Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!decwrl!ucbvax!van-bc!rsoft!mindlink!a35
From: a35@mindlink.UUCP (Bruce Atherton)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Virtual Memory for the Heap
Message-ID: <2698@mindlink.UUCP>
Date: 2 Aug 90 16:01:58 GMT
Organization: MIND LINK! - British Columbia, Canada
Lines: 41

6600m00n@ucsbuxa.ucsb.edu (Rob Blair) writes:
> 
> The main problem with swapping data is that
> it is accessed 'asyncronously'. i.e.  You never know when you will need
> to be in, since pointers can point anywhere.  Code is always accesed
> with calls, and can be routed through a overlay maneger.

As long as a function was called every time a pointer was dereferenced,
the time that the data was accessed wouldn't be a problem.  Of course,
that function would somehow have to be able to tell what page the
pointer belonged to, and swap it in if necessary.

You could do that three ways that I can think of off of the top of my
head.  First, you could have a special memory model that had 6 byte
pointers.

Second, you could use the Zortech method of assuming that pointers will
never want to point into the BIOS.  This strikes me as dangerous, since I can
think of reasons why I might want a pointer to point there.

The third method, and one I am sorry they didn't include as part of
VROOMM, would be to use normalized pointers and store the page
information in the unused part.

In Turbo C++, there is a Huge memory model that automatically normalizes
your pointers, so the offset is always between 0 and 15.  That means the
pointers look like this -

Bit FEDCBA9876543210:FEDCBA987654210   s = segment
    ssssssssssssssss:xxxxxxxxxxxoooo   o = offset
                                       x = unused

Why couldn't there be a variant of the Huge memory model called Virtual,
that used those unused bits to store page information for a memory
manager?  Some function must be ensuring that the pointers stay
normalized.  If that function is called whenever a pointer is
dereferenced, a virtual memory manager would be simple to implement and
almost invisible to the programmer.

I know, I know.  That ain't the way Turbo C++ works.  Is there anything
that does, other than Zortech?