In order for a parent program to use the EXEC function to load a child program, sufficient unallocated memory must be available in the transient program area.
When the parent itself was loaded, MS-DOS allocated it a variable amount of memory, depending upon its original file type——.COM or .EXE——and any other information that was available to the loader. (See Chapter 11 for further details.) Because the operating system has no foolproof way of predicting how much memory any given program will require, it generally allocates far more memory to a program than is really necessary.
Therefore, a prospective parent program's first action should be to use Int 21H Function 4AH (Resize Memory Block) to release any excess memory allocation of its own to MS-DOS. In this case, the program should call Int 21H Function 4AH with the ES register pointing to the program segment prefix of the program releasing memory and the BX register containing the number of paragraphs of memory to retain for that program. (See Figure 11-1 for an example.)
WARNING:
A .COM program must move its stack to a safe area if it is reducing its memory allocation to less than 64 KB.