Extended File Control Block
	A DOS File Control Block which has had an additional seven
bytes prepended to permit control of file attributes (which are stored
in the appendage).  See also FCB.

Extended Memory
	Memory beyond the one megabyte address which is available only
on 80286 and higher machines.  Except for a small portion (the High
Memory Area), extended memory is only accessible from protected mode.

Extended Memory Specification
	A specification devised by Microsoft which allows multiple
programs to share extended (above 1 megabyte) memory and noncontiguous
memory above 640K.  See also Upper Memory Block.

External Device Interface
	A series of calls made by the DESQview multitasker on various
"interesting" events, which may be intercepted by programs which wish to
keep track of the current system state.	 See also Callout.

FAT
	see File Allocation Table

FCB
	see File Control Block

File Allocation Table
	A data structure on disk that records which clusters are free,
which are unusable, and which have been allocated.  The clusters occupied
by a file are linked into a list in the file allocation table, allowing
DOS to find the contents of the file.

File Control Block
	A data record in the calling program's address space which is
used by DOS 1.x functions to record the state of an open file.	See
also INT 21h Function 13h.

File Handle
	A small positive integer used to identify the previously-opened
file on which a program wishes to perform an operation.

Flush
	To force the copying of any data still stored in temporary
buffers to its final destination.

FM
	(Frequency Modulation)	A method of encoding data as a series of
magnetic flux reversals on disk or tape, commonly known as single-density
recording.  In frequency modulation, a series of clock pulses are written
at regular intervals, with one data bit for each clock pulse.  See also
MFM, RLL.

Formatting
	Preparing a storage medium (usually magnetic media such as a
disk or tape) for storing data.	 Low-level or physical formatting
writes all necessary housekeeping data to enable the storage device
to read the media and may also initialize the storage units on the
media to a known state.	 High-level or logical formatting writes data
used by the operating system, such as allocation information and
directories onto media which has already been physically formatted.
Formatting programs often perform both a low-level and a high-level
format.

FOSSIL
	(Fido/Opus/Seadog Standard Interface Layer)  A standardized API
for performing serial I/O, originally used by the Fido and Opus bulletin-
board software and Seadog bulletin-board mailer, but now in wider use.

FTP
	(File Transfer Protocol)  The standard protocol for copying files
from one machine to another on a TCP/IP (Internet) network.  Also the
program of the same name with which a user may transfer files.

Gather-Write
	see Scatter/Gather

Handle
	A short identifier, usually a small integer or a pointer, for
some other object which is maintained or controlled by the operating
system or environment; a particular handle may be valid system-wide or
may have meaning only for a particular process.	 See also File Handle.

HGC
	(Hercules Graphics Card) A monochrome video adapter capable
of 720x352 monochrome graphics.	 The HGC was the first non-IBM video
adapter for the IBM PC.	 See also CGA, MDA.

High Memory Area
	The first 65520 bytes (64K less 16 bytes) of extended memory.
This area is accessible from real mode on the 80286 and higher
processors because these processors do not wrap addresses at one
megabyte as the 8088 and 8086 do.  See also A20, INT 2Fh Functions
4A01h.

HMA
	see High Memory Area

Horizontal Retrace
	When a monitor has finished displaying a single scan line, it must
move it electron beam(s) back to the left edge of the CRT, during which time
it turns off the beam.	On the original CGA (and some early clones), the
only time one could access the display memory without causing "snow" was
during the horizontal or vertical retrace periods, as the display adapter
was not itself accessing the display memory during those times.	 See also
Vertical Retrace.

I2C
	(also IIC; the "2" is superscripted) Inter-Integrated Circuit
Bus -- A moderate-speed serial communications bus originally invented
by Philips in the early 1980s for consumer-electronics applications,
such as inter-chip communication in a television set or high-end
stereo.	 The I2C bus has recently appeared on PCs in video capture
boards and similar devices, as well as (surprisingly) SDRAM DIMMs (for
the on-board serial EEPROM).  The ACCESS.bus is a derivative of the I2C
bus which forms the physical layer of the Universal Serial Bus.	 Similary,
the SMBus (System Management Bus) also uses I2C as its physical layer.

IDE
	(Integrated Drive Electronics) A type of disk drive interface
which essentially extends the PC's expansion bus all the way to the
drive and places the drive controller on the disk drive itself.	 See
also ESDI.

IDT
	see Interrupt Descriptor Table

IFS
	see Installable File System

IIC
	see I2C.

Installable File System
	An Installable File System which allows non-DOS format media to
be used by DOS.	 In most ways, an IFS is very similar to a networked
drive, although an IFS would typically be local rather than remote.
See also INT 21h Function 52h.

Interrupt Descriptor Table
	The protected-mode table of descriptors specifying the handlers
for up to 256 interrupts, and how to transfer control to each one.  See
also Interrupt Vector Table.

Interrupt Vector Table
	The real-mode table of 256 four-byte interrupt handler addresses,
located in the lowest 1024 bytes of the address space.	See also Interrupt
Descriptor Table.


IP
	(Internet Protocol) The lower level (transport layer) of the
TCP/IP protocol suite.	See also TCP, TCP/IP.

IPC
	(Inter-Process Communication) Any one of numerous methods for
allowing two or more separate processes to exchange data.

IPL
	(Initial Program Load) See Boot.

IPX
	(Internetwork Packet Exchange) A low-level layer of Novell's
NetWare networking software.

IRQ
	(Interrupt ReQuest) A hardware line connected to the interrupt
controller chip which signals that a CPU interrupt should be generated.

ISA
	(Industry-Standard Architecture) The expansion bus used by the
IBM PC/AT.  See also EISA.

