Which of the following system software resides in the main memory always?

Most system software is loaded into main memory only when it is needed and removed once its task is done. Text editors, assemblers, and linkers are all examples of this: an assembler is loaded to translate assembly code, then removed; a linker is loaded to combine object modules into an executable, then removed; a text editor is loaded only while a file is being edited.

The loader is different. Its job is to place a program's machine code into main memory and prepare it for execution, and part of the operating system's loading function (the resident portion) must always be present in memory so that any program, including the operating system itself, can be brought into memory and run. Because starting any process depends on this loading function being available, it always resides in main memory rather than being swapped in only on demand.

Exam takeaway: distinguish software that is loaded temporarily to perform a translation or editing task from the loader, whose core function of placing programs into memory must be permanently resident for the system to run at all.