The MS-DOS EXEC function (Int 21H Function 4BH) allows a program (called the parent) to load any other program (called the child) from a storage device, execute it, and then regain control when the child program is finished.
A parent program can pass information to the child in a command line, in default file control blocks, and by means of a set of strings called the environment block (discussed later in this chapter). All files or devices that the parent opened using the handle file-management functions are duplicated in the newly created child task; that is, the child inherits all the active handles of the parent task. Any file operations on those handles by the child, such as seeks or file I/O, also affect the file pointers associated with the parent's handles.
MS-DOS suspends execution of the parent program until the child program terminates. When the child program finishes its work, it can pass an exit code back to the parent, indicating whether it encountered any errors. It can also, in turn, load other programs, and so on through many levels of control, until the system runs out of memory.
The MS-DOS command interpreter, COMMAND.COM, uses the EXEC function to run its external commands and other application programs. Many popular commercial programs, such as database managers and word processors, use EXEC to run other programs (spelling checkers, for example) or to load a second copy of COMMAND.COM, thereby allowing the user to list directories or copy and rename files without closing all the application files and stopping the main work in progress. EXEC can also be used to load program overlay segments, although this use is uncommon.