Stay in Your Own Memory Area

It's obvious that the currently executing program utilizes certain areas of memory. Its program code resides in its code segment(s) within memory. Some of the data that it acts upon is stored within the processor's registers and much of it in the areas of memory designated as its data segments. When the program needs to store the information from a register briefly so that it can use the register for something else, it typically stores the data in the area of memory designated as its stack segment.

The currently executing program is typically only aware of two entities—itself and the OS that created it. It is completely unaware of the existence of any other tasks that are currently suspended. The currently executing program should only access its own memory. If it were permitted to perform memory writes anywhere in memory, it is entirely probable that it will corrupt the code, stack or data areas of programs that reside in memory but are currently suspended. Consider what would happen when the OS resumes execution of a task that had been corrupted while in suspension. Its program and/or data would have been corrupted, causing it to behave unpredictably when it resumes execution.

The OS must protect suspended tasks (including itself!) from the currently executing task. If it doesn't, multitasking will not work reliably.