SCSI Terminology  

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

[back to SCSI Terminology index page]

M___________________________

Magneto-Optical
See MO.

Mainboard
See Motherboard.

Mainframe
An extremely large (occupying the space of entire rooms) and costly computer used for supporting many users running programs simultaneously. The IBM S370/3031 is an example of a mainframe.

Main Memory
The part of a computers memory which is directly accessible by the CPU (usually synonymous with RAM).

Mastering
Technically, refers to the process of creating a glass master from which compact discs will be reproduced in quantity. In desktop recordable CD systems, mastering is done together with premastering by the desktop CD recorder, and the term is generally used to mean "recording."

Max Out
Slang term meaning to use fully.

Mb
See Megabyte.

Mbps
Megabits per second (one million bits per second). Speed at which data is transferred. Hopefully as modems and serial transmissions become faster, Mbps will be used to discuss their speed.

MCA (Micro Channel Architecture)
See Micro Channel Architecture.

MDRAM
Short for Multibank DRAM, a relatively new memory technology developed by MoSys Inc.   MDRAM utilizes small banks of DRAM (32 KB each) in an array, where each bank has its own I/O port that feeds into a common internal bus. Because of this design, data can be read or written to multiple banks simultaneously, which makes it much faster than conventional DRAM.

Another advantage of MDRAM is that memory can be configured in smaller increments, which can reduce the cost of some components. For example, it's possible to produce MDRAM chips with 2.5 MB, which is what is required by video adapters for 24-bit color at a resolution of 1,024x768. With conventional memory architectures, it's necessary to jump all the way to 4 MB. Currently, MDRAM is used in some video adapters and graphics accelerators.

Megabyte
1024 kilobytes (about one million bytes hence megabyte). Usually abbreviated Mb or MB as in 30 Mb.

Megahertz (Mhz)
Describes the speed at which a computer runs. Abbreviated MHz or Mhz. Do not take the speed at which a processor runs to be absolute.  For instance a machine with a PowerPC processor running at 90 MHz may perform some tasks more quickly than an Pentium machine running at 120 MHz, and the Pentium may be faster at others.

Memory
A temporary storage area for information and applications. RAM, ROM, conventional memory, expanded memory, and extended memory are all different types of memory. RAM and hard disk space should not be confused. RAM is temporary storage while the hard disk offers semi-permanent storage.

MHz
See Megahertz

Micro Channel Architecture
Early in the PC revolution, IBM added the capability to give add-in devices control of PCs - first giving its XT a special slot for the purpose and then giving the AT the capability to let any slot give over control to an external master. The shortcoming of these designs was that only one device could take control; multiple add-in masters could not be accommodated safely. Consequently, even before the AT and its improved expansion bus were introduced, IBM began working on a total redesign of personal computer expansion in 1983.

Micro Channel brought many mainframe computer design ideas to PC bus expansion and absolved many of the original sins of the PC bus. Micro Channel's architects completely redesigned the bus with high speed operation in mind, relocating and redefining signals. Unlike the PC and AT buses, the Micro Channel put a ground or power supply conductor within three pins of every signal to shield against radio frequency interference.

Most importantly, Micro Channel took bus control from the system microprocessor and gave it a circuit IBM called the central Arbitration Point. Transfers across the bus were managed by devices called bus masters, of which the microprocessor was but one.   Others could operate from individual expansion slots. Multiple bus masters were permitted, and the bus provided a dynamic hierarchical method of assigning priorities and bus access to each.

Far from stripping the microprocessor of power, however, this design lightened its overhead so that the microprocessor had more time for microprocessing. Although the Micro Channel design did little to improve performance of the bus, it gave the overall system more potential. The nominal bus clock was upped to 10 MHz, and the data path expanded to 32bits, providing a peak data rate of 20 MB/sec. IBM designed Micro Channel to be microprocessor-independent and operate asynchronously, so that the speed could be varied. Using what IBM called Matched Memory Cycles, some 32-bit memory boards operated at 16 MHz (peak data rate, 32 MB/sec).

The new bus architecture required devices to negotiate for every access to the bus, for every transfer made. To cut this overhead, IBM added what it called a Burst Mode that permitted a single device to maintain control of the bus without renegotiation for up to about 12 milliseconds. Even with this Burst Mode, all Micro Channel transfers each still required two clock cycles - one for addressing; one for data transfer.

The original Micro Channel was a full 32-bit design with 32-bit addressing that allowed up to 4 GB of memory on the bus, but initial IBM products only allowed 16 MB to be accessed by a computer's DMA controller. That limitation tied down Micro Channel with effective memory-handling capabilities no better than the AT bus because most products assume that DMA reaches all memory addresses. On the other hand, all Micro Channel expansion boards were required to decode all 65, 536 I/O ports addressable by Intel microprocessors.

Although Micro Channel did not change the number of available interrupts, it allowed the existing interrupts to be shared. To facilitate sharing and improve reliability, interrupts were maintained by level-sensitive signals instead of being edge-triggered To prevent any attempt at using old bus cards in Micro Channel machines, IBM specified a new miniaturized connector for expansion boards. This incompatibility and the initial high royalty IBM demanded to use proprietary aspects of Micro Channel technology dampened enthusiasm for the new bus. Whereas most industry insiders viewed these measures as IBM trying to regain control of the PC market by promoting a proprietary standard, IBM actually documented Micro Channel more fully than it ever did the AT bus.

