Glossary of Acronyms

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RAID - Redundant Array of Independent Disks. RAID is an established technology that provides capacity, reliability, high performance, and economy for the storage of on line data. The RAID system manages a set of disks, but appears to the user as a single large disk. The advantage of multiple disks is that if any single disk fails, the data is transferred to a neighboring disk, and the system continues to operate without any loss of data. Multiple disks also can be searched simultaneously and quickly. Fault tolerance is higher with RAID. Individual disks can be replaced while the system continues to function. With hot swapping capability, network managers can save time and avert potential problems before they threaten system operation. There are several levels or types of RAID to accommodate different storage needs.

RAID 1 - This level had duplicate disks operating side by side in parallel disk mirroring. System reliability is very high. If one disk fails, the other can supply any data needed. However, only 50% of the drive capacity is available for storage.

RAID 2 - It is not used due to its incompatibility with current drive disks.

RAID 3 - This level sues data striping and a dedicated parity drive. When data is written to the array, one byte goes to each disk. Each drive is accessed at the same time. The advantage is a very high transfer rate. The downside is that since every drive is used, only one I/O transaction can be processed at a time. RAID 3 is best for large data requests.

RAID 4 (RAID 0) - In this level, blocks of data are striped across the array of disks, thus the disks can be accessed in parallel mode. RAID 4 has a higher I/O rate than RAID 3, but data transfer is slower. Parity drives may be sued to provide tolerance to data drive failure. RAID 4 with no parity is known as RAID 0.

RAID 5 - Unlike RAID 3, which accesses all drives at the same time for the same read or write, RAID 5 can access as many drives as possible at the same time for different reads or writes. As a result, RAID 5 offers the highest I/O transfer rate of all the RAID levels.

RAM - Random Access Memory. Stands for Random Access Memory, which is used by the computer for a working area or as working storage. When programs are run and files are created or modified, they are loaded into RAM by the microprocessor. Same as DRAM. , the PC uses five types of RAM. The memory contains the instructions that tell the Central Processor what to do, as well as the data created by its activities. Since the computer works with the binary system, memory chips work by keeping electronic switches in one state or the other for however long they are required. Actually, they consist of a capacitor and a transistor; the capacitor stores a charge (data), which represents a 1, and the transistor acts as a switch that turns the charge on or off. Where these states can be changed at will, it is called Random Access Memory, or RAM. The term derives from when magnetic tapes were used for data storage, and the information could only be accessed sequentially. A ROM, on the other hand, is a memory chip with its electronic switches permanently on of off, so they can't be changed, hence Read Only Memory.

RAMDAC - A graphics chip that usually has three 8 bit DAC's and a small RAM to store color translations. The DACs generate red, green, and blue outputs for an analog monitor. The RAM translates a 4 or 8 bit color specifier into 24 bit output.

RAT - Register Alias Table. Renames programmer variable register reference to internal physical registers. This mapping is done at runtime.

RDRAM - RAMBUS Dynamic Random Access Memory.

RFI - Radio Frequency Interference. A high frequency signal radiated by improperly shielded conductors, particularly when signal path lengths are comparable to or longer than the signal wavelengths. The FCC now regulates RFI in computer equipment sold in the U.S. under FCC Regulations Part 1 5, Subpart J.

RISC - Reduced Instruction Set Computers. A type of CPU chip design where instructions and data are all the same size. The instructions are simplified and therefore easier to decode. Room is then left on the chip for other functions. RISC processors are much faster than ordinary (CISC) processors. Reduced instruction set refers to the number of clock cycles it takes a processor to issue an instruction. An ordinary chip takes several clock cycles to issue just one instruction. A RISC chip can execute one instruction for each clock cycle. Refers to processors with limited addressing modes and short (less than 32 bits), fixed length instructions that typically execute in a single cycle.  RISC processors have simple instruction sets requiring only one or a few execution cycles. These simple instructions can be utilized more effectively than CISC systems with appropriately designed software, resulting in faster operations.

RLL - Run Length Limited. A method used on some hard disks to encode data into magnetic pulses. RLL requires more processing, but stores almost 50 percent more data per disk than the older MFM (modified frequency modulation) method. Actually accomplished by recording fluxes for every byte, but packing them more tightly onto the surface. Often called 2,7 RLL because the recording scheme involves patterns with no more than 7 successive zeros and no less than two. Can support up to 17 x 512 byte sectors per track.

A type of encoding that derives its name from the fact that the techniques used limit the distance (run length) between magnetic flux reversals on the disk platter. Several types of RLL encoding techniques exist, although only two are commonly used. (1, 7)RLL encoding increases storage capacity by about 30 percent over MFM encoding and is most popular in the very highest capacity drives due to a better window margin, while (2,7)RLL encoding increases storage capacity by 50 percent over MFM encoding and is used in the majority of RLL implementations. Most IDE, ESDI, and SCSI hard disks use one of these forms of RLL encoding.

ROB - Reorder Buffer - The Pentium Pro Processor functional unit where initial uops wait, speculative results are collected, and then are retired.

ROM - Read Only Memory. Read only memory is memory inside a computer that contains either instructions or information that is permanently stored. It cannot be changed or written to. A ROM, is a memory chip with its electronic switches permanently on of off, so they can't be changed, hence Read Only Memory. PROM can be programmed only once. EPROM can be erased by exposure to ultraviolet light and then reprogrammed. EEPROM can be electronically erased but takes much longer to program than to read.

RS-232 An interface introduced in August 1969 by the Electronic Industries Association. The RS-232 interface standard provides an electrical description for connecting peripheral devices to computers.

RTC - Real Time Clock.

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Compiled by Scott McArdle, MagnaCom Limited. I hope this list has helped you and if there is an item that should be on this list, please let me know. Thanks. PS, I've spent 100's of hours maintaining this list, please don't be a LAMER.