vfork

[Henry Spencer]

> ...it's not because the segments are too large.  It's because you
> must have contiguous physical addressing on the pdp, otherwise
> swapping becomes a nightmare (very important on pdp's remember) and
> physical i/o becomes impossible (only the cpu goes through the
> virtual memory map, not the i/o devices)...

Not true, actually.  Few processes use physical i/o; they can always
be forced to be contiguous.  How bad the swapping issue is depends on
the tradeoffs between disk speed and memory scarcity (but note that on
an 11 with a Unibus map (70, 44, 24[?]) there is no problem at all).
Non-contiguous allocation isn't trivial, but it's not impossible either.
Proof by example:  it's been done.

The problem with copy-on-write "smart" forking on the 11 is, as stated,
that there are big segments and relatively few of them, so you probably
end up copying most of the program anyway.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry