Computer Main Memory, Secondary Memory and Backup Memory
The main memory in a computer is called Random Access Memory. It is also known as RAM. This is the part of the computer that stores operating system software, software applications and other information for the central processing unit (CPU) to have fast and direct access when needed to perform tasks. It is called “random access” because the CPU can go directly to any section of main memory, and does not have go about the process in a sequential order.
RAM is one of the faster types of memory, and has the capacity to allow data to be read and written. When the computer is shut down, all of the content held in RAM is purged. Main memory is available in two types: Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).
Dynamic random access memory (DRAM) is the most common kind of main memory in a computer. It is a prevalent memory source in PCs, as well as workstations. Dynamic random access memory is constantly restoring whatever information is being held in memory. It refreshes the data by sending millions of pulses per second to the memory storage cell.
Static Random Access Memory (SRAM) is the second type of main memory in a computer. It is commonly used as a source of memory in embedded devices. Data held in SRAM does not have to be continually refreshed; information in this main memory remains as a “static image” until it is overwritten or is deleted when the power is switched off. Since SRAM is less dense and more power-efficient when it is not in use; therefore, it is a better choice than DRAM for certain uses like memory caches located in CPUs. Conversely, DRAM’s density makes it a better choice for main memory.
Secondary memory is computer memory that is non-volatile and persistent in nature and is not directly accessed by a computer/processor. It allows a user to store data that may be instantly and easily retrieved, transported and used by applications and services.
Secondary memory is also known as secondary storage.
Characteristics of Secondary Memory
These are some characteristics of secondary memory, which distinguish it from primary memory −
- It is non-volatile, i.e. it retains data when power is switched off
- It is large capacities to the tune of terabytes
- It is cheaper as compared to primary memory
Hard Disk Drive
Hard disk drive is made up of a series of circular disks called platters arranged one over the other almost ½ inches apart around a spindle. Disks are made of non-magnetic material like aluminum alloy and coated with 10-20 nm of magnetic material.
Standard diameter of these disks is 14 inches and they rotate with speeds varying from 4200 rpm (rotations per minute) for personal computers to 15000 rpm for servers. Data is stored by magnetizing or demagnetizing the magnetic coating. A magnetic reader arm is used to read data from and write data to the disks. A typical modern HDD has capacity in terabytes (TB).
CD stands for Compact Disk. CDs are circular disks that use optical rays, usually lasers, to read and write data. They are very cheap as you can get 700 MB of storage space for less than a dollar. CDs are inserted in CD drives built into CPU cabinet. They are portable as you can eject the drive, remove the CD and carry it with you. There are three types of CDs −
- CD-ROM (Compact Disk – Read Only Memory): The data on these CDs are recorded by the manufacturer. Proprietary Software, audio or video are released on CD-ROMs.
- CD-R (Compact Disk – Recordable): Data can be written by the user once on the CD-R. It cannot be deleted or modified later.
- CD-RW (Compact Disk – Rewritable): Data can be written and deleted on these optical disks again and again.
DVD stands for Digital Video Display. DVD are optical devices that can store 15 times the data held by CDs. They are usually used to store rich multimedia files that need high storage capacity. DVDs also come in three varieties – read only, recordable and rewritable.
Pen drive is a portable memory device that uses solid state memory rather than magnetic fields or lasers to record data. It uses a technology similar to RAM, except that it is nonvolatile. It is also called USB drive, key drive or flash memory.
Blu-Ray Disk (BD) is an optical storage media used to store high definition (HD) video and other multimedia filed. BD uses shorter wavelength laser as compared to CD/DVD. This enables writing arm to focus more tightly on the disk and hence pack in more data. BDs can store up to 128 GB data.
Backup storage refers to a storage device, medium or facility that is used for storing copies and instances of backup data. Backup storage enables the maintenance, management, retrieval and restoration of backup data for any individual, application, computer, server or any computing device.
Backup storage is primarily an additional storage device used for keeping backup data. Typically, it is external to the system, server or device for which the backup data is created, such as a local/remote storage server. The backup storage itself can be a hard disk drive, tape drive, compact disk drive or any mass storage medium installed within a computer or storage server. In enterprise IT environments, the backup storage medium/technology used can be RAID, a storage area network or a network-attached storage system. Backup software or a backup manager is used to create, store, manage and retrieve backup data to and from the backed up application/device and the backup storage location.