Complex sentence: Normally in the MS-DOS operating system, only one program can be running at any given time, and when it wants to stop running, it relinquishes the control to DOS's shell program, COMMAND.COM, using the system call INT 21H/4CH.
Split into simpler sentences: Normally in the MS-DOS operating system, only one program can run at any given time. To stop running, it gives control back to the DOS shell program, COMMAND.COM, using the system call INT 21H/4CH.