Most C compilers that target DOS, Windows 3.1x, Win32, OS/2, Novell NetWare or DOS extenders supply this header and the library functions in their C library.
VDMs generally also implement support for running 16- and 32-bit protected mode software (DOS extenders), which has to conform to the DOS Protected Mode Interface.
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Unlike Ultima VII which used Intel's undocumented "Big real mode" which became known as Voodoo Memory at Origin, Ultima VIII used a more conventional DOS extender.