QEP is a universal, UML-compliant event processor that enables developers to code UML state machines in highly readable ANSI-C, in which every state machine element is mapped to code precisely, unambiguously, and exactly once (traceability). QEP fully supports hierarchical state nesting, which is the fundamental mechanism for reusing behavior across many states instead of repeating the same actions and transitions over and over again.
QF is a portable, event-driven, real-time framework for execution of active objects (concurrent state machines) specifically designed for real-time embedded (RTE) systems.
QS is software tracing system that enables developers to monitor live event-driven QP applications with minimal target system resources and without stopping or significantly slowing down the code. QS is an ideal tool for testing, troubleshooting, and optimizing QP applications. QS can even be used to support acceptance testing in product manufacturing.
QV is a simple **cooperative** kernel (previously called "Vanilla" kernel). This kernel executes active objects one at a time, with priority-based scheduling performed before processing of each event. Due to naturally short duration of event processing in state machines, the simple QV kernel is often adequate for many real-time systems.
The QV scheduler is engaged after every RTC step of any @termref{active object} to choose the next active object to execute. The QV scheduler always chooses the highest-priority active object that has any events in its event queue. The QV scheduler then extracts the next event from this queue and dispatches it to the state machine associated with the active object. The state machine runs to completion, after which the QV scheduler runs and the cycle repeats.
Please note that because the state machines always return to the QV scheduler after each RTC step, a single stack can be used to process all state machines (memory-friendly architecture).
The QV scheduler can also very easily detect when all event queues are empty, at which point it can call the idle callback to let the application put the CPU and peripherals to a low-power sleep mode (power-friendly architecture).
Given the simplicity, portability, and low-resource consumption, the QV scheduler is very attractive. It allows you to partition the problem into active objects and execute these active objects orderly. The task-level response of this scheduler is the longest RTC step in the whole system, but because event-driven active objects don’t block, the RTC steps tend to be very short (typically just a few microseconds). Also, often you can break up longer RTC steps into shorter pieces, by posting an event to self and returning (“Reminder” state pattern). The self-posted event then triggers the continuation of longer processing.
QK is a tiny **preemptive**, priority-based, non-blocking kernel designed specifically for executing active objects. QK runs active objects in the same way as prioritized interrupt controller (such as NVIC in ARM Cortex-M) runs interrupts using the single stack (MSP in Cortex-M). Active objects process their events in run-to-completion fashion and remove themselves from the call stack, the same way as nested interrupts. At the same time high-priority active objects can preempt lower-priority active objects, in the same way as interrupts can preempt each other under a prioritized interrupt controller. QK meets all the requirement of the Rate Monotonic Scheduling (a.k.a. Rate Monotonic Analysis RMA) and can be used in hard real-time systems.
Sometimes it is not practical to break up long RTC steps, and consequently the task-level response of the simple @ref "QV kernel" might be too slow. In this cases you need to use a *preemptive* kernel. The big advantage of preemptive kernel is that it effectively decouples high-priority task from low-priority tasks in the time domain. The timeliness of execution of high-priority task is almost independent on the low-priority tasks. But of course there is no such thing as a free lunch. Preemptive kernels open the whole new class of problems related to race conditions. So you need to be very careful about sharing any resources.
QXK is a small, preemptive, priority-based, dual-mode **blocking** kernel that executes active objects like the @ref qk "QK kernel", but can also execute traditional __blocking__ threads (extended threads). In this respect, QXK behaves exactly as a conventional __RTOS__ (Real-Time Operating System). QXK has been designed specifically for mixing event-driven active objects with traditional blocking code, such as commercial middleware (TCP/IP stacks, UDP stacks, embedded file systems, etc.) or legacy software.
