Free DOS Extenders
Run 32 bit protected mode programs on old MSDOS
Free DOS Extenders and DPMI Hosts
DOS extenders allow you to run protected mode programs under MSDOS. Most of the free DOS extenders are 32 bit DOS extenders, allowing you to run 32 bit protected mode programs under DOS. Some extenders rely on DPMI services to be available, others can work under VCPI or require XMS and/or EMS services to be available (requiring you to load, say HIMEM.SYS and EMM386.SYS), while yet still others can work under plain MSDOS. DOS extenders were often used for programs (like games) that require more memory than was allowed under plain MSDOS, ie it allowed you to break the 640K barrier, so to speak.
Free DOS Extenders and DPMI Hosts
- WDOSX DOS Extender
This extender was originally developed for 32 bit programs generated by Borland C++ and Delphi. It has since been extended to support programs compiled and linked with Visual C++ and Watcom C/C++ as well as programs assembled using NASM. It provides most of the DPMI services, extends the most frequently used int 21h services as well as some of the int 33h (mouse) services. The extender is prepended to your executable and you need not ship any separate EXE files. A full-screen debugger is included. You can distribute applications bound with the extender royalty free even if they are commercial programs. Source code for the extender is provided.
- DOS/32 Advanced DOS Extender
DOS/32 Advanced DOS Extender is a drop-in replacement for the Tenberry DOS/4GW DOS extender that comes with the Watcom C/C++ compilers (which are now free). It is compatible with MSDOS, Windows 9x/ME/NT/2k/XP, and IBM OS/2. It comes with documentation, source code, binaries, utilities to compress the executable, a debugger, a bind utility, etc.
- Causeway DOS Extender
The famous Causeway DOS extender for Watcom C/C++ has now been released into the public domain, sources and all. This means that it is not only free, you can do whatsoever you want with it (it's now public property). If you have a copy of Watcom C/C++ (which is free, and can be obtained through our Free C/C++ Compilers page), or if you use assembly language and want to write in 32 bit protected mode, this is the MSDOS extender to get. The DOS extender has a small footprint, is very fast, and has a good selection of useful features. It comes with a linker and symbolic debugger, all of which include sources. The complete manual is also available from their web site (you have to save it page by page). This was my personal favourite among all available DOS extenders (back when I still wrote MSDOS software). Highly recommended.
- DPMIONE - DPMI 1.0 Host
DPMIONE is a standalone DPMI 1.0 host that can run under real mode or VCPI. You can load it directly from the command line or as a device driver under DOS. It supports both 16 bit and 32 bit DPMI clients, provides full DOS int 21h and mouse int 33h translation services, supports the use of a swapfile, etc. It requires you to have an XMS driver loaded (eg, HIMEM.SYS). Note that this extender is only free for non-commercial use.
- CSDPMI DPMI Host
This program is not really a DOS extender, but a DPMI host. It is useful if you need to distribute MSDOS programs that need DPMI services. It comes complete with source code. It is the DPMI server that comes with the free DJGGP C compiler (see the Free C and C++ Compilers page for more information about DJGPP). Sources for the DPMI host are also available here (csdpmi5s.zip). Note that if you get a file not found error, you may wish to search here for the file yourself. The filename at the time I wrote this was csdpmi5b.zip (binaries) and csdpmi5s.zip (sources), but the filename may change when a new version is released.
PMODE/W is a DOS extender that you can use with that Watcom and OpenWatcom C/C++ compiler. It is designed to be a drop-in replacement for DOS4GW.EXE. You can also get PMODE/W by downloading the OpenWatcom C/C++ Compiler listed on the Free C/C++ Compilers page.
- X-32 and X-32VM
[Update: this program no longer seems to be available.] These two DOS extenders are apparently based on the DOSX extender that used to be distributed with the Zortech C Compiler and the Symantec C Compiler. Besides the usual ability to run protected mode programs, it also has support for running protected mode TSRs. The X-32VM extender has a virtual memory manager with memory mapped file I/O. The extenders are compatible with XMS, VCPI and DPMI memory managers, and they will work under any MSDOS compatible system (even embedded ones). The site also claims that most X-32 and X-32VM applications will work under Windows 95, 98, ME, NT and 2000. Note: you can get a drop-in replacement for the Symantec C Compiler from the Free C/C++ Compilers page.
- 386Power 32-bit DOS Extender
[Update: this program no longer seems to be available.] If you write programs in assembly language, you may find this 32 bit DOS extender useful. It comes with source code which you can link with your assembly code to make a program that can run in 32 bit protected mode.
- HX DOS Extender
[Update: This program no longer seems to be available.] HX DOS extender is a 32-bit and 16-bit DOS extender that not only provides DPMI services for DOS programs but also provides a Win32 API layer that enables many Win32 console programs to work under DOS. It even provides limited support for DirectDraw, GDI and OpenGL graphics. It supports the Windows PE file format as well as the DOS MZ executable file format. Source code is provided. At the time this review was written, it is one of the few, if not the only, DOS extender still being maintained.
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