* @file rtems/rtems/mainpage.h
* This file exists to provide a top level description of RTEMS for Doxygen.
* COPYRIGHT (c) 1989-2014.
* On-Line Applications Research Corporation (OAR).
* The license and distribution terms for this file may be
* found in the file LICENSE in this distribution or at
* The RTEMS real-time operating systems is a layered system with each of the
* public APIs implemented in terms of a common foundation layer called the
* SuperCore. This is the Doxygen generated documentation for the RTEMS CPU
* Kit including the Classic API, POSIX API and SuperCore.
* @page RTEMSPreface RTEMS History and Introduction
* In recent years, the cost required to develop a software product has
* increased significantly while the target hardware costs have decreased. Now
* a larger portion of money is expended in developing, using, and maintaining
* software. The trend in computing costs is the complete dominance of software
* over hardware costs. Because of this, it is necessary that formal
* disciplines be established to increase the probability that software is
* characterized by a high degree of correctness, maintainability, and
* portability. In addition, these disciplines must promote practices that aid
* in the consistent and orderly development of a software system within
* schedule and budgetary constraints. To be effective, these disciplines must
* adopt standards which channel individual software efforts toward a common
* The push for standards in the software development field has been met with
* various degrees of success. The Microprocessor Operating Systems Interfaces
* (MOSI) effort has experienced only limited success. As popular as the UNIX
* operating system has grown, the attempt to develop a standard interface
* definition to allow portable application development has only recently begun
* to produce the results needed in this area. Unfortunately, very little
* effort has been expended to provide standards addressing the needs of the
* real-time community. Several organizations have addressed this need during
* recent years.
* The Real Time Executive Interface Definition (RTEID) was developed by
* Motorola with technical input from Software Components Group. RTEID was
* adopted by the VMEbus International Trade Association (VITA) as a baseline
* draft for their proposed standard multiprocessor, real-time executive
* interface, Open Real-Time Kernel Interface Definition (ORKID). These two
* groups are currently working together with the IEEE P1003.4 committee to
* insure that the functionality of their proposed standards is adopted as the
* real-time extensions to POSIX.
* This emerging standard defines an interface for the development of real-time
* software to ease the writing of real-time application programs that are
* directly portable across multiple real-time executive implementations. This
* interface includes both the source code interfaces and run-time behavior as
* seen by a real-time application. It does not include the details of how a
* kernel implements these functions. The standard's goal is to serve as a
* complete definition of external interfaces so that application code that
* conforms to these interfaces will execute properly in all real-time
* executive environments. With the use of a standards compliant executive,
* routines that acquire memory blocks, create and manage message queues,
* establish and use semaphores, and send and receive signals need not be
* redeveloped for a different real-time environment as long as the new
* environment is compliant with the standard. Software developers need only
* concentrate on the hardware dependencies of the real-time system.
* Furthermore, most hardware dependencies for real-time applications can be
* localized to the device drivers.
* A compliant executive provides simple and flexible real-time
* multiprocessing. It easily lends itself to both tightly-coupled and
* loosely-coupled configurations (depending on the system hardware
* configuration). Objects such as tasks, queues, events, signals, semaphores,
* and memory blocks can be designated as global objects and accessed by any
* task regardless of which processor the object and the accessing task reside.
* The acceptance of a standard for real-time executives will produce the same
* advantages enjoyed from the push for UNIX standardization by AT&T's System V
* Interface Definition and IEEE's POSIX efforts. A compliant multiprocessing
* executive will allow close coupling between UNIX systems and real-time
* executives to provide the many benefits of the UNIX development environment
* to be applied to real-time software development. Together they provide the
* necessary laboratory environment to implement real-time, distributed,
* embedded systems using a wide variety of computer architectures.
* A study was completed in 1988, within the Research, Development, and
* Engineering Center, U.S. Army Missile Command, which compared the various
* aspects of the Ada programming language as they related to the application
* of Ada code in distributed and/or multiple processing systems. Several
* critical conclusions were derived from the study. These conclusions have a
* major impact on the way the Army develops application software for embedded
* applications. These impacts apply to both in-house software development and
* contractor developed software.
