Glossary of PC terms: R

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RAMAC (Random Access Method of Accounting and Control)	Introduced in 1957, IBM-made  RAMAC was the first hard disk drive. It consisted of 50 magnetic disks 24 inches in diameter, rotating at 1200 RPM. Two air-bearing supported magnetic heads accessed all 50 disks. The storage capacity of this system was 5 MB with a data rate of 12.5 kB/s, and the system was rented to the end user for $130 a month. The system used Aluminum sliders, metal heads, had 20 tracks per inch and 0.002 Mb per inch squared, and the slider / disk spacing was about 20 um (micrometers, or 20,000 nm).
RAM (Random Access Memory)	An integrated circuit memory chip that allows data to be stored and retrieved by a microprocessor or controller. The data can be stored and accessed in any order, and all storage locations are equally accessible.
RAID (redundant arrays of independent/inexpensive disks)	RAID is an assembly of disk drives, known as a disk array, that operates as one storage unit. In general, the drives could be any storage system with random data access, such as magnetic hard drives, optical storage, magnetic tapes, etc. When the speed (data transfer rate) is an issue, the fastest SCSI hard drives are typically used. For details go here.
Removable storage	A type of storage with removable media, such as magnetic removable storage, optical storage, magneto-optical storage, etc..
Remnant magnetization (Mr)	The magnetization that remains in a material after the magnetic field is removed. See also Coercivity.
Read/write element	See magnetic Head.
ROM (Read Only Memory)	The memory unit containing programs and data that can be read but cannot be modified. A CD-ROM is one example of read only memory.
Random Access	An access method when the data can be stored and accessed in any order, and all storage locations are equally accessible.
Rigid disk drives	See Hard disk.
RISC	RISC is a type of CPU that sacrifices completeness of instructions in return for a very high speeds.
RLL (Run Length Limited) code	An encoding scheme used in hard disks drives and CD drives to encode data into magnetic pulses or pits.  RLL requires more processing, but stores almost 50 percent more data per disk than the older MFM (modified frequency modulation) method.
RPM (Revolutions Per Minute)	Rotational speed.  Related to linear velocity in the following way: V = pRN/30, where V is linear velocity [m/s], R is the radius [m],  p=3.14..., and N is rotation speed measured in RPM.

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