4. Overview of embedded OS types

For the average user, an operating system is generally perceived as an all-knowing program that performs tasks such as launching applications or managing a mouse or keyboard. In reality, this software layer is nothing more than a set of functions whose specific role is to ensure the interface between software and hardware. These functions are used to manipulate data structures that basically represent the notion of "task" as conceived by the system, and the notion of task manager, to ensure selection and launch. So it's through calls to functions that applications share out processor time, memory and peripherals. The special feature of these functions is that, in practice, they are called explicitly only very rarely. These calls are made either through library functions, or through the interrupt handler.