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From: 6600m00n@ucsbuxa.ucsb.edu (Jihad 'R US)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Virtual Memory for the Heap
Message-ID: <6017@hub.ucsb.edu>
Date: 27 Jul 90 10:39:20 GMT
References: <2628@mindlink.UUCP>
Sender: news@hub.ucsb.edu
Reply-To: 6600m00n@ucsbuxa.ucsb.edu
Organization: UCSB, Cell 105a
Lines: 43
In-reply-to: a35@mindlink.UUCP's message of 25 Jul 90 16:34:43 GMT

In article <2628@mindlink.UUCP> a35@mindlink.UUCP (Bruce Atherton) writes:


   I'm looking for the best method of implementing virtual memory on 80x86
   machines under MSDOS.  Specifically, it has to allow swapping of the
   heap, rather than code.  The method that is the simplest to implement
   and least buggy would be preferable.

   I just received Turbo C++ and was thinking of using VROOMM, but it
   appears to only swap overlays of code.  Is there any way to allow it to
   swap from the heap?  Do I have to overload the -> operator so that it
   works with pages, or can VROOMM take care of that for me (and if so,
   how?).

VROOM will not swap data.  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. ( setjmp,
longjmp could cause problems . . .)   One way of dealing with this is to
only use access to memory through 'handles' ( picked the idea up from
_mac_ programing.) and have a byte that says if the info is in main
memory ( in which case you use the pointer in the handle structure) or
not, in which case you use the (ems|xms|ext|disk) handle you have setup
to load the handle into main memory( and load out any needed to make
space). You could even put a byte in for 'locking' the pointer so it
does not get swapped out. 

   What about using _handle in Zortech C++?  It sounds like it would be
   easy for bugs to creep in, from the description I read.  I would also
   like to hear about any heap manager toolboxes and how they work.

   Finally, is there a speed advantage to any of these methods over using a
   disk caching system?

Yes:  disk caching doesn't do diddly for speeding up writing data.
Not so important with code( how often do you write self modifing code)
very important with data.

  I would be grateful for any information on these questions.
happy to reply.

Rob Blair 6600m00n@ucsbuxa.ucsb.edu
'Yea, brother . . .'