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RTEMS
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This group contains the Classic API groups. More...
Modules | |
| Barrier Manager | |
| The Barrier Manager provides a unique synchronization capability which can be used to have a set of tasks block and be unblocked as a set. | |
| Basic Types | |
| This group contains basic types of the Classic API. | |
| Cache Manager | |
| The Cache Manager provides functions to perform maintenance operations for data and instruction caches. | |
| Chains | |
| Chain API. | |
| Clock Manager | |
| The Clock Manager provides support for time of day and other time related capabilities. | |
| Directive Attributes | |
| This group contains the Classic API directive attributes. | |
| Directive Options | |
| This group contains the Classic API directive options. | |
| Directive Status Codes | |
| This group contains the Classic API status codes and support functions. | |
| Dual-Ported Memory Manager | |
| The Dual-Ported Memory Manager provides a mechanism for converting addresses between internal and external representations for multiple dual-ported memory areas (DPMA). | |
| Event Manager | |
| The Event Manager provides a high performance method of inter-task communication and synchronization. | |
| Fatal Error Manager | |
The Fatal Error Manager processes all fatal or irrecoverable errors and other sources of system termination (for example after exit()). Fatal errors are identified by the fatal source and error code pair. | |
| Free-Running Counter and Busy Wait Delay | |
| Free-running counter and busy wait delay functions. | |
| I/O Manager | |
| The Input/Output (I/O) Manager provides a well-defined mechanism for accessing device drivers and a structured methodology for organizing device drivers. | |
| Initialization and Shutdown | |
| This group contains directives to initialize and shutdown the RTEMS executive. | |
| Interrupt Manager | |
| Any real-time executive must provide a mechanism for quick response to externally generated interrupts to satisfy the critical time constraints of the application. The Interrupt Manager provides this mechanism for RTEMS. This manager permits quick interrupt response times by providing the critical ability to alter task execution which allows a task to be preempted upon exit from an ISR. | |
| Local Packages | |
| Local packages. | |
| Message Manager | |
| The Message Manager provides communication and synchronization capabilities using RTEMS message queues. | |
| Object Services | |
| RTEMS provides a collection of services to assist in the management and usage of the objects created and utilized via other managers. These services assist in the manipulation of RTEMS objects independent of the API used to create them. | |
| Partition Manager | |
| The Partition Manager provides facilities to dynamically allocate memory in fixed-size units. | |
| Rate-Monotonic Manager | |
| The Rate-Monotonic Manager provides facilities to implement tasks which execute in a periodic fashion. Critically, it also gathers information about the execution of those periods and can provide important statistics to the user which can be used to analyze and tune the application. | |
| Region Manager | |
| The Region Manager provides facilities to dynamically allocate memory in variable sized units. | |
| Semaphore Manager | |
| The Semaphore Manager utilizes standard Dijkstra counting semaphores to provide synchronization and mutual exclusion capabilities. | |
| Signal Manager | |
| The Signal Manager provides the capabilities required for asynchronous communication. | |
| Support Services | |
| Items of this group should move to other groups. | |
| Task Manager | |
| The Task Manager provides a comprehensive set of directives to create, delete, and administer tasks. | |
| Task Modes | |
| This group contains the Classic API task modes. | |
| Timecounter Support | |
| Timer Manager | |
| The Timer Manager provides support for timer facilities. | |
| User Extensions Manager | |
| The User Extensions Manager allows the application developer to augment the executive by allowing them to supply extension routines which are invoked at critical system events. | |
| Version | |
| The Version API provides functions to return the version or parts of the version of RTEMS you are using. | |
Files | |
| file | rtems.h |
| This header file defines the RTEMS Classic API. | |
This group contains the Classic API groups.
The facilities provided by RTEMS are built upon a foundation of very powerful concepts. These concepts must be understood before the application developer can efficiently utilize RTEMS. The purpose of this chapter is to familiarize one with these concepts.
RTEMS provides directives which can be used to dynamically create, delete, and manipulate a set of predefined object types. These types include tasks, message queues, semaphores, memory regions, memory partitions, timers, ports, and rate monotonic periods. The object-oriented nature of RTEMS encourages the creation of modular applications built upon re-usable "building block" routines.
All objects are created on the local node as required by the application and have an RTEMS assigned ID. All objects have a user-assigned name. Although a relationship exists between an object's name and its RTEMS assigned ID, the name and ID are not identical. Object names are completely arbitrary and selected by the user as a meaningful "tag" which may commonly reflect the object's use in the application. Conversely, object IDs are designed to facilitate efficient object manipulation by the executive.
An object name is an unsigned 32-bit entity associated with the object by the user. The data type rtems_name is used to store object names.
Although not required by RTEMS, object names are often composed of four ASCII characters which help identify that object. For example, a task which causes a light to blink might be called "LITE". The rtems_build_name() routine is provided to build an object name from four ASCII characters. The following example illustrates this:
However, it is not required that the application use ASCII characters to build object names. For example, if an application requires one-hundred tasks, it would be difficult to assign meaningful ASCII names to each task. A more convenient approach would be to name them the binary values one through one-hundred, respectively.
RTEMS provides a helper routine, rtems_object_get_name(), which can be used to obtain the name of any RTEMS object using just its ID. This routine attempts to convert the name into a printable string.
An object ID is a unique unsigned integer value which uniquely identifies an object instance. Object IDs are passed as arguments to many directives in RTEMS and RTEMS translates the ID to an internal object pointer. The efficient manipulation of object IDs is critical to the performance of RTEMS services. Because of this, there are two object ID formats defined. Each target architecture specifies which format it will use. There is a 32-bit format which is used for most of the supported architectures and supports multiprocessor configurations. There is also a simpler 16-bit format which is appropriate for smaller target architectures and does not support multiprocessor configurations.
