Memory Hierarchy

A number of devices used to store data in a computer. You may wonder why there are such a large variety of storage devices. The primary reason is the tradeoff between cost per byte of storage and access time and the secondary reason is whether the storage is volatile or not. A computer system needs all the storage devices described in this tutorials in a balanced configuration. The memory system is organized as a hierarchy consisting of a number of levels. At the top level is high speed, low capacity, volatile memory called a cache and at the bottom is high capacity, slow access, non-volatile tap storage.

This figure shows different level of memory store.

Memory Type	Average capacity in bytes	Technology	Average time to access a byte	Purpose in a compute system	Relative cost per byte in units
Cache	16 M	High speed integrated circuits	0.5 ns	Instruction and data to be immediately used	10
Main memory	512 M	Integrated circuits(DRAM)	5 ns	Program and data	1
Disk Memory (hard disk)	750 G	Magnetic surfaces on hard disks	8 ms	Large data files and program overflow from main memory	1/100
Flash memory EEPROM	32 G	Integrated circuit	1 µ	Storing personal program and data files	1/1000
LTO-6	6.25T	Long 1/2” tape would on cartridge	NS	Historical files. Backup for disk. Data and program exchange between installations.	1/1000
CDROM	600 M	Laser Disk	500 ms	Storage large texts, pictures and audio. Software distribution.	1/100000
DVDROM	8 G	Laser Disk	500 ms	Video files	1/10000
Blue-ray disc	50 g	Laser Disk	500 ms	Video files	1/10000

                                 Table 1. Comparative characteristics of memories

In table 1, we have summarized the average capacity of each of the devices, their access times and the relative cost per byte of storage. From the table, it is seen that small semiconductor memories using high speed transistor flip- flops known as cache memories are the most expensive as well as the fastest in the set. At the other end. At the other end, we have CDROMs, the DVDROMs, Blue-ray discs, and tapes with high storage capacity, low speed serial access, and low cost per byte of storage. In a computer system, it is necessary to use a judicious combination of all these types of storage devices for best results. The situation is similar to the method used to store files in offices. The most urgent files are kept for immediate attention on table of a manager. This is analogous to the data stored in a high speed memory used. Files which need to be examined on a given day by a manager would be normally kept in an in-tray on the side of the table and can be immediately accessed by the manager. The main semiconductor random access memory of a computer is similar to this. It stores the program currently being executed. Files which do not have to be referred to immediately but which should be within easy reach of the manager are kept in an organized manner in a filing cabinate. The disk storage in a computer is similar to this. It stores programs and data which would be needed in the near future. It also stores the data which would be needed for immediate reference. As it is non-volatile, programs and data are periodically backed up on disks. Files which require attention only rarely and those which need to be preserved for audit and legal requirements are kept in big index files in the main office away from the manager’s room.

The magnetic tape storage system in a computer fulfils a similar purpose. Archival data is stored on tapes. Finally, legal and reference books are in a library. CDROM, DVDROM, and Blu-ray discs fulfil a special purpose. Duplicate copies of programs and data are stored in CDs, DVDs Blu-ray discs.

The most appropriate tape for backing up achieving data from a disk today is LTO-6 Ultrium tapes. Software or data distribution is best done on CDROMs or DVDOMs. Large high definition video files are distributed using Blu-ray disks. The best medium now storing personal files and retrieving them easily is pen drive.