Microsoft run-time library routines are designed to maintain maximum compatibility between DOS, Windows, and UNIX or XENIX systems. The run-time library offers a number of operating-system interface routines that allow you to take advantage of specific DOS and Windows features. Functions that are DOS and Windows compatible are marked, respectively, as DOS and WIN in the compatibility section. Note that for Windows the compatibility section also contains information on dynamic-link library (DLL) compatibility.
Many run-time library functions are designed to work with the Microsoft DOS Extender. The DOS Extender is a shell between a program and DOS that allows the program to run in the 32-bit flat memory model. Currently, the Microsoft C and C++ compilers are hosted under the DOS Extender; when Microsoft C/C++ provides 32-bit targeting, you can use the functions listed as DOS32X compatible to develop and run 32-bit flat model programs under DOS.