Data Dictionary - M

M

  • Magnetic Disk
  • Mail Merge
  • Manchester Coding
  • Mask
  • Mass Storage
  • Matching
  • Media
  • Mega
  • Megabyte (MB)
  • Megahertz (Mhz)
  • Memory
  • Memory Model
  • Mesh Network
  • Metafile
  • MFM
  • Mirroring
  • MMC Card
  • Monochrome
  • MS-DOS
  • MTBF
  • Multitasking
  • Multithreading

Magnetic Disk
A computer disk enclosed in a protective case (hard disk) or jacket (floppy disk) and coated with a magnetic material that permits flux changes on many small sections (magnetic domains) of the disk surface. Flux changes are changes in magnetic polarity and are used to encode information in binary form (one polarity equals 1, the opposite equals 0). The changes are produced at high speed by the read/write head of the disk drive as it passes over the surface of the disk. Because of its properties, a computer disk should be protected from exposure to sources of magnetism, which can damage or destroy the information it holds.

Mail Merge
A mass-mail facility that takes names, addresses, and (sometimes) pertinent facts about recipients and merges the information into a form letter or another such basic document.

Manchester Coding
A method of encoding data used in communications – for example, on some local area networks – that combines both data and timing signals in a stream of transmitted bits. The data value of each bit is indicated during the first half of the bit period (time required to define the bit) by the signal state (high for 1, low for 0); a transition to the opposite state at the middle of the bit period acts as a timing signal.

Mask
A binary value used to selectively screen out or let through certain bits in a data value. Masking is performed by using a logical operator (AND, OR, XOR, NOT) to combine the mask and the data value. For example, the mask 00111111, when used with the AND operator, removes (masks off) the two uppermost bits in a data value but does not affect the rest of the value. This is shown in the figure, which uses the data value 11010101.

11010101 Data Value
AND  00111111 Mask
-----------------------
00010101 Resulting Value

Mass Storage
A generic term for disk or tape storage of computer data, so called for the large masses of data that can be stored in comparison with computer memory capacity. Optical disc storage is also a form of mass storage.

Matching
The process of testing whether two data items are identical or of finding a data item that is identical to a key.

Media
A collective word for the physical material, such as paper, disk, and tape, used for storing computer-based information. Media is plural; medium is singular.

Mega
Abbreviated M. A prefix meaning 1 million (base 10 to the power of 6). In computing, which is based on the binary (base-2) numbering system, mega- has a literal value of 1,048,576 bytes, which is the power of 2(base 2 to the power of 20) closest to one million.

Megabyte (MB)
See Byte

Megahertz (MHz)
A 200 MHz processor can (theoretically) execute 200,000 instructions in a millisecond. See Hz

Memory
Circuitry that allows information to be stored and retrieved. In the most general sense, memory can refer to external systems such as disk drives or tape dives; in common usage, it refers only to the fast semiconductor storage (RAM) directly connected to the processor.

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Memory Model
The approach used to address the code and the data that are used in a computer program. The memory model dictates how much memory can be used in a program for code and how much for data. Most computers with a flat address space support only a single memory model. Computers with a segmented address space usually support multiple memory models.

Mesh Network
A communications network having two or more paths to any node.

Metafile
A file that contains or defines other files. Many operating systems use metafiles to contain directory information about other files on a given storage device.

MFM
Stands for modified frequency modulation, an encoding method used to store data in hard disks.

Mirroring
Also known as RAID 1. One drive is used to store a duplicate of the data of the original drive, providing redundancy. No data is lost if either drive fails.

MMC card
Multimedia Card. A type of flash memory card for data storage commonly used in portable devices measuring 24mm x 32mm x 1.5mm.

Monochrome
In reference to computers, a term applied to a monitor that displays images in only one color – black on white (as is standard on monochrome Apple Macintosh screens) or amber or green on black (as is common on IBM and other monochrome monitors). The term is also applied to a monitor that displays only variable levels of gray. High-quality monochrome monitors are commonly considered to be clearer and more readable than colour monitors of the same resolution.

MS-DOS
Pronounced “em-ess doss”; acronym for Microsoft Disk Operating System. MS-DOS – like other operating systems – oversees operations such as disk input and output, video support, keyboard control, and many internal functions related to program execution and file maintenance. MS-DOS is a single-tasking, single-user operating system with a command-line interface.

MTBF
Mean Time Between Failure.

Multitasking
A mode of operation offered by an operating system in which a computer works on more than one task at a time. There are several types of multitasking. Context switching is a very simple type of multitasking in which two or more applications are loaded at the same time but only the foreground application is given processing time; to activate a background task, the user must bring the window or screen containing that application to the front. In cooperative multitasking, exemplified by the Macintosh operating system, background tasks are given processing time during idle times in the foreground task (such as when the application waits for a keystroke), and only if the application allows it. In time-slice multitasking, exemplified by os/2, each task is given the microprocessor’s attention for a fraction of a second. To maintain order, tasks are either assigned priority levels or processed in sequential order. Because the user’s sense of time is much slower than the processing speed of the computer, time-slice multitasking operations seem to be simultaneous.

Multithreading
The running of several processes in rapid sequence (multitasking) within a single program. In data manipulation, a technique in which nodes in a tree data structure contain pointers to higher nodes to make traversal of the structure more efficient.

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