When older versions (before September 1986) of TOPS for the Macintosh are
used together with AppleShare, an endless loop of error messages ("The Disk
need minor repairs" and "Desktop file could not be created") appear when you
try to mount an AppleShare volume. Newer versions of TOPS fix the problem.
There is an external file system hook that both AppleShare and TOPS use.
Since there is only one hook, and more than one application needs to access
it, the solution implemented is to daisy-chain the routines that latch into
it. This means that any routine that puts its address in the hook should
preserve the address that was previously there, and then call it after it's
done performing its tasks. That way, all applications have a chance to
examine a filing call if all other applications are well-behaved. While early
versions of TOPS daisy-chained properly, and passed on the filing command to
other routines, they didn't preserve register values.