BSD vs. System V, one last thing...

[Doug Gwyn]

In article <21106 at cornell.UUCP> murthy at cs.cornell.edu (Chet Murthy) writes:
>Actually, v7 didn't have virtual memory built-in.
>At least, not paged virtual memory.  sysV diesn't
>have it because, as I heard it, it is derived from
>PWB/UNIX, and not v7.  PWB was the same UNIX,
>so I have heard, that spawned 32v, which went on to
>become 2bsd, 3bsd, 4bsd, etc.  However, sysV did
>get paging in version V.2.2.  And also in V.3
>System V did indeed have swapping, though.  But
>then swapping can be done with a minimum of hardware.

This is mostly wrong.  UNIX/32V sprang directly from 7th Edition
UNIX.  UNIX System V has resulted from several evolutionary streams,
not especially PWB.  The original PWB/UNIX and 7th Edition UNIX had
essentially the same memory management (swapping) strategy anyway.
The PDP-11 had sufficient hardware (KT11) for demand paging.
(It was missing page reference bits, but so was the VAX-11/780.)

You have to be careful with UNIX System V version numbers.  Release
2 Version 2 for the 3B2. for example, did NOT implement demand paging
(that occurred with Release 2 Version 4 for 3B2).  I don't think
there was a Release 2.2 for any architecture, but there was a Release
2.1 for the 3B2.

UNIX System V used a partial-swapping strategy on most architectures
until the demand-paging versions came out.  Most implementations of
UNIX have some form of swapping, even if they also provide demand
paging.