What is a real-time executive?

The core of all operating systems is called a REAL TIME EXECUTIVE (also known as the kernel). Some of the functions that it performs are

• switching between programs
  
• hardware device control and programming
  
• memory management
  
• process management
  
• scheduling (deciding what programs to run)
  
• inter-process communication
  
• processing of exceptions and interrupts
  

Our simple security monitoring system would not have all of the above, as it would probably be a single task system, running only one program. As such, it would not need to perform scheduling of more than one program or allow communication to take place between programs (called inter-process communication). Memory management would be unnecessary, as the program would easily fit into the available memory of the computer. An operating system designed to handle a large number of people would need a real-time executive that performs all of the above. User programs are generally stored on disk, thus need to be loaded into memory before being executed. This presents the need for memory management, as the memory of the computer would need to be searched for a free area in which to load a person's program into. When the user was finished running the program, the memory consumed by it would need to be freed up and made available for another user when required.

Process scheduling and management is also necessary, so that all programs would be executed and run fairly. There is no point if a program by a specific user runs to such an extent that it denies the running of any other programs, making every other user wait. In addition, some programs might need to be executed more frequently than others, for example, checking network communications or printing. Some programs may need to be temporarily halted, and then restarted again later, so this introduces the need for inter-program communication.