Path: utzoo!attcan!uunet!husc6!bbn!rochester!cornell!batcomputer!itsgw!nyser!cmx!billo
From: billo@cmx.npac.syr.edu (Bill O)
Newsgroups: comp.arch
Subject: Re: VM needed for rapid startup
Keywords: paging virtual-memory speed
Message-ID: <508@cmx.npac.syr.edu>
Date: 25 May 88 12:47:29 GMT
References: <463@cvaxa.sussex.ac.uk> <1016@cresswell.quintus.UUCP>
Reply-To: billo@cmx.npac.syr.edu (Bill O'Farrell)
Organization: Northeast Parallel Architectures Center, Syracuse NY
Lines: 36

In article <1016@cresswell.quintus.UUCP> ok@quintus.UUCP (Richard A. O'Keefe) writes:
>In article <463@cvaxa.sussex.ac.uk>, aarons@cvaxa.sussex.ac.uk (Aaron Sloman) writes:
>> Perhaps there are memory management techniques I am unaware of that
>> make it easy for portions of a large program to be read in as required,
>> without the cost of full virtual memory.
>...
>The B6700 could and did page large data arrays, but it couldn't page
>code.  Since each procedure was a separately swappable segment, that
>didn't matter much.  No page tables, but easy partial loading.

The B6700, at least the one that I knew and loved :-), did not do
"paging" of either data or code.  Yes, procedures were in their own
segment -- that is the normal logical partitioning of code space.  And
data was also partitioned logically into segments; examples: rows of
an array, stack trunks, heaps, etc. The "swapping" of these segments
(code or data) is analogous to paging, but substitutes
hardware-supported manipulation and checking of segment descriptors
for TLB's and the like. (There's really no "free lunch" in the world
of demand-driven loading and virtual memory.)

It should also be said that demand-driven loading is as old as the
hills, and pre-dates virtual memory. It used to be called "overlays".
To do overlays one instructed the linker to link certain procedure
calls to special procedures which loaded the appropriate code segment.
The special procedure could be set-up to first check and see if the
segment was already loaded, and just jump to it in that case.  The
cost here was a more expensive calling protocol, but not for every
call -- only the ones which were requested to be overlays.  The word
overlay refers, of course, to the "virtual memory" aspect of this
method: newly loaded procedures could replace ones already loaded.
Overlays could be a really effective memory management tool in some
cases, and I think they may still available as an option in some pc
linkers.

Bill O'Farrell, Northeast Parallel Architectures Center at Syracuse University
(billo@cmx.npac.syr.edu)