Brief History
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. The memory and system resources used by the program are marked as unused, effectively making it impossible to summon parts of it again without reloading it from scratch. However, if a program ends with the system call INT 27H or INT 21H/31H, the operating system does not reuse a certain specified part of the program's memory.
Read more about this topic: Terminate And Stay Resident
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