A part of the system software, which under all circumstances must reside in the main memory is:

The details of an interrupt cycle are shown in Figure
Given that an interrupt input arrives every $$1$$ $$msec,$$ what is the percentage of the total time that the $$CPU$$ devotes for the main program execution.

What does the following code do?

var a, b : integer;
begin
    a:=a+b;
    b:=a-b;
    a:=a-b;
end;