CIGAL Reference Manual, Chapter 4 (Variables): opsys
OPSYS, a read-only character string variable, is set automatically at start-up based on the operating system's internal version flags.
On Windows, OPSYS is set by a call to the system's GetVersion() function. The low byte from GetVersion is the major version number (stored as xx.0 in OPSYS), the high 16-bits from GetVersion form the subversion (stored as 0.xxx in OPSYS).
See Also:
LIST(2),
MACROS(1),
MENU(2),
RUN(2),
STRINGS(1)