IVT
	see Interrupt Vector Table

JFT
	see Job File Table

Job File Table
	The Job File Table (also called Open File Table) stored in a
program's PSP which translates handles into SFT numbers.  See also INT
21h Function 26h.

LCD
	(Liquid Crystal Display)

List of Lists
	An internal DOS table of lists and other tables through which
most DOS-internal data structures may be reached.  See INT 21h Function
52h.

Little-Endian
	One of the two major ways of organizing multi-byte numeric
values in memory.  A little-endian layout places the least significant
byte of the value in the lowest (first) memory location, i.e. 12345678h
is stored as 78h 56h 34h 12h.  Intel processors are little-endian.
Compare Big-Endian.

LPT
	Abbreviation for Line PrinTer.

MCB
	see Memory Control Block

MCGA
	(Multi-Color Graphics Array) The low-end color adapter offered
in IBM's early PS/2 series machines.

MDA
	(Monochrome Display Adapter) A text-only video adapter introduced
together with the original IBM PC.  See also CGA, HGC.

Memory Control Block
	The data structure containing the length and owner (among other
things) of a portion of the memory managed by DOS.  See INT 21h
Function 52h.

MFM
	(Modified Frequency Modulation) A method of encoding data as a
series of magnetic flux reversals on disk or tape, commonly known as
double-density recording.  In contrast to FM, modified frequency
modulation omits all clock pulses except those between pairs of zero
bits.  See also FM, RLL.

Mickey
	The smallest increment of motion a mouse can sense.

MIDI
	(Musical Instrument Digital Interface) A standardized interface
for controlling musical instruments with a computer.

Modem
	(contraction of MOdulator/DEModulator)

MSR
	(Model-Specific Register) Additional, indirectly-accessible,
registers containing control or status information about various
aspects of the processor such as caches, performance counters, and the
like.	These registers, accessible via the RDMSR and WRMSR
instructions, were added with the Pentium and later-model 486
processors.

Multitasking
	Any of a number of methods by which multiple programs may
execute concurrently, with rapid switching between the programs giving
the appearance that all are executing simultaneously.

MZ
	The letters M and Z appear in numerous places in DOS (memory
control blocks, .EXE header, etc.); the conventional explanation is
that these are the initials of Mark Zbikowski, one of the principal
architects of MSDOS 2.0.

NCB
	see Network Control Block

NDIS
	(Network Driver Interface Specification) A hardware-independent
network interface developed by Microsoft and 3com.  See also Packet
Driver, TCP/IP.

NetBIOS
	One of a number of low-level device-independent network
interfaces; the other major interfaces are Novell's IPX and the
Internet's IP (Internet Protocol, the lower-level portion of TCP/IP).

Network Control Block
	A Network Control Block used to pass requests to NetBIOS and
receive status information from the NetBIOS handler.

Network Redirector
	A program which permits access to network devices (disks,
printers, etc.) using the MSDOS kernel network redirector interface.
See also Network Shell, Redirector Interface.

Network Shell
	A program which permits access to network devices (disks,
printers, etc.) by intercepting DOS calls before they reach the DOS
kernel and handling those operating on network devices while passing
through actions on local devices.  See also Network Redirector.

NMI
	see Non-Maskable Interrupt

Non-Maskable Interrupt
	An interrupt which can not be disabled by clearing the CPU's
interrupt enable flag, unlike most normal interrupts.  Non-maskable
interrupts are typically used to signal calamities which require
immediate action, such as a hardware failure or imminent loss of power.

Non-Volatile RAM
	Memory which can be modified like normal RAM but does not lose
its contents when the system's power is turned off. This memory may be
powered by a battery when the system power if off, or it may be a type
of memory which does not need electricity to maintain its contents,
such as EEPROM or bubble memory.

NVRAM
	see Non-Volatile RAM

ODI
	(Open Data-link Interface) A hardware-independent network
interface developed by Novell, Inc.  See also NDIS, Packet Driver.

OEM
	(Original Equipment Manufacturer) a company which purchases
components that are resold as part of its own products under the company's
own brand name, e.g. a Gateway 2000-branded monitor may actually be a Mag
or NEC monitor.

Open File Table
	see Job File Table

Overscan Area
	The "border" between the edge of the area where graphics or text can
be displayed and the actual edge of the area the video adapter can illuminate
on the monitor's screen.

Overscan Register
	On a display adapter, the control register which specifies the color
to be displayed in the overscan area.

Packet Driver
	Any one of the numerous drivers conforming to FTP Software's
Packet Driver Specification, which provides a hardware-independent
network interface.  See also NDIS, ODI.

Page Fault
	A CPU-generated signal, and the operating system's reaction to it,
generated when a program accesses a page of virtual memory which is not
located in RAM at the time.  The operating system's response is to load in
the required page, possibly writing some other page out to disk in order
to make room.  See also INT 0E in INTERRUP.LST.

Page Register
	A peripheral register or I/O port used to extend the addressing
range of some other register or I/O port.  The prime example are
the DMA page registers, which allow the DMA controller to address
more than 64K (since the DMA controller only contains 16 address
lines; this is the cause of the 64K DMA boundaries).

Palette Register
	A memory location on the video controller which specifies the actual
color displayed for a particular color number.

Paragraph
	A 16-byte unit of memory.  This size is the result of the 8086's
segmentation model, where incrementing the segment by one advances the
absolute location by 16 bytes.

Park
	To move a hard disk's read/write heads to a position in which
it is safe to turn off the power and transport the disk drive.	Many
drives also lock the heads into position when they are parked,
providing additional protection from sudden movement.

Pel
	see Pixel