Memory

Computer Memory

There are different types of memories, which are used in computer. Primary memory has other names as Main memory or Random Access Memory (RAM), CPU uses it to store or retrieve data and instructions for processing. Secondary Memory or Auxiliary memory is used to store the data and instructions permanently. Hard disk and (floppy disk are the example of secondary memory. Compact Disk Read Only Memory (CD ROM) is also used to store the data permanently but St requires CD writer to store data into it. Cache memory is used to enhance the performance of Computer system.
Main Memory
Data and instructions are required to be stored in main memory from where it can be retrieved by central processing unit for processing results. Main memory serves as a temporary storage area for data and instructions. It is made up of a large number of cells, with each cell capable of storing one bit. The cells may be organized as a set of addressable words, each word storing a sequence of bits. The time to store and retrieve a word is independent of the address of the word. This organization is also called RAM or Random Access Memory. The main memory is very fast. Its storage capacity is in megabytes. Nowadays most of the Computers have minimum of 128 Mbytes of RAM. All the modern computers are now built with semiconductor main memory because of its fast access time, low cost, small physical size and low power consumption. RAM is further classified as either dynamic RAM (DRAM) or static RAM (SRAM)
The larger capacity chips are normally constructed as dynamic RAM because the dynamic RAM requires less number of transistors than the static RAM and hence can be packed more densely on a single chip. Thus dynamic RAM can achieve higher storage density. DRAM is implemented using capacitors which tend to loose their charge with time and need periodic refreshing. So, it needs extra control circuitry. This refreshing process slows down memory access speed slightly. SRAM, on the other hand, does not need any such refreshing circuitry. Thus a SRAM is faster than a DRAM. DRAM is cheaper than SRAM and consumes less power than SRAM.
Secondary Memory
The programs & data that are required to be used again & again are kept in auxiliary or secondary memory. Floppy disk, Hard disk are the examples of it. The present day capacity of Hard disk is 300 Giga bytes. Because of its relatively slow speed auxiliary storage is used for storing programs and data not immediately needed by the Computer. The programs and data are then copied into faster main memory when the Computer is ready to process them. Secondary storage devices are online to the processor. They accept data or program instructions from the processor, retain them, and then write them back to the processor as needed to complete processing tasks. Examples of Secondary memory are floppy disk, hard disk, magnetic tapes, charge-coupled devices, magnetic bubble memories, optical memories etc.
Floppy Disk
The floppy disk gets its name from the fact that it is made out of a flexible plastic material. The plastic base is coated with an iron-oxide recording substance that's similar to the material applied to the plastic ribbon of a magnetic tape. Data are recorded as tiny invisible magnetic spots on this coating. The disk is loaded and rotated inside its envelope tiny electromagnetic heads in the disk reader access the data through the slot in the jacket. A floppy disk is divided into tracks (these are a series of concentre rings on the surface of the disk) and each track is further divided into sectors. The number of tracks that a disk can hold is measured in tracks per inch (tpi) and it is always mentioned on any floppy disk you buy. A diskette is inexpensive and can be erased and reused many times.
Hard Disk
The disk described so far is the type of disks which can be removed from the disk drive and carried from place to place. Some disks however, are built into the computer or a special disk drive. These are fixed disks. They are used in just the same way as removable disks, but tend to be used for storing programs or data which have to be permanently available. Hard disks can store anywhere from 20 MB to more than 40GB. Hard disks are also from 10 to 100 times faster than floppy disk.
Magnetic Tapes
Magnetic tapes are mounted on reels or a cartridge or a cassette of tape to store large volumes or backup data. These are cheaper and since these are removable from the drive, they provide unlimited storage capacity. Since the recording is like that in a tape recorder used in audio systems, information retrieval will be only sequential and not random as in the case of a disk. These are not suitable for online retrieval of data, since sequential searching will take long time. These are convenient for archival storage for backup.
CD-ROM
The CD-ROM (Compact Disk Read Only Memory) is a direct extension of audio CD. CD-ROM players are more rugged and have errorcorrection facility. This ensures proper data transfer from CD-ROM to the RAM of the computer. CD-ROM is written using a CD writer. Information is retrieved from a CD-ROM using a low power laser, which ingenerate in an on optical disk drive unit. In CD-ROMs the information is stored evenly across the disk in segments of the same size. Therefore, in CD-ROMs data stored on a track increases as we go towards outer surface of disk. Thus, the CD-ROMs are rotated at variable speeds for the reading process.
Cache Memory
CPU speed is quite high as compared to the access time of main memory. In many situations, the performance of the processor suffers because of the low speed of the main memory. This problem is solved by placing small fast memory called cache memory between the processor and the main memory whose access time is close to the processing speed of the CPU. Cache memory is faster than the main memory. The cache memories although are fast yet are very expensive memories and are used in only small size. Thus, small cache memories are intended to provide fast speed of memory retrieval without sacrificing the size of memory. Cache memory stores a copy of the instructions and data to be immediately used of main memory. The memory reads or writes operation is first checked with cache memory and if the desired location data is available in cache memory then used by the CPU directly.