* A conclusion of the analysis, which has been previously recognized by other
* agencies attempting to utilize Ada in a distributed or multiprocessing
* environment, is that the Ada programming language does not adequately
* support multiprocessing. Ada does provide a mechanism for multi-tasking,
* however, this capability exists only for a single processor system. The
* language also does not have inherent capabilities to access global named
* variables, flags or program code. These critical features are essential in
* order for data to be shared between processors. However, these drawbacks do
* have workarounds which are sometimes awkward and defeat the intent of
* software maintainability and portability goals.
* Another conclusion drawn from the analysis, was that the run time executives
* being delivered with the Ada compilers were too slow and inefficient to be
* used in modern missile systems. A run time executive is the core part of the
* run time system code, or operating system code, that controls task
* scheduling, input/output management and memory management. Traditionally,
* whenever efficient executive (also known as kernel) code was required by the
* application, the user developed in-house software. This software was usually
* written in assembly language for optimization.
* Because of this shortcoming in the Ada programming language, software
* developers in research and development and contractors for project managed
* systems, are mandated by technology to purchase and utilize off-the-shelf
* third party kernel code. The contractor, and eventually the Government, must
* pay a licensing fee for every copy of the kernel code used in an embedded
* The main drawback to this development environment is that the Government
* does not own, nor has the right to modify code contained within the kernel.
* V&V techniques in this situation are more difficult than if the complete
* source code were available. Responsibility for system failures due to faulty
* software is yet another area to be resolved under this environment.
* The Guidance and Control Directorate began a software development effort to
* address these problems. A project to develop an experimental run time kernel
* was begun that will eliminate the major drawbacks of the Ada programming
* language mentioned above. The Real Time Executive for Multiprocessor Systems
* (RTEMS) provides full capabilities for management of tasks, interrupts,
* time, and multiple processors in addition to those features typical of
* generic operating systems. The code is Government owned, so no licensing
* fees are necessary. RTEMS has been implemented in both the Ada and C
* programming languages. It has been ported to the following processor
* - Altera NIOS II
* - Analog Devices Blackfin
* - ARM
* - Freescale (formerly Motorola) MC68xxx
* - Freescale (formerly Motorola) MC683xx
* - Freescale (formerly Motorola) ColdFire
* - Intel i386 and above
* - Lattice Semiconductor LM32
* - MIPS
* - PowerPC
* - Renesas (formerly Hitachi) SuperH
* - Renesas (formerly Hitachi) H8/300
* - SPARC
* - Texas Instruments C3x/C4x
* - UNIX
* Support for other processor families, including RISC, CISC, and DSP, is
* planned. Since almost all of RTEMS is written in a high level language,
* ports to additional processor families require minimal effort.
* RTEMS multiprocessor support is capable of handling either homogeneous or
* heterogeneous systems. The kernel automatically compensates for
* architectural differences (byte swapping, etc.) between processors. This
* allows a much easier transition from one processor family to another without
* a major system redesign.
* Since the proposed standards are still in draft form, RTEMS cannot and does
* not claim compliance. However, the status of the standard is being carefully
* monitored to guarantee that RTEMS provides the functionality specified in
* the standard. Once approved, RTEMS will be made compliant.
* @page RTEMSOverview RTEMS Overview
* @section RTEMSOverviewSecIntroduction Introduction
* RTEMS, Real-Time Executive for Multiprocessor Systems, is a real-time
* executive (kernel) which provides a high performance environment for
* embedded military applications including the following features:
* - multitasking capabilities
* - homogeneous and heterogeneous multiprocessor systems
* - event-driven, priority-based, preemptive scheduling
* - optional rate monotonic scheduling
* - intertask communication and synchronization
* - priority inheritance
* - responsive interrupt management
* - dynamic memory allocation
* - high level of user configurability
* This manual describes the usage of RTEMS for applications written in the C
* programming language. Those implementation details that are processor
* dependent are provided in the Applications Supplement documents. A
* supplement document which addresses specific architectural issues that
* affect RTEMS is provided for each processor type that is supported.
