1. What is the primary storage?
2. What is used in modern computer as secondary storage?
3. Why is tertiary storage useful for extraordinarily large data stores?
4. What computer data storage is not under the control of a processing unit?
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
Secondary storage
Tertiary storage
Off line storage
CPU central processing unit
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 () lasers and greater numerical apertures. The Blu-ray disc uses blue-violet lasers of greater aperture (), for use with discs with smaller pits and lands, thereby greater data storage capacity.
