OS/2 2.0 provides a redesigned DOS compatibility environment known as
Enhanced DOS Sessions. Features of Enhanced DOS Sessions include:
- The ability to run DOS applications as V86 mode tasks. This
eliminates the operating system overhead of switching between real
mode and protect mode, and provides a fully protected system
environment.
- The ability to start many concurrent DOS sessions, each operating
in its own independent 1MB linear address space.
- The ability to customize the operation of DOS sessions through DOS
settings.
- The ability to run DOS applications in windows in the PM
environment.
- Increased available base memory over previous versions of the
operating system.
- Support for EMS and XMS. This allows DOS applications to access
memory above the 1MB real mode addressing limit, to have total
code and data space larger than the available base memory, and to
have very large code or data objects loaded into memory for
enhanced execution speed or compatibility.
The Enhanced DOS Session mechanism is composed of three modules, DOS
Emulation, 8086 Emulation, DOS Session Manager, which provides a full
set of control program interfaces known as Virtual Device Helper
services. These services are invoked by VDD modules. VDD modules
provide hardware-specific support, such as hardware virtualization,
BIOS emulation, and other low-level system functions.