Technically, Micro Channel rated as a masterstroke, essentially embodying the best of mainframe technology distilled down to PC size. But in a fit of corporate hubris, IBM tied use of MCA's proprietary technologies with hefty licensing fees that the rest of the PC industry, accustomed to taking advantage of IBM-developed technology for free, balked at. Moreover, IBM cursed Micro Channel with marketing so inept it might have besmirched the reputation of Florence Nightingale, and failed ever to satisfactorily explain why backward compatibility with PC and AT expansion boards was such an ill-considered concept. IBM's rationale was that the truly bad engineering behind the old bus and boards would hold back performance of new MCA systems. Advanced features, such as level-sensitive interrupts would not tolerate old boards that used edge-triggered interrupts. Starting with a brand new standard opened the opportunity for doing things right, entirely re-engineering the bus for optimum high speed operation and reliability. Most people, however, believed the Micro Channel design was more a means for IBM to steal back the industry by making the products of other manufacturers obsolete.

Microchip
A small, silicon object containing microscopic circuitry.

Microcomputer
A computer that is small enough to fit on a desktop and consisting of very few microchips. The PC and Macintosh are examples of microcomputers.

MiniCAM
A scaled-down, limited-functionality version of NCR's DOSCAM driver.

Minicomputer
A term coined in the early 1970s to describe a small, low-priced computer (relative to the humongous and extremely expensive mainframe computers). The PDP-11, VAX-11, and HP3000 are examples of minicomputers.

Mixed-Mode Disc
A compact disc including both computer data (CDROM tracks) and audio (CD-DA) tracks.   The data is all contained in Track 1, and the audio in one or more of the following tracks.

MO (Magneto-Optical)
Magneto-optical (MO) technology uses a combination of magnetics and optics to read, write, and erase information. A laser beam heats a layer of material on the disk until the material takes on the polarity of an electromagnet located in the drive mechanism. Data is written to the disk by rapidly switching the polarity of
the electromagnet.

To read data, the laser beam is polarized and the reflected light is detected, measured, and translated into data. To erase data, the laser beam heats the disk sector until it returns to a pre-write state.

MotherBoard
The main circuit board in a computer on which the CPU, main memory, system BIOS, custom integrated circuits, and other built-in electronic components that make a computer work. It also has expansion slots for host adapters and other expansion boards.

Mount
To install a compact disc so that the computer recognizes its presence and can read data from it.

MPEG
Moving Pictures Experts Group. A standard for compressed audio and video data. This is a "lossy" compression scheme, which means the data loses some quality in compression. The standard specifies a bandwidth of 150 Kbytes per second, which is the data rate of audio CD's.

MSCDEX
Microsoft DOS extensions for CD-ROM. Allows the DOS operating system to recognize a CD-ROM as a DOS volume.

MS-DOS
Microsoft Disk Operating System. The most common operating system for IBM and compatible PCs. Produced by Microsoft. Sometimes referred to simply as DOS.

Multi-function Drives
Multi-function drives are large capacity storage devices that provide long term storage and flexibility of working in either WORM or Rewritable mode. The drive determines whether you're using a WORM or Rewritable cartridge, and then operates in the appropriate mode.

Multimedia
Computer applications involving text, graphics, audio and video, or computers that are (or claim to be) designed to provide high quality graphics and audio. Multimedia is largely associated with CD-ROM, because the large amounts of data involved are provided best by CD-ROM.

Multiprocessor
A computer system that uses more than one CPU running in parallel for faster performance.

MultiRead
MultiRead is an OSTA specification that recognizes the importance of having future CD readers capable of reading a wide variety of CD-based media compared to older generations of CD readers. MultiRead CD-ROM drives, for instance, will be able to read newer CD-RW media as well as CD-R and CD-ROM.

Multisession Disc
If a disc contains more than one session, then the disc is called a Multisession Disc.  It is a compact disc to which data is added incrementally in more than one recording session. If data is linked between sessions, all data on a multisession disc, when read on a multisession CD-ROM drive, may be seen as part of a single logical structure.

Multi-tasking
The ability to run more than one program at the same time. 

There are different types of multitasking. Cooperative multitasking requires a program to be written to allow other programs to access the system. In preemptive multitasking, the system can suspend any program to allow other programs access. Preemptive multitasking provides better performance, as programs can switch with less overhead. The Macintosh and Windows 3.1 use cooperative multitasking. Windows 95 and Unix use preemptive multitasking.

The executing of more than one command at the same time. This allows programs to operate in parallel.

Multi-threading
The ability to have more than one task occurring in an application. For example, you could have a large spreadsheet calculate values while continuing to use that spreadsheet or print a complicated document while continuing to work on that document. OS/2 has multithreading capabilities. Applications must be written according to certain specifications in order to multithread.

The simultaneous accessing of data by more than one SCSI device. This increases the data throughput.

MultiSync
In a monitor, the ability to run at different scan rates, allowing the monitor to display different resolutions.

Multi-Volume
Multi-volume is used to organize multiple sessions as completely separate volumes, each with its own directory of information.

[back to SCSI Terminology index page]

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

home | company info | products | inquiry | tech support
driver & bios updates | where to buy | contact ioi