"The Atari A to Z"
                              by Mark S Baines
                      Copyright (c) 1998 Mark S Baines
                             All Rights Reserved


       YOU MUST READ "READ_ME.NOW" BEFORE YOU LOOK AT ANY OF THIS FILE
*****************************************************************************


X


X number
Such as X.25. A series of CCITT Recommendations concerning public data 
networks, PAD facilities and their interfaces.


XBIOS
eXtended BIOS. The machine-specific extension to the BIOS that supports the 
special hardware features of the ST-Falcon range, being the lowest level of 
access in TOS to the hardware. Interfacing is provided to the RS-232C port, 
the disk drive, sound, video SHIFTER and BLiTTER chips, keyboard etc. There 
are no naming consistencies with XBIOS functions, unlike GEMDOS and BIOS.


XBRA
A protocol for writing TOS-based programs that install vectors at certain 
memory locations (usually system variables and exception vectors in low 
memory) which are intercepted and the routine pointed to by that vector 
executed. The protocol ensures that any vector already installed at that 
location is stored by the program so that when it has finished it can pass 
control to the old vector thus giving that routine a chance of execution. In 
this manner a chain of vectors is maintained by compliant programs.


XCONTROL panel
eXtensible CONTROL panel. The Control Panel accessory designed for TOS 2 and 3 
but capable of being used on other TOS versions. The program takes up one 
accessory slot but has the capability of loading up many separate CPX modules 
each performing a specific task, such as formatting a disk, providing a 
calendar or setting the Desktop colours. This overcomes the problem of limited 
accessory space and provides a uniform style for the CPX accessories.


XGA
eXtended Graphics Array. A screen display format found on the IBM PC 
compatibles which offers a resolution of 1,024 x 678 pixels with 256 colours.


Xmodem
An almost universal data transfer protocol enabling the transfer of files 
between remote computers. It uses small blocks of data, typically 256 bytes 
although a 1 K version is now common. The original 1-byte checksum version was 
replaced by a 2-byte cyclical redundancy check (CRC-16) version to provide 
modern error detection protection. See Ymodem, Zmodem.


XNAM
A protocol similar to that for the XBRA protocol except that the old vector is 
not stored by any vector grabbing program. It is used as an identification 
header to programs running under interrupts (usually VBLs) that do not need to 
be chained.


XT
eXtended Technology. An IBM PC computer launched in 1983 as an upgrade of the 
original IBM PC. It had 640 K of memory and a 10 Mb hard disk.


X Window
A network transparent window system for the UNIX operating system developed by 
MIT in the late 1980s and available on the TT as part of its UNIX system. See 
TT UNIX workstation.


XY plotter
A flatbed plotter on which the pen is controlled in two dimensions and the 
paper does not move.


X
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