DOS Extenders.

   Sad history of the subject.
   When first 8086/8088 processors appeared they can address only 1
megabyte (1024K) of memory. First PCs that were created by IBM reserved 384K
of upper memory for BIOS, videomemory, etc, so a program can address only
640K.
   80186 processor didn't add much to this.

   80286 did add, though. Intel invented special "protected" mode. In this
mode a program can access a dozen of megabytes of memory. The problem was
compatibility. MsDOS/PcDOS didn't support the new mode. So some firms
that were producing compilers, libraries and other development tools started to
make special "wrappers" for programs to help developers to write in
protected mode.
   The problem was that after protected program finishes the processor must
be returned to "normal", non-protected mode - for MsDOS to operate
properly. It was near to be impossible, as 80286 processor had no such
provisions. The processor has only "reset" signal, after which entire
computer must be rebooted.
   Well, BIOS manufacturers invented a trick (probably, with help from
Intel). The program that wants to go from protected mode to non-protected
put some information into reserved area of lower memory. And reset the
processor. The computer starts to boot and passes control to the boot
program in BIOS. BIOS looks into that reserved area and instead of booting
returns control to MsDOS (that was already in memory)!

   Then came 80386! It was really huge leap forward. It can address much
more memory, and what is more important, it can address it in "flat mode" -
no need to address memory by 64K chunks - you can address whatever you
want. And even more important - the processor had soft switch between
protected and non-protected modes.
   MsDOS, though, still didn't support protected mode, so developers
continued to use wrappers and special libraries. Of course, such wrappers
require help from compilers, so the firms that were producing compilers started
to bundle these wrappers and protected mode libraries with their compilers.
Soon these wrappers and libraries become like mini-operating systems.
Consortiums of developers invented few standard sets of API, most notable of which
was DPMI (DOS Protected Mode Interface ) version 0.9. Few very mature DPMI
implementations appeared, and soon they got the name (let us finally return
to our muttons) DOS Extenders!

   The best known DOS Extender was PharLap, but it had the problem that it
didn't support DPMI. Without support of proper standard it failed.
   The second best known was DOS/4G, created by Rational Systems. I asked
Google where it is now - and hurray! - it is now at http://www.tenberry.com/.
Its primary compiler was Watcom C: http://www.tenberry.com/dos4gw/. It was
very successful DOS Extender, in the end of DOS era almost every commercial
protected mode program used it.

   There was shareware Extender PMODE/W, that was 99% compatible with
DOS/4GW. It was free for non-commercial use, so I downloaded it. It had
tools to replace DOS/4GW in a protected .EXE. I used it to replace DOS/4GW
in perl.exe and python.exe that I used. It worked perfectly. I know there
were some free and commercial DOS/4GW replacement, ZRDX, for example. Its
pages are in Russian, so I am not providing URL here.

   Other compilers also had free and/or commercial extenders. DJGPP (DOS
port of GCC) has one, for example. I put some additional information on
using python with extenders: http://phdru.name/Software/Python/py4dos.txt.