QXK distinguishes two kinds of threads: **basic**-threads (non-blocking, run-to-completion activations) and **extended**-threads (blocking, typically structrued as endless loops). The basic-threads all nest on the same stack (Main Stack Pointer in ARM Cortex-M). The extended-threads use private per-thread stacks, as in conventional RTOS kernels. Any switching from basic- to extended-thread or extended- to extended-thread requires full context switch. On the other hand, switching from basic-thread to another basic-thread requires only activation of the basic-thread, which is much simpler and faster.
<li><span class="tag">0</span> The abstract ::QActive class represents active objects in QP. This class contains the @c thread object of the underlying kernel (QXK thread-control-block in this case) as well as the event queue and the unique priority of the active object.
<li><span class="tag">1</span> The ::QXThread class represents the "naked" blocking threads of the QXK kernel. It inherits ::QActive, so that extended-threads can be treated as active objects internally in the framework. However, the extended-threads do not implement state machines. Instead, the data fields used for storing the current state in active objects are re-used to store the private stack of the extended-thread. The ::QXThread class also contains the @c timeEvt object (see ::QTimeEvt) for generating timeouts when the extended-thread is blocked.
<li><span class="tag">2</span> The ::QXMutex class represents the @ref qxk_mutex "priority-ceiling mutex" of the QXK kernel. The mutex can be used by both the extended-threads and active object threads (in case they share resources that need to be protected). However, using any blocking mechanism inside active objects is not recommended, because it delays run-to-completion event processing.
<li><span class="tag">3</span> The ::QXSemaphore class represents the @ref qxk_sema "counting semaphore" of the QXK kernel. The semaphore can be waited on only in the extended-threads and the QXK kernel would assert if an active object thread would attempt to wait on a semaphore. On the other hand, a semaphore can be signaled from anywhere in the code, including active objects and ISRs.
The main takeaway from the QXK class diagram is QXK's **optimal, tight integration** with the QP/C framework. The QXK kernel reuses all mechanisms already provided in QP, thus avoiding any code duplication, inefficient layers of indirection, and additional licensing costs, which are inevitable when using @ref ports_rtos "3rd-party RTOS kernels" to run QP/C applications.
As you can see in the list below, <span class="highlight">QXK provides most features you might expect of a traditional blocking **RTOS** kernel and is <strong>recommended</strong> as the preferred RTOS kernel for QP/C applications</span> that need to mix active objects with traditional blocking code.
<ul class="tag">
<li><span class="bullet">></span>Preemptive, priority-based scheduling of up to 64 threads. Each thread must be provided with its own private stack space and must be assigned its own unique priority (1..::QF_MAX_ACTIVE);
</li>
> NOTE: QXK always executes the highest-priority thread that is ready to run (is not blocked). The scheduling algorithm used in QXK meets all the requirement of the Rate Monotonic Scheduling (a.k.a. Rate Monotonic Analysis — RMA) and can be used in hard real-time systems.
<li><span class="bullet">></span>All threads in QXK are capable of **blocking**. However, QXK distinguishes between two types of threads:
</li>
- active object threads that can block only while waiting on events from the private event queue and should not block otherwise (e.g., inside state machines). QXK asserts when an active object thread attempts to use a timed-delay or attempts to block on a semaphore. This is because active objects have non-blocking alternatives for these operations (time-events and event posting, respectively).
- extended-threads, that can block anywhere in their thread-handler function, whereas QXK provides a typical assortments of blocking primitives for extended-threads, such as time-delay, message queue, semaphore, or a mutex;
<li><span class="bullet">></span>Deterministic fixed-size memory pools for dynamic memory management available both to extended-threads and active objects;
Files from this directory need to be added to the project, to build the QP/C framework from source code.
@attention
Not all QP/C source files should be added to every project. For example, native QP/C ports to the preemptive QK kernel should **not** contain the file <span class="img file_c">qv.c</span> and conversely, QP/C ports to the cooperative QV kernel should not contain the files <span class="img file_c">qk.c</span> and <span class="img file_c">qk_mutex.c</span>.
The QP/C <span class="img folder">source</span> directory needs to be added to the compiler's include path in the applications using QP/C, because QP/C is built from sources.
