Hierarchy of 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. Any data actively operated on is also stored there in uniform manner. Main memory is directly or indirectly connected to the CPU via a memory bus, today sometimes referred to as a front side bus. It is actually comprised of two buses (not on the diagram): an address bus and a data bus. The CPU firstly sends a number through an address bus, a number called memory address, that indicates the desired location of data. Then it reads or writes the data itself using the data bus.

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. Modern computer systems typically have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there.In modern computers, hard disks are usually used as secondary storage.

Tertiary storage or tertiary memory, provides a third level of storage. 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 library.

Off-line storage, also known as disconnected storage, is computer data storage on a medium or a device that is not under the control of a processing unit. 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.

Primary storage – первичная память

Secondary storage – вторичная память

Tertiary storage – третичную память

Off – line storage – автономное ЗУ

CPU – central processing unit – процессор

Exercise 15. Read the text without a dictionary. Express its contents by 5-6 sentences:

Optical disc

An optical disc is a flat, circular disc where data are stored in the pits in its flat surface . The data are accessed in the disc when a special material (often aluminum) is illuminated with a laser diode. The pits distort the reflected laser light.

Write-once optical discs commonly use an organic dye, and re-writable discs use phase change alloys.

Initially, optical discs were for storing music and computer software. The laser disc format stored analog video signals, other first-generation disc formats are designed to store digital data.One example of high-density data storage capacity is 700MB of net user data for a 12cm compact disc.

Second-generation optical discs were for storing great amounts of data, including broadcast-quality digital video. Such discs usually are read with a visible-light laser (usually red);. In the DVD format, this allows 4.7GB storage on a standard 12cm, single-sided disc

Third-generation optical discs are in development, meant for distributing high-definition video and support greater data storage capacities, accomplished with short-wavelength (коротковолновой) lasers and greater numerical apertures. The Blu-ray disc uses blue-violet lasers of greater aperture (носитель), for use with discs with smaller pits and lands, thereby greater data storage capacity.

Exersice 16. Translate the text in a written form with a dictionary.

Virtual memory is a computer system technique which gives an application program the impression that it has contiguous working memory, while in fact it may be physically fragmented and may even overflow on to disk storage. Systems that use this technique make programming of large applications easier and use real physical memory (e.g. RAM) more efficiently than those without virtual memory.

Note that "virtual memory" is not just "using disk space to extend physical memory size". Extending memory is a normal consequence of using virtual memory techniques, but can be done by other means such as overlays or swapping programs and their data completely out to disk while they are inactive. The definition of "virtual memory" is based on tricking programs into thinking they are using large blocks of contiguous addresses.

All modern general-purpose computer operating systems use virtual memory techniques for ordinary applications, such as word processors, spreadsheets, multimedia players, accounting, etc. Few older operating systems, such as DOS of the 1980s, or those for the mainframes of the 1960s, had virtual memory functionality - notable exceptions being the Atlas and B5000.

In computer science, dynamic memory allocation is the allocation of memory storage for use in a computer program during the runtime of that program. It can be seen also as a way of distributing ownership of limited memory resources among many pieces of data and code.

Dynamically allocated memory exists until it is released either explicitly by the programmer, exiting a block, or by the garbage collector. This is in contrast to static memory allocation, which has a fixed duration. It is said that an object so allocated has a dynamic lifetime.