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DOSEMU, stylized as dosemu, is a compatibility layer software package that enables DOS operating systems (e.g., MS-DOS, DR-DOS, FreeDOS) and application software to run atop Linux on x86-based PCs (IBM PC compatible computers).
DOSBox is a free and open-source emulator which runs software for MS-DOS compatible disk operating systems—primarily video games. [5] It was first released in 2002, when DOS technology was becoming obsolete. Its adoption for running DOS games is widespread, with it being used in commercial re-releases of those games as well.
The DOS version returns 5.00 or 5.50, depending on which API function is used to determine it. Utilities from MS-DOS 5.00 run in this emulation without modification. The very early beta programs of NT show MS-DOS 30.00, but programs running in MS-DOS 30.00 would assume that OS/2 was in control.
Virtual DOS machines can operate either exclusively through typical software emulation methods (e.g. dynamic recompilation) or can rely on the virtual 8086 mode of the Intel 80386 processor, which allows real mode 8086 software to run in a controlled environment by catching all operations which involve accessing protected hardware and forwarding them to the normal operating system (as exceptions).
The hard bug exists in recent versions of HIMEM.SYS from MS-DOS and Windows 9x for handling /a20control:off option causing a hang-up. There is an unofficial patch for this. History
The environment of an EXE program run by DOS is found in its Program Segment Prefix.. EXE files normally have separate segments for the code, data, and stack. Program execution begins at address 0 of the code segment, and the stack pointer register is set to whatever value is contained in the header information (thus if the header specifies a 512 byte stack, the stack pointer is set to 200h).
This strategy worked well enough until Microsoft decided to make some changes in the way the underlying operating system worked. And as more and more PC started supporting 64-bit operating systems, the ability to keep dBASE for DOS running on maintainable hardware became difficult.
A DOS extender is a program that "extends" DOS so that programs running in protected mode can transparently interface with the underlying 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.