* @section RTEMSOverviewSecRealtimeApplicationSystems Real-time Application Systems
* Real-time application systems are a special class of computer applications.
* They have a complex set of characteristics that distinguish them from other
* software problems. Generally, they must adhere to more rigorous
* requirements. The correctness of the system depends not only on the results
* of computations, but also on the time at which the results are produced. The
* most important and complex characteristic of real-time application systems
* is that they must receive and respond to a set of external stimuli within
* rigid and critical time constraints referred to as deadlines. Systems can be
* buried by an avalanche of interdependent, asynchronous or cyclical event
* Deadlines can be further characterized as either hard or soft based upon the
* value of the results when produced after the deadline has passed. A deadline
* is hard if the results have no value or if their use will result in a
* catastrophic event. In contrast, results which are produced after a soft
* deadline may have some value.
* Another distinguishing requirement of real-time application systems is the
* ability to coordinate or manage a large number of concurrent activities.
* Since software is a synchronous entity, this presents special problems. One
* instruction follows another in a repeating synchronous cycle. Even though
* mechanisms have been developed to allow for the processing of external
* asynchronous events, the software design efforts required to process and
* manage these events and tasks are growing more complicated.
* The design process is complicated further by spreading this activity over a
* set of processors instead of a single processor. The challenges associated
* with designing and building real-time application systems become very
* complex when multiple processors are involved. New requirements such as
* interprocessor communication channels and global resources that must be
* shared between competing processors are introduced. The ramifications of
* multiple processors complicate each and every characteristic of a real-time
* @section RTEMSOverviewSecRealtimeExecutive Real-time Executive
* Fortunately, real-time operating systems or real-time executives serve as a
* cornerstone on which to build the application system. A real-time
* multitasking executive allows an application to be cast into a set of
* logical, autonomous processes or tasks which become quite manageable. Each
* task is internally synchronous, but different tasks execute independently,
* resulting in an asynchronous processing stream. Tasks can be dynamically
* paused for many reasons resulting in a different task being allowed to
* execute for a period of time. The executive also provides an interface to
* other system components such as interrupt handlers and device drivers.
* System components may request the executive to allocate and coordinate
* resources, and to wait for and trigger synchronizing conditions. The
* executive system calls effectively extend the CPU instruction set to support
* efficient multitasking. By causing tasks to travel through well-defined
* state transitions, system calls permit an application to demand-switch
* between tasks in response to real-time events.
* By proper grouping of responses to stimuli into separate tasks, a system can
* now asynchronously switch between independent streams of execution, directly
* responding to external stimuli as they occur. This allows the system design
* to meet critical performance specifications which are typically measured by
* guaranteed response time and transaction throughput. The multiprocessor
* extensions of RTEMS provide the features necessary to manage the extra
* requirements introduced by a system distributed across several processors.
* It removes the physical barriers of processor boundaries from the world of
* the system designer, enabling more critical aspects of the system to receive
* the required attention. Such a system, based on an efficient real-time,
* multiprocessor executive, is a more realistic model of the outside world or
* environment for which it is designed. As a result, the system will always be
* more logical, efficient, and reliable.
* By using the directives provided by RTEMS, the real-time applications
* developer is freed from the problem of controlling and synchronizing
* multiple tasks and processors. In addition, one need not develop, test,
* debug, and document routines to manage memory, pass messages, or provide
* mutual exclusion. The developer is then able to concentrate solely on the
* application. By using standard software components, the time and cost
* required to develop sophisticated real-time applications is significantly
* @section RTEMSOverviewSecApplicationArchitecture RTEMS Application Architecture
* One important design goal of RTEMS was to provide a bridge between two
* critical layers of typical real-time systems. As shown in the following
* figure, RTEMS serves as a buffer between the project dependent application
* code and the target hardware. Most hardware dependencies for real-time
* applications can be localized to the low level device drivers.
* @todo Image RTEMS Application Architecture
* The RTEMS I/O interface manager provides an efficient tool for incorporating
* these hardware dependencies into the system while simultaneously providing a
* general mechanism to the application code that accesses them. A well
* designed real-time system can benefit from this architecture by building a
* rich library of standard application components which can be used repeatedly
* in other real-time projects.
