Computer data storage, often called storage or memory, refers to computer components, devices, and recording media that retain digital data used for computing for some interval of time.
Usually the memory is consider of semiconductor storage known as random access memory(RAM),these are the primary storage memory but use for temporary storage,But the permanent storage for data is also required these can be gain through optical disk,magnetic disk like hard disk .
Hierarchy of storage
1 Primary storage
2 Secondary storage
3 Tertiary storage
4 Off-line storage
Primary storage
Primary storage, presently known as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them.
The invention of transistor makes new technology development in the storing capability. This led to a modern random access memory (RAM). It is small-sized, light, but quite expensive at the same time. (The particular types of RAM used for primary storage are also volatile, i.e. they lose the information when not powered).
Beside of RAM their is two other type of memory also-:
a)Register memory-
Processor registers are located inside the processor. Each register typically holds a word of data (often 32 or 64 bits). Registers are technically among the fastest of all forms of computer data storage, being switching transistors integrated on the CPU's chip, and functioning as electronic "flip-flops".
b)cache memory-
Processor cache is an intermediate stage between ultra-fast registers and much slower main memory. It's introduced solely to increase performance of the computer. Most actively used information in the main memory is just duplicated in the cache memory, which is faster, but of much lesser capacity. Multi-level hierarchical cache setup is also commonly used—primary cache being smallest, fastest and located inside the processor; secondary cache being somewhat larger and slower.
Main memory is directly or indirectly connected to the CPU via a memory bus.
As the RAM types used for primary storage are volatile (cleared at start up), a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small startup program (BIOS) is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. A non-volatile technology used for this purpose is called ROM, for read-only memory (the terminology may be somewhat confusing as most ROM types are also capable of random access).
Secondary storage
Secondary storage, or storage in popular usage, differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile.
Hard disks are usually used as secondary storage.
The secondary storage is often formatted according to a filesystem format, which provides the abstraction necessary to organize data into files and directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, the access permissions, and other information.
Most computer operating systems use the concept of virtual memory, allowing utilization of more primary storage capacity than is physically available in the system. As the primary memory fills up, the system moves the least-used chunks (pages) to secondary storage devices (to a swap file or page file), retrieving them later when they are needed. As more of these retrievals from slower secondary storage are necessary, the more the overall system performance is degraded.
Tertiary storage
Tertiary storage or tertiary memory,[2] provides a third level of storage. Typically it involves a robotic mechanism which will mount (insert) and dismount removable mass storage media into a storage device according to the system's demands; this data is often copied to secondary storage before use. It is primarily used for archival of rarely accessed information since it is much slower than secondary storage (e.g. 5-60 seconds vs. 1-10 milliseconds). This is primarily useful for extraordinarily large data stores, accessed without human operators. Typical examples include tape libraries and optical jukeboxes.
Off-line storage
Off-line storage, also known as disconnected storage, is a computer data storage on a medium or a device that is not under the control of a processing unit.[3] The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted or connected by a human operator before a computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction.
