Conventional Memory - DOS Extenders

DOS Extenders

The barrier was only overcome with the arrival of DOS extenders, which allowed DOS applications to run in extended memory, but these were not very widely used outside the computer game area. As games began to use digital sound and digital image textures, they performed better if these large data components could be preloaded into megabytes of memory before playing the game rather than constantly loading the data from external storage.

The first PC operating systems to integrate such technology were Compaq DOS 3.31 (via CEMM) and Windows/386 2.1, both released in 1988. Since the 80286 version of Windows 2.0 (Windows/286), Windows applications did not suffer from the 640 KB barrier. Prior to DOS extenders, if a user installed additional memory and wished to use it under DOS, they would first have to install and configure drivers to support either expanded memory specification (EMS) or extended memory specification (XMS).

EMS was a specification available on all PCs, including the Intel 8086 and Intel 8088 which allowed add-on hardware to page small chunks of memory in and out of the "real mode" addressing space. (0x0400–0xFFFF). This required that a hole in real memory be available, typically (0xE000–0xEFFF). A program would then have to explicitly request the page to be accessed before using it. These memory locations could then be used arbitrarily until replaced by another page. This is very similar to modern virtual memory. However, in a virtual memory system, the operating system handles all paging operations: the programmer, for the most part, does not have to consider this.

XMS provided a basic protocol which allowed the client program to load a custom protected mode kernel. This was available on the Intel 80286 and newer processors. The problem with this approach is that while in 286 protected mode, direct DOS calls could not be made. The work around was to implement a callback mechanism, requiring a reset of the 286. On the 286, this was a major problem. The Intel 80386, which introduced "Virtual86 mode", allowed the guest kernel to emulate the 8086 and run the host operating system without having to actually force the processor back into "real mode".

Windows installs its own version of Himem.sys on DOS 3.3 and higher. Windows himem.sys launches 32-bit protected mode XMS (n).0 services provider for the Windows Virtual Machine Manager, which then provides XMS (n-1).0 services to DOS boxes and the 16-bit Windows machine (e.g. DOS 7 Himem.sys is XMS 3.0 but running 'Mem' command in a Windows 95 DOS Window shows XMS 2.0 information).

The latest DOS extension is DOS Protected Mode Interface (DPMI), a more advanced version of XMS which provided many of the services of a modern kernel, obviating the need to write a custom kernel. It also permitted multiple protected mode clients. This is the standard target environment for the DOS port of the GCC compilers.

There are a number of other common DOS extenders. The most notable of which is the runtime environment for the Watcom compilers, DOS/4GW, which was very common in games for DOS. Such a game would consist of either a DOS/4GW 32-bit kernel, or a stub which loaded a DOS/4GW kernel located in the path or in the same directory and a 32-bit "linear executable". Utilities are available which can strip DOS/4GW out of such a program and allow the user to experiment with any of the several, and perhaps improved, DOS/4GW clones. Another popular extender often used in DOS games was VCPI.