* @section RTEMSOverviewSecInternalArchitecture RTEMS Internal Architecture
* RTEMS can be viewed as a set of layered components that work in harmony to
* provide a set of services to a real-time application system. The executive
* interface presented to the application is formed by grouping directives into
* logical sets called resource managers. Functions utilized by multiple
* managers such as scheduling, dispatching, and object management are provided
* in the executive core. The executive core depends on a small set of CPU
* dependent routines. Together these components provide a powerful run time
* environment that promotes the development of efficient real-time application
* systems. The following figure illustrates this organization:
* @todo Image RTEMS Architecture
* Subsequent chapters present a detailed description of the capabilities
* provided by each of the following RTEMS managers:
* - initialization
* - task
* - interrupt
* - clock
* - timer
* - semaphore
* - message
* - event
* - signal
* - partition
* - region
* - dual ported memory
* - I/O
* - fatal error
* - rate monotonic
* - user extensions
* - multiprocessing
* @section RTEMSOverviewSecUserCustomization User Customization and Extensibility
* As 32-bit microprocessors have decreased in cost, they have become
* increasingly common in a variety of embedded systems. A wide range of custom
* and general-purpose processor boards are based on various 32-bit
* processors. RTEMS was designed to make no assumptions concerning the
* characteristics of individual microprocessor families or of specific support
* hardware. In addition, RTEMS allows the system developer a high degree of
* freedom in customizing and extending its features.
* RTEMS assumes the existence of a supported microprocessor and sufficient
* memory for both RTEMS and the real-time application. Board dependent
* components such as clocks, interrupt controllers, or I/O devices can be
* easily integrated with RTEMS. The customization and extensibility features
* allow RTEMS to efficiently support as many environments as possible.
* @section RTEMSOverviewSecPortability Portability
* The issue of portability was the major factor in the creation of RTEMS.
* Since RTEMS is designed to isolate the hardware dependencies in the specific
* board support packages, the real-time application should be easily ported to
* any other processor. The use of RTEMS allows the development of real-time
* applications which can be completely independent of a particular
* microprocessor architecture.
* @section RTEMSOverviewSecMemoryRequirements Memory Requirements
* Since memory is a critical resource in many real-time embedded systems,
* RTEMS was specifically designed to automatically leave out all services that
* are not required from the run-time environment. Features such as networking,
* various filesystems, and many other features are completely optional. This
* allows the application designer the flexibility to tailor RTEMS to most
* efficiently meet system requirements while still satisfying even the most
* stringent memory constraints. As a result, the size of the RTEMS executive
* is application dependent.
* RTEMS requires RAM to manage each instance of an RTEMS object that is
* created. Thus the more RTEMS objects an application needs, the more memory
* that must be reserved. See Configuring a System Determining Memory
* Requirements for more details.
* @todo Link to Configuring a SystemDetermining Memory Requirements
* RTEMS utilizes memory for both code and data space. Although RTEMS' data
* space must be in RAM, its code space can be located in either ROM or RAM.
* @section RTEMSOverviewSecAudience Audience
* This manual was written for experienced real-time software developers.
* Although some background is provided, it is assumed that the reader is
* familiar with the concepts of task management as well as intertask
* communication and synchronization. Since directives, user related data
* structures, and examples are presented in C, a basic understanding of the C
* programming language is required to fully understand the material presented.
* However, because of the similarity of the Ada and C RTEMS implementations,
* users will find that the use and behavior of the two implementations is very
* similar. A working knowledge of the target processor is helpful in
* understanding some of RTEMS' features. A thorough understanding of the
* executive cannot be obtained without studying the entire manual because many
* of RTEMS' concepts and features are interrelated. Experienced RTEMS users
* will find that the manual organization facilitates its use as a reference
* @addtogroup ClassicAPI
* 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.
* @section ClassicRTEMSSecObjects Objects
* 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.
