Atari ST series Technical Reference Manual page 116

GEMDOS also provides a function that returns the GEM­
DOS version number. This number refers only to the version
of GEMDOS, not to the GEM or TOS version in general. The
function used to find the GEMDOS version number is Sver-
sion(), and it's called like this:
int version;
version = Sversion();
where version is a 16-bit code which indicates the GEMDOS
version number. This number is stored in the same format as
MS-DOS uses for its version number, with the major version
number in the low byte, and the minor revision number in
the high byte. The first version of the TOS ROMs all show
the GEMDOS version to be 0x1300, or version 0.19.
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CHAPTER 5