The first line of an sccs file contains the checksum, preceded by ‘^Ah’. The checksum is in decimal and is generated by adding together the values of all the characters in the file, and taking the result modulo 65536. A checksum line might look like this:-
On systems whose C implementation considers the
char type to be
unsigned, characters with their highest bit set appear to be considered
positive, and on machines with a signed
char type, these
characters appear to be considered negative. This seems to mean that
these two types of machines will not agree on the correctness of an
sccs file's checksum.
The BitKeeper suite uses ‘^AH’ to introduce its checksum line rather than ‘^Ah’, but the checksum is computed in the same way.