The 32-bit format for an object ID is composed of four parts: API, object class, node, and index. The data type rtems_id is used to store object IDs.
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The most significant five bits are the object class. The next three bits indicate the API to which the object class belongs. The next eight bits (16 .. 23) are the number of the node on which this object was created. The node number is always one (1) in a single processor system. The least significant 16-bits form an identifier within a particular object type. This identifier, called the object index, ranges in value from one to the maximum number of objects configured for this object type.
The 16-bit format for an object ID is composed of three parts: API, object class, and index. The data type rtems_id is used to store object IDs.
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The 16-bit format is designed to be as similar as possible to the 32-bit format. The differences are limited to the elimination of the node field and reduction of the index field from 16-bits to 8-bits. Thus the 16-bit format only supports up to 255 object instances per API/Class combination and single processor systems. As this format is typically utilized by 16-bit processors with limited address space, this is more than enough object instances.
The components of an object ID make it possible to quickly locate any object in even the most complicated multiprocessor system. Object ID's are associated with an object by RTEMS when the object is created and the corresponding ID is returned by the appropriate object create directive. The object ID is required as input to all directives involving objects, except those which create an object or obtain the ID of an object.
The object identification directives can be used to dynamically obtain a particular object's ID given its name. This mapping is accomplished by searching the name table associated with this object type. If the name is non-unique, then the ID associated with the first occurrence of the name will be returned to the application. Since object IDs are returned when the object is created, the object identification directives are not necessary in a properly designed single processor application.
In addition, services are provided to portably examine the subcomponents of an RTEMS ID. These services are described in detail later in this manual but are prototyped as follows:
An object control block is a data structure defined by RTEMS which contains the information necessary to manage a particular object type. For efficiency reasons, the format of each object type's control block is different. However, many of the fields are similar in function. The number of each type of control block is application dependent and determined by the values specified in the user's Configuration Table. An object control block is allocated at object create time and freed when the object is deleted. With the exception of user extension routines, object control blocks are not directly manipulated by user applications.
In real-time multitasking applications, the ability for cooperating execution threads to communicate and synchronize with each other is imperative. A real-time executive should provide an application with the following capabilities
Most RTEMS managers can be used to provide some form of communication and/or synchronization. However, managers dedicated specifically to communication and synchronization provide well established mechanisms which directly map to the application's varying needs. This level of flexibility allows the application designer to match the features of a particular manager with the complexity of communication and synchronization required. The following managers were specifically designed for communication and synchronization:
The semaphore manager supports mutual exclusion involving the synchronization of access to one or more shared user resources. Binary semaphores may utilize the optional priority inheritance algorithm to avoid the problem of priority inversion. The message manager supports both communication and synchronization, while the event manager primarily provides a high performance synchronization mechanism. The signal manager supports only asynchronous communication and is typically used for exception handling.
The development of responsive real-time applications requires an understanding of how RTEMS maintains and supports time-related operations. The basic unit of time in RTEMS is known as a tick. The frequency of clock ticks is completely application dependent and determines the granularity and accuracy of all interval and calendar time operations.
By tracking time in units of ticks, RTEMS is capable of supporting interval timing functions such as task delays, timeouts, timeslicing, the delayed execution of timer service routines, and the rate monotonic scheduling of tasks. An interval is defined as a number of ticks relative to the current time. For example, when a task delays for an interval of ten ticks, it is implied that the task will not execute until ten clock ticks have occurred. All intervals are specified using data type rtems_interval.
A characteristic of interval timing is that the actual interval period may be a fraction of a tick less than the interval requested. This occurs because the time at which the delay timer is set up occurs at some time between two clock ticks. Therefore, the first countdown tick occurs in less than the complete time interval for a tick. This can be a problem if the clock granularity is large.
The rate monotonic scheduling algorithm is a hard real-time scheduling methodology. This methodology provides rules which allows one to guarantee that a set of independent periodic tasks will always meet their deadlines – even under transient overload conditions. The rate monotonic manager provides directives built upon the Clock Manager's interval timer support routines.
Interval timing is not sufficient for the many applications which require that time be kept in wall time or true calendar form. Consequently, RTEMS maintains the current date and time. This allows selected time operations to be scheduled at an actual calendar date and time. For example, a task could request to delay until midnight on New Year's Eve before lowering the ball at Times Square. The data type rtems_time_of_day is used to specify calendar time in RTEMS services. See Clock Manager Time and Date Data Structures.
Obviously, the directives which use intervals or wall time cannot operate without some external mechanism which provides a periodic clock tick. This clock tick is typically provided by a real time clock or counter/timer device.
RTEMS memory management facilities can be grouped into two classes: dynamic memory allocation and address translation. Dynamic memory allocation is required by applications whose memory requirements vary through the application's course of execution. Address translation is needed by applications which share memory with another CPU or an intelligent Input/Output processor. The following RTEMS managers provide facilities to manage memory:
RTEMS memory management features allow an application to create simple memory pools of fixed size buffers and/or more complex memory pools of variable size segments. The partition manager provides directives to manage and maintain pools of fixed size entities such as resource control blocks. Alternatively, the region manager provides a more general purpose memory allocation scheme that supports variable size blocks of memory which are dynamically obtained and freed by the application. The dual-ported memory manager provides executive support for address translation between internal and external dual-ported RAM address space.
1.8.14