HX DOS Extender

A DOS extender is a computer software program running under DOS that enables software to run in a protected mode environment even though the host operating system is only capable of operating in real mode.

DOS extenders were initially developed in the 1980s following the introduction of the Intel 80286 processor (and later expanded upon with the Intel 80386), to cope with the memory limitations of MS-DOS and its derivatives.

A DOS extender is a program that "extends" DOS so that programs running in protected mode can transparently interface with the underlying MS-DOS API. This was necessary because many of the functions provided by DOS require 16-bit segment and offset addresses pointing to memory locations within the first 640 kilobytes of memory. Protected mode, however, uses an incompatible addressing method where the segment registers (now called selectors) are used to point to an entry in the Global Descriptor Table which describes the characteristics of the segment. The two methods of addressing are mutually exclusive, with the processor having to make costly switches to real (or V86) mode to service non-protected mode requests.

In addition to setting up the environment and loading the actual program to be executed, the DOS extender also provides (amongst other things) a translation layer that maintains buffers allocated below the 1MB real mode memory barrier. These buffers are used to transfer data between the underlying real mode operating system and the protected mode program. Since switching between real/V86 mode and protected mode is a relatively time consuming operation, the extender attempts to minimize the number of switches by duplicating the functionality of many real mode operations within its own protected mode environment. As DOS uses interrupts extensively for communication between the operating system and user level software, DOS extenders intercept many of the common hardware (e.g. the real-time clock and keyboard controller) and software (e.g. DOS itself and the mouse API) interrupts. Some extenders also handle other common interrupt functions, such as video BIOS routines.

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