The Formal Design Model of a Real-Time Operating System (RTOS+): Conceptual and Architectural Frameworks

Introduction

An operating system is a set of integrated system software that organizes, manages, and controls the resources and computing power of a computer or a computer network. It also provides users a logical interface for accessing the physical machine in order to run applications. A general-purpose operating system may be perceived as an agent between the hardware and resources of a computer and the applications and users. An operating system may be divided into three subsystems known as those of the kernel or system management, the resource management, and the process management (Dijkstra, 1968; Brinch-Hansen, 1971; Liu & Layland, 1973; Peterson & Silberschantz, 1985; Anderson et al., 1989; McDermid, 1991; Deitel & Kogan, 1992; Tanenbaum, 1994; Viscarola & Mason, 2001; Silberschatz et al., 2003; Wang, 2004, 2007). The kernel of an operating system is a set of central components for computing, including CPU scheduling and process management. The resource management subsystem is a set of supporting functions for various system resources and user interfaces. The process management subsystem is a set of transition manipulation mechanisms for processes and threads interacting with the system kernel and resources.

A real-time operating system (RTOS) is an operating system that supports and guarantees timely responses to external and internal events of real-time systems (Laplante, 1977; Sha et al., 1990; ISO/IEC, 1996; Ford, 1997; Bollella et al., 2002; Kreuzinger et al., 2002; Ngolah, Wang, & Tan, 2004). An RTOS monitors, responds to, and controls an external environment, which is connected to the computer system through sensors, actuators, or other input-output (I/O) devices. In a real-time system in general and an RTOS in particular, the correctness of system behaviors depends not only on the logical results of computation but also on the time point at which the results are obtained. Real-time systems can be divided into hard and soft real-time systems. In the former, a failure to meet timing constraints will be of serious consequences, while in the latter, a timing failure may not significantly affect the functioning of the system.

A great variety of RTOS’s have been developed in the last decades (Lewis & Berg, 1998; Labrosse, 1999; Yodaiken, 1999; Rivas & Harbour, 2001; Lamie, 2008; ETTX, 2009). The existing RTOS’s are characterized as target-machine-specific, implementation-dependent, and not formally modeled. Therefore, they are usually not portable as a generic real-time operating system to be seamlessly incorporated into real-time or embedded system implementations. Problems often faced by RTOS’s are CPU and tasks scheduling, time/event management, and resource management. Efficient and precise methodologies and notations for describing solutions to these problems are critical in RTOS design and implementation. Generally, RTOS’s require multitasking, process threads, and a sufficient number of interrupt levels to deal with the random interrupt mechanisms in real-time systems. In modern RTOS’s, multitasking is a technique used for enabling multiple tasks to share a single processor. A thread is individual execution of a process in order to handle concurrent tasks. In addition, an interrupt is a request of an external device or internal process that causes the operating system to suspend the execution of a current low priority task, serve the interrupt, and hand control back to the interrupted process.