Variables used in the nucleus subroutines and in the program subroutines are stored in a system common. The system common declarations appear in the file XMACRO. When this file is included at the start of each program, the system common is extruded as the file AACOMN. AACOMN is then inserted in the same way that the program common file is applied. Note that system common variables are usually needed when any of the system macros are applied, even if they are not shown explicitly in the code.

The contents of the system common may be summarized as follows:

line character buffers	CHRIN,CHRIP,CHROT
line field buffers	BUFIN,BUFOT
line field order & pointer arrays	BUFORD,BUFPNT
title character buffer	TITLE
line ID, line counters, etc.	LINID,LINCT,LINRM,..
character and page variables	NPAGE,CHRMAX,CHRCOL,..
binary file signals	IOUNIT,IOMARK,IOPRCT,..
QX memory allocation variables	QXMAX,QXSTR,QXCUR,..
QX general data storage array	QX

These variables are declared in four labelled commons.

SYSI contains the integer variables, 

SYSF the real variables (except QX), 

SYSC the character variables and 

QXDATA contains the general data storage array QX.