XA        Extended Architecture , Extended Attribute
XAPIA     X.400 Application Program Interface Association
X2B       Hexadecimal to Binary [REXX]
.XBM      X-Windows Bitmaps (file name extension) (black and white)
.XPM      X-Windows Pixelmaps (file name extension) (color)
X2C       Hexadecimal to Character [REXX]
XCHG      Exchange
XCMD      External Command
XCOPY     Extended Copy
X2D       Hexadecimal to Decimal [REXX]
XDF       Extended Density Format [IBM]
XDR       Extended/External Data Representation
XFCN      External Function
XGA       Extended Graphics Array [IBM]
XID       Exchange Identifier
XIOS      Extended Input/Output System
XLAT      Translate
XLM       Excel Macro Language [Microsoft]
XMIT      Transmit
XMM       Extended Memory Manager [LIM/AST]
XMS       Extended Memory Specification [LIM/AST]
XNS       Xerox Network System (protocol) [Xerox]
XOFF      Transmitter Off
XON       Transmitter On
XOR       Exclusive OR (Also EOR)
XPRM      Xerox Print Resources Manager [Xerox]
XRT       Extensions for Real-Time
XSMD      Extended Storage Module Drive (interface)
XTAL      Crystal
XTCLK     External Transmit Clock