> [...]   The distinction between (2) and (3) is silly.   It existed
> in ancient unix purely because of the radically different calling sequence
> for a sys call compared to a libc function when called from asssembler code
> (back in the days when a good fraction of unix userland was written in as).

there was also, at that time, a desire to make the syscalls minimal to keep
the size of the kernel down.  anything that didn't need to be a syscall was
pushed out to pure usermode.  and even though we don't have a 64KB boundary,
or 64KB segments, or any other resource barrier that makes this necessary,
it is still highly desireable for security auditing and overall reliability
to keep the kernel minimal.  (remember, i was at digital at the time they put
X10 into the ultrix kernel, so i've actually got this tire track down my back.)