DOS Protected Mode Interface


In computing, the DOS Protected Mode Interface is a specification introduced in 1989 which allows a DOS program to run in protected mode, giving access to many features of the new PC processors of the time not available in real mode. It was initially developed by Microsoft for Windows 3.0, although Microsoft later turned control of the specification over to an industry committee with open membership. Almost all modern DOS extenders are based on DPMI and allow DOS programs to address all memory available in the PC and to run in protected mode.

Overview

DPMI allows a program to run in protected mode on 80286 series and later processors and make calls back to the operating system running in Real Mode. For example, a DOS program can "turn on" protected mode, but still make calls back to DOS for service. The most important reason for doing this is to allow protected mode programs to function under DOS, whose kernel used 16-bit real mode throughout its commercial life. Because MS-DOS was never updated for 32 bits, nor enabled 24- or 32-bit addressing, DPMI served as a "patch" to allow advanced protected mode programs to run on the stable MS-DOS platform. DPMI can be thought of as a "translator" that takes protected mode DOS calls and translates the registers to and from real and protected mode, including taking 16-bit segmented addresses and translating them to a suitable protected mode format.
DPMI also allows, for some time, 32-bit programs to run under Windows 3.x, since Windows itself was 16-bit. This capability remains today in a 32-bit Windows "DOS box" all the way to the Windows 10 operating system for backwards compatibility reasons.
DPMI constitutes the only officially supported method to run protected mode DOS programs in most DOS-compatible multitasking operating systems. While Windows has long had native support for 32-bit programs, the many DOS compatible systems that still exist must typically use DPMI to provide protected mode services or risk being incompatible. Systems such as FreeDOS still have a place in embedded systems programming, where their simplicity and their low implementation and resource costs, due to the abundance of API documentation and compiler tools, are important.
A DPMI service can be 16-bit, 32-bit, or "universal" and is called the DPMI kernel, DPMI host, or DPMI server. It is provided either by the host operating system or by a DOS extender. The DPMI kernel can be part of a DOS extender such as in DOS/4GW or DOS/32A, or separate, like CWSDPMI or HDPMI.

History

The first DPMI specification drafts were published in 1989 by Microsoft's Ralph Lipe. While based on a prototypical version of DPMI for Windows 3.0 in 386 enhanced mode, several features of this implementation were removed from the official specification, including a feature named MS-DOS Extensions or DOS API translation that had been proposed by Ralph Lipe in the original drafts. Most of it was implementing DOS and BIOS interfaces. DPMI version 0.9 was published in 1990 by the newly formed DPMI Committee. The version number 0.9 of the resulting specification was chosen to reflect the stripped-down nature and incomplete status of the standard the members of the DPMI Committee could agree upon. While Windows reports DPMI version 0.9 for compatibility, it actually implements the other parts as well, since they present a vital part of the system. This undocumented full nature of DPMI has become known as "true DPMI" in the industry. The DPMI standard was not the only effort to overcome the shortcomings of the VCPI specification. At the same time that Microsoft developed DPMI for Windows 3.0, another industry alliance including Intel's Software Focus Group, Lotus, Digital Research, Interactive Systems and others developed a specification named Extended VCPI to make the memory management and multitasking capabilities of the 386 available for extended DOS applications.
When it turned out that Microsoft's DPMI proposal addressed a number of similar issues and was supported by Windows, these efforts led to the creation of the DPMI Committee in February 1990 during a meeting at Intel in Santa Clara.
In 1991, the DPMI Committee revised DPMI to version 1.0 in order to incorporate a number of clarifications and extensions, but it still did not include the missing "true DPMI" bits implemented in Windows. In fact, "true DPMI" never became part of the official DPMI specification, and Windows likewise never implemented the DPMI 1.0 extensions.
While DPMI is tailored to run extended DOS application software in protected mode and extended memory, it is not particularly well suited for resident system extensions. Another specification named DPMS, developed by Digital Research / Novell around 1992, specifically addresses requirements to easily relocate modified DOS driver software into extended memory and run them in protected mode, thereby reducing their conventional memory footprint down to small stubs. This is also supported by Helix Cloaking.
The DPMI "method" is specific to DOS and the IBM PC. Other computer types were upgraded from 16-bit to 32-bit, and the advanced program support was provided by upgrading the operating system with a new 32-bit "API" and new memory management/addressing capabilities. For example, the OS/2 core system supports 32-bit programs, and can be run without the GUI. The DPMI solution appears to be mainly needed to address third party need to get DOS protected mode programs running stably on Windows 3.x before the dominant operating system vendor, Microsoft, could or would address the future of 32-bit Windows. In addition, Microsoft didn't see the answer to the 32-bit transition as a 32-bit DOS, but rather a 32-bit Windows with a completely different API.

Compatibility

While Windows 3.0 implements "true DPMI" and reports support for DPMI 0.9, DPMI version 1.0 was never implemented in Microsoft Windows, so most programs and DOS extenders were mostly only written for version 0.9. Few extenders, however, implement "true DPMI".
Beta versions of Qualitas 386MAX implemented "true DPMI" and could run Windows' KRNL386.EXE from the command line, but it was claimed that was disabled in the released product in an internal email. However, DPMIONE can do it. Currently DPMIONE and 386MAX is also the only DPMI host which supports DPMI 1.0 completely and they are the main supporter of DPMI 1.0.
The KRNL386.SYS of DR DOS "Panther" and "StarTrek", which was under development since 1991, and the EMM386.EXE memory managers of Novell DOS 7, Caldera OpenDOS and DR-DOS 7.02 and higher have built-in support for DPMI when loaded with the /DPMI option. KRNL386.SYS even had a command line option /VER=0.9|1.0 to provide support for either DPMI 1.0 or 0.9. DOS API translation was referred to as "called interrupt 21 from protected mode". Multiuser DOS, System Manager and REAL/32 support DPMI as well.
The most famous separate DPMI kernel is probably CWSDPMI; it supports DPMI 0.9, but no undocumented "DOS API translation".
Another variant called PMODE by "TRAN" aka Thomas Pytel was popular with 32-bit programmers during the demo scene of the 1990s.
Many games used DOS/4GW, which was developed by Rational Systems as a subset of DOS/4G and was distributed with the Watcom C compiler.
HDPMI provides "DOS API translation" and almost complete DPMI 1.0 implementation.

DPMI Committee

The DPMI 1.0 Committee met between 1990 through 1991 and consisted of 12 groups: