Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!pilchuck!dataio!bright
From: bright@Data-IO.COM (Walter Bright)
Newsgroups: comp.lang.c++
Subject: Re: Zortech C++ Version 2.1
Message-ID: <2558@dataio.Data-IO.COM>
Date: 29 Jun 90 17:50:48 GMT
References: <25252@unix.cis.pitt.edu>
Reply-To: bright@Data-IO.COM (Walter Bright)
Organization: Data I/O Corporation; Redmond, WA
Lines: 32

In article <25252@unix.cis.pitt.edu> rozin@unix.cis.pitt.edu (R Rozin) writes:
<I just received an upgrade order form from Zortech, for version 2.1. 
<Two of the advertised new features which seem unclear to me are:
<    1. Virtual code manager, which is supposed to let you write programs
<       up to 4 megabytes in size in real mode.
<    2. Rational systems Dos extender, which is supposed to let you write
<       proprams which access memory beyond 640k.
<If you have 1., why would you need 2.?  Does anyone know more about these
<features?  Would they let you write programs whose data is about 2-3 mbytes,
<without worrying about expanded memory, and without using the __handle?
<How does it compare to the facility of Turbo C++ which provides virtual
<memory (in a way)?

VCM is really a 'software virtual memory' system. Code segments are swapped
in and out of memory as needed. VCM programs run in real mode (and will work
on 8088 machines). VCM does not virtualize data, data is still restricted
to 640k. For a complete technical description of how VCM works, I wrote an
article about it for the July C Programmers Journal.

DOS extenders work in protected mode, and can directly access up to 16 mb of
code and data. A DOS extender app will run much faster than a VCM app, but
requires a 286/386. Building a DOS extender app also requires that you
obtain the DOS extender itself from Rational Systems.

VCM is similar to Borland's VROOM. One difference is that in VROOM, you must
set aside a 'swap space' for the code segments. With VCM, this is not
necessary, as VCM actually calls malloc to get memory for a code segment.
When malloc runs out of memory, it calls the VCM manager to discard code
segments.

I have not seen a technical description of how VROOM works, nor has Borland
shipped me TC++ yet, so I am not positive about how it works.