* @subsection ClassicRTEMSSubSecObjectNames Object Names
* An object name is an unsigned 32-bit entity associated with the
* object by the user. The data type @ref 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:
* rtems_name my_name = rtems_build_name('L', 'I', 'T', 'E');
* 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.
* @subsection ClassicRTEMSSubSecObjectIdentifiers Object Identifiers
* 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.
* @subsubsection ClassicRTEMSSubSec32BitObjectIdentifierFormat 32-Bit Object Identifier Format
* The 32-bit format for an object ID is composed of four parts: API,
* object class, node, and index. The data type @ref rtems_id is used to store
* object IDs.
* <td colspan=5>Class</td><td colspan=3>API</td><td colspan=8>Node</td><td colspan=16>Object Index</td>
* 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.
* @subsubsection ClassicRTEMSSubSec16BitObjectIdentifierFormat 16-Bit Object Identifier Format
* The 16-bit format for an object ID is composed of three parts: API, object
* class, and index. The data type @ref rtems_id is used to store object IDs.
* <td colspan=5>Class</td><td colspan=3>API</td><td colspan=8>Object Index</td>
* 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
* @subsection ClassicRTEMSSubSecObjectIdentiferDescription Object Identifer Description
* 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:
* - rtems_object_id_get_api()
* - rtems_object_id_get_class()
* - rtems_object_id_get_node()
* - rtems_object_id_get_index()
* 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.
* @section ClassicRTEMSSecComSync Communication and Synchronization
* 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
* - data transfer between cooperating tasks,
* - data transfer between tasks and ISRs,
* - synchronization of cooperating tasks, and
* - synchronization of tasks and ISRs.
* 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:
* - @ref ClassicSem
* - @ref ClassicMessageQueue
* - @ref ClassicEvent
* - @ref ClassicSignal
* 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
* @section ClassicRTEMSSecTime Time
* 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 @ref 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
* 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 @ref rtems_time_of_day is used to specify
* calendar time in RTEMS services. See Clock Manager Time and Date Data
* @todo Link to 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
* @section ClassicRTEMSSecMemoryManagement Memory Management
* 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:
* - @ref ClassicRegion
* - @ref ClassicPart
* - @ref ClassicDPMEM
* 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.
* @addtogroup ClassicTasks
* @section ClassicTasksSecTaskDefinition Task Definition
* Many definitions of a task have been proposed in computer literature.
* Unfortunately, none of these definitions encompasses all facets of the
* concept in a manner which is operating system independent. Several of the
* more common definitions are provided to enable each user to select a
* definition which best matches their own experience and understanding of the
* task concept:
* - a "dispatchable" unit.
* - an entity to which the processor is allocated.
* - an atomic unit of a real-time, multiprocessor system.
* - single threads of execution which concurrently compete for resources.
* - a sequence of closely related computations which can execute concurrently
* with other computational sequences.
* From RTEMS' perspective, a task is the smallest thread of execution which
* can compete on its own for system resources. A task is manifested by the
* existence of a task control block (TCB).
* @section ClassicTasksSecTaskControlBlock Task Control Block
* The Task Control Block (TCB) is an RTEMS defined data structure which
* contains all the information that is pertinent to the execution of a task.
* During system initialization, RTEMS reserves a TCB for each task configured.
* A TCB is allocated upon creation of the task and is returned to the TCB free
* list upon deletion of the task.
* The TCB's elements are modified as a result of system calls made by the
* application in response to external and internal stimuli. TCBs are the only
* RTEMS internal data structure that can be accessed by an application via
* user extension routines. The TCB contains a task's name, ID, current
* priority, current and starting states, execution mode, TCB user extension
* pointer, scheduling control structures, as well as data required by a
* blocked task.
* A task's context is stored in the TCB when a task switch occurs. When the
* task regains control of the processor, its context is restored from the TCB.
* When a task is restarted, the initial state of the task is restored from the
* starting context area in the task's TCB.
* @section ClassicTasksSecTaskStates Task States
* A task may exist in one of the following five states:
* - executing - Currently scheduled to the CPU
* - ready - May be scheduled to the CPU
* - blocked - Unable to be scheduled to the CPU
* - dormant - Created task that is not started
* - non-existent - Uncreated or deleted task
* An active task may occupy the executing, ready, blocked or dormant state,
* otherwise the task is considered non-existent. One or more tasks may be
* active in the system simultaneously. Multiple tasks communicate,
* synchronize, and compete for system resources with each other via system
* calls. The multiple tasks appear to execute in parallel, but actually each
* is dispatched to the CPU for periods of time determined by the RTEMS
* scheduling algorithm. The scheduling of a task is based on its current state
* and priority.
* @section ClassicTasksSecTaskPriority Task Priority
* A task's priority determines its importance in relation to the other tasks
* executing on the same processor. RTEMS supports 255 levels of priority
* ranging from 1 to 255. The data type rtems_task_priority() is used to store
* task priorities.
* Tasks of numerically smaller priority values are more important tasks than
* tasks of numerically larger priority values. For example, a task at priority
* level 5 is of higher privilege than a task at priority level 10. There is no
* limit to the number of tasks assigned to the same priority.
* Each task has a priority associated with it at all times. The initial value
* of this priority is assigned at task creation time. The priority of a task
* may be changed at any subsequent time.
* Priorities are used by the scheduler to determine which ready task will be
* allowed to execute. In general, the higher the logical priority of a task,
* the more likely it is to receive processor execution time.
* @section ClassicTasksSecTaskMode Task Mode
* A task's execution mode is a combination of the following four components:
* - preemption
* - ASR processing
* - timeslicing
* - interrupt level
* It is used to modify RTEMS' scheduling process and to alter the execution
* environment of the task. The data type rtems_task_mode() is used to manage
* the task execution mode.
* The preemption component allows a task to determine when control of the
* processor is relinquished. If preemption is disabled (@c
* RTEMS_NO_PREEMPT), the task will retain control of the
* processor as long as it is in the executing state -- even if a higher
* priority task is made ready. If preemption is enabled (@c RTEMS_PREEMPT)
* and a higher priority task is made ready, then the processor will be
* taken away from the current task immediately and given to the higher
* priority task.
* The timeslicing component is used by the RTEMS scheduler to determine how
* the processor is allocated to tasks of equal priority. If timeslicing is
* enabled (@c RTEMS_TIMESLICE), then RTEMS will limit the amount of time the
* task can execute before the processor is allocated to another ready task of
* equal priority. The length of the timeslice is application dependent and
* specified in the Configuration Table. If timeslicing is disabled (@c
* RTEMS_NO_TIMESLICE), then the task will be allowed to
* execute until a task of higher priority is made ready. If @c
* RTEMS_NO_PREEMPT is selected, then the timeslicing component is ignored by
* the scheduler.
* The asynchronous signal processing component is used to determine when
* received signals are to be processed by the task. If signal processing is
* enabled (@c RTEMS_ASR), then signals sent to the task will be processed
* the next time the task executes. If signal processing is disabled (@c
* RTEMS_NO_ASR), then all signals received by the task will
* remain posted until signal processing is enabled. This component affects
* only tasks which have established a routine to process asynchronous signals.
* The interrupt level component is used to determine which interrupts will be
* enabled when the task is executing. @c RTEMS_INTERRUPT_LEVEL(n) specifies
* that the task will execute at interrupt level n.
* - @ref RTEMS_PREEMPT - enable preemption (default)
* - @ref RTEMS_NO_PREEMPT - disable preemption
* - @ref RTEMS_NO_TIMESLICE - disable timeslicing (default)
* - @ref RTEMS_TIMESLICE - enable timeslicing
* - @ref RTEMS_ASR - enable ASR processing (default)
* - @ref RTEMS_NO_ASR - disable ASR processing
* - @ref RTEMS_INTERRUPT_LEVEL(0) - enable all interrupts (default)
* - @ref RTEMS_INTERRUPT_LEVEL(n) - execute at interrupt level n
* The set of default modes may be selected by specifying the @ref
* RTEMS_DEFAULT_MODES constant.
* @section ClassicTasksSecAccessingTaskArguments