Geoff Chappell - Software Analyst
This function was introduced for MS-DOS 3.00 as part of implementing new kernel functionality outside the kernel as optional extensions. The first were SHARE.EXE, which was distributed with DOS, and REDIR.EXE, which was instead left to be distributed by the makers of networking software. For file sharing, the kernel calls SHARE through subfunctions of int 21h function 10h. For network redirection, and more generally for file I/O that does not fit the built-in file system, the kernel calls out through subfunctions of int 21h function 11h. The handlers of these calls out can call back to the kernel through subfunctions of int 21h function 12h.