The velOSity real-time OS is implemented as a library of C-callable routines. Applications invoke velOSity services by making subroutine calls from their C applications. The library implementation enables designers to maximize performance and minimize space requirements by selecting and embedding only those RTOS services that they need for their application.

The INTEGRITY real-time OS is implemented as a stand-alone kernel that exists apart from the user application. INTEGRITY leverages the memory management facilities of hardware MMUs to provide a secure, memory protected environment. In this environment, user processes request kernel services via a trap mechanism that prevents them from accessing critical data structures. Through the trap mechanism, the kernel is able to enforce process access permissions for objects such as memory, semaphores, and pipes. This prevents one process from accessing and corrupting either the kernel or other user processes.

Both velOSity and INTEGRITY share a common set of system services. They also share a common API, which makes it easy to port applications between the two platforms. The two real-time operating systems employ a pre-emptive, real-time, priority-based multitasking scheduler. Processes of equal priority can run until completion or in a round-robin fashion. Programmers can also set aside a fixed amount of run time in order to guarantee sufficient CPU access for critical processes at a given priority level.

The velOSity and INTEGRITY real-time operating systems are optimized for fast deterministic response. Both real-time OSs run with interrupts enabled all the time. Competitive RTOSs must disable interrupts for some period of time in order to update critical internal data structures, thereby compromising the system's ability to respond to critical external interrupts.

To facilitate interprocess communications and synchronization, both INTEGRITY and velOSity support semaphores and pipes. Semaphores provide an efficient means of implementing interprocess communications via shared memory. Pipes provide a simpler, more secure means of conducting interprocess communications that decouples processes from the details of the underlying IPC mechanism.

To enhance reliability and security, velOSity and INTEGRITY prevent kernel re-entrance - that is, multiple user processes may not invoke the same kernel service at the same time. "In the past two years," explains Green Hills President Dan O'Dowd, "a number of RTOS vendors have announced re-entrant kernels with the goal of making kernel services pre-emptable and improving scheduling flexibility. In doing so, however, they have opened a Pandora's Box that makes their kernels inherently unreliable and untestable, and therefore ill-suited for use in mission-critical applications."

velOSity and INTEGRITY are tightly integrated with the entire Green Hills suite of software development tools. The Green Hills MULTI® Development Environment and family of optimizing compilers support C, C++, Fortran and Ada. The window-oriented MULTI Development Environment features an RTOS-aware source-level debugger that lets programmers program at a system and task level. MULTI also features an execution profiler, source and version control software, and run-time error checking.

Green Hills Software's MULTI Development Environment supports all major advanced microprocessor families and target environments, including instruction set simulators, ROM monitors, commercial and home-grown real-time operating systems and instruction set simulators.

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