD-ROM (Compact Disc-Read Only Memory)

This is a type of optical disk, capable of storing latge amounts of data up to 1 GB (although the most common size is 650 MB). A single CD-ROM has a storage capacity of 700 floppy disks.

The vendor stamps CD-ROMs with data and once stamped, they cannot be erased and filled with new data. to read a CD, you need a Cd-ROM player. all CD-ROMs are made to a standard size and format, so one can load any type of CD-ROM into any CD-ROM player. The data on a CD-ROM is permanent and can be read any number of times, but annot be modified.

WORM(Write Once Read Many)

WORM drives contain disks that you can record on only once. whatever you record on the disk remains there permanently. After that, the WORM disk behaves just like a CD-ROM. These drives are perfect for achieving large amounts of data and are frequently used by banks and accounting firms. To write data onto it, a laser beam of modest intensity is employed. IBM developed the 20GB WORM for its PS/2 system; it has a large storage capacity and a longer life. It is more reliable - the only drawback being the longer access time.

EO(Erasable Optical)

These are optical disks that can be erased and loaded with new data, just like magnetic disks. Both lasers and electromagnets are used to record information on a cartridge, the surface of which contains tiny embedded magnets.

DVD(Digital Video Disk)

DVD stands for Digital Video disk. The DVD is also a member of optical disk family. It has the same dimensions of a CD but has significantly higher storage capacity. DVDs are double-sided unlike simple CDs which are single sided. However, DVD drivers can read most of the CD media. Many types of DVDs are avaialble.

1) DVD ROM is just like a large CD-ROM, allowinng data and infrastructure material as well as audio and video. It requires a DVD-ROM drive installed in computer. DVD-ROM drivers can play DVD Video movies.

2) DVD-R is a write-once version for creating masters.

3) DVD-RAM is the rewritable DVD.

4) DVD-R/W is also a rewritable version but is more an extension to DVD-R than a competitor to DVD-RAM.

5) DVD-Video is a read-only DVD disk used in full lenght movies.

Primary Memory : RAM / ROM

Memory is the storage place where data and instructions are stored. They can be retrieved from memory whenever required. Every computer comes with a certain amount of physical memory, usually referred to as Main Memory or RAM. You can think of main memory as an array of cells, each cell holding a single bit of information. this menas a computer with 1MB of memory can hold about 1 million bytes of information.

RAM(Random Access Memory)

It is a read/write (R/W)memory which is volatile. This means when power is turned off, all the contents are destroyed. This is memory that can be accessed randomly: that is, any byte of memory can be accessed without touching the preceding bytes. RAM is synonymous with main memory, the memory avaialble to programs. RAm is the  most common type of memory found in computers and other devices such as printers. There are two basic types of RAM: Dynamic RAM (DRAM) and Static RAM(SRAM)

DRAM(Dynamic RAM) : Dynamic RAM is more common type. Dynamic RAM needs to be refreshed thousands of times per second. DRAM stores a bit of data using a transistor and capacitor pair, which together comprise a memory cell. The capacitor holds a high or low charge (1 or 0, respectively), and the transistor acts as a switch that lets the control circuitry on the chip read the capacitor's state of charge or change it. As this form of memory is less expensive to produce than static RAM, it is the predominant form of computer memory used in modern computers.

SRAM(StaticRAM) : Static RAM does not need to be refreshed, which makes it faster, but it is more expensive than dynamic RAM. In static RAM, a bit of data is stored using the state of a flip-flop. This form of RAM is more expensive to produce, but is generally faster and requires less power than DRAM and, in modern computers, is often used as cache memory for the CPU.

ROM (Read Only Memory)

ROM is non-volatile which menas it retains the stored information even if power is turned off. this memory is used to store programs that boot the computer and perform diagnostics. therefore, we can also call ROM as the read-only RAM.

ROM is of four types:

Masked ROM: In this ROM a bit pattern is permanently recorded by a marking and metalization process, which is an expensive and specialized one. Memory manufacturers are generally equipped to undertake this process.

PROM(Programmable ROM): A PROM is a memory chip on which data can be written onto only once. Once a program is written onto a PROM chip, it remains there forever. Unlike RAM, PROM retains its contents when the computer is turned off. The difference between a PROM and a ROM is that a PROM is manufactured as blank memory and programmed later with a special device called PROM programmer or the PROM burner, whereas the ROM is programmed during manufacturing process. the process of programming a PROM is sometimes called burning a PROM.

EPROM(Erasable Programmable ROM): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light. Once erased, it can be reprogrammed. An EPROM is similar to a PROM except that it requires ultravilolet radiation to be erased.

EEPROM(Electrically Erasable Programmable ROM): EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge. Like other types of PROM, EEPROM retains its contents even when the power is turned off. Also, like other types of ROM, EEPROM is not as fast as RAM. EEPROM is similar to Flash Memory (sometimes called flash EEPROM). the principal difference is that EEPROM requires data to be written or erased one byte at a time whereas flsh memory allows sata to be written or erased in blocks.

Cache Memory:

The speed of the CPU is extremely high as compared to the access time of main memory. the slowness of main memory inhibits the performance of CPU. To decrease the mismatch in operating speed, a small memory chip is attached between the CPU and the main memory, whose access time is close to the processing speed of the CPU. It is called cache memory. Cache memory is accessed more quickly than conventional RAM. It is used to store programs or data currently being executed or temporary data frequently used by the CPU.

Arithmetic Logic Unit (ALU)

It is one of the main components of the Central Processing Unit. This is where all arithmetic and logic operations are done. Data received from an input device is stored in primary memory before being passed on to ALU for processing.
Memory
Memory is the location where data and instructions are stores. They can be retrieved from memory whenever required. Memory is used to:
1. Hold the data received from input device temporarily and ready it for processing.
2. Hold data that has been processed and the intermediate results generated within.
3. Hold the finished results of processed data, until released to output devices.
4. Hold the system software and application software in use.

Data is stored in memory as bytes. A byte is made up of eight bits. A bit is the smallest unit of memory. Other units of memory are Kilobytes(KB), Megabytes(MB), Gigabytes(GB) and Terra bytes (TB) where:

1 KB = 1024 bytes
1 MB = 1024 KB = 1024 * 1024 bytes
1 GB = 1024 MB
1 TB = 1024 GB

Memory can be classified into two groups:
1. Main Memory
2. Secondary Memory

Main Memory
Every computer comes with a certain amount of physical memory, usually referred to as the main memory of RAM( Random Access Memory)
There are different kinds of main memory:
RAM (Read/write or R/W). this type of memory is also known as random access memory. This is volatile, which means, when the power is turned off, all the contents are destroyed. This memory is again of two types: Static RAM and Dynamic RAM.
ROM (Read Only Memory). The ROM is non-volatile, which means, it retains the stored information even if the power is turned off.
ROM is again of four types:
Masked ROM, PROM (Programmable ROM), EPROM( Erasable Programmable ROM) and EEPROM(Electrically Erasable Programmable ROM).
Secondary Memory:
Secondary Memory includes storage devices like cassette tape, magnetic tape, floppy disk, hard dist, Winchester disk, etc.