COMPUTER MEMORY

Memory in computer system is required  for storage the data and instructions. It stores program, data, output or any other kind of information. there are two types of computer memory.

                *  Main memory (Primary memory)

                *  Secondary memory

MAIN MEMORY 

A primary or internal storage section is basic to all computers. It is made up of several small storage areas called locations or cells. Each of these locations can store a fixed number of bits called word length of that particular primary storage. This number is called the address of the location and is used to identify the location. Each location can hold either a data item or an instruction, and its address remains the same regardless of its contents.

There are two types of main memory.

            *  RAM

            *  ROM

RAM

RAM (random access memory) chip is made with MOS (metal oxide semiconductor). We can select a location on this chip rendomly and can be use it to store retrieve data and instructions directly. A memory unit is a collection of storage registers, needed to transfer information in and out of the registers.

                                                            Figure 5.1- RAM

ROM 

ROM (read only memory) does not have a write capability. This implies the binary information stored in a ROM is made permanent during the hardware production of the unit and can not be altered by writing different words into it. While a ROM is a general-purpose device whose contents can be altered during the computational process, a ROM is restricted to reading words that are permanently stored within the unit.

Read only memory is a type of memory chip that we can read only and we can not write on it. ROM provides permanent storage for program instruction. The most important ROM chip in any computer is ROM BIOS (basic input output system). ROM is most often used in microprocessors that always execute the same program such as BOOT STRAPLOADER.

SECONDARY MEMORY  

Secondary storage devices provide a facility for permanently storing the information contained in temporary memory. There are several types of secondary storage devices. The data and program information are stored on circular metal or plastic disks as a series of electromagnetic spots. 

The disk can be internal hard disk that is housed within the system unit or an external floppy disk, in addition, the CD-ROM disk is other types of secondary storage device commonly found in computer system.

MAGNETIC DISK

A magnetic disk is a circular platter of plastic which is coated with magnet sable material. One of the key components of a magnetic disk is a conducting coil named as Head which performs the job of reading and writing on the magnetic surface, the Head remains stationary while the disk rotates below it for reading or writing operation.

MAGNETIC TAPE CASSETTES AND CARTRIDGES  

There are also large tape cartridges which contain long strips of magnetic tape and which resemble large cassettes. These cartridges provide a more convenient way to package tape, and greatly simplify the mounting of tape which is normally a problem with conventional reels of tape that have to be manually mounted on the mechanism.

HARD DISK  

The hard disk is a permanent fixture containing one or more circular metallic disks that are used to store data files and software programs.

                                                                 Figure 5.2- Hard Disk Drive 

FLOPPY DISK  

The second main type of secondary storage is the floppy disk. Floppy disk store data on a flexible plastic disk. It allows saving data and retrieves data from disk that mostly used for backup. The main advantage of floppy disk is that is transportable.

                                                                 Figure 5.3- Floppy Disk 

CD-ROM DISK  

(Compact disk read only memory) This optical disk's data is imprinted by the disk manufacture. The user cannot erase it, change it, the user can only "read" the data This type of optical disk is used primarily for making huge amounts of prerecorded data-------such as government statistics, encyclopedias medical reference books, dictionaries, and legal libraries----  immediately available to the user. write once the disk have been written on, however they can only be read from then on--again, on changes can be made.

                                                                     Figure 5.4- CD

                                                                 Figure 5.5- CD-ROM 

UNITS OF MEMORY  

   1Bit                                   = 1 (single character)

   1Byte                                = 8 Bit

   1Kilo byte                        = 1024 Bytes

   1 Mega byte                     = 1024 kilo byte

   1 Giga byte                      = 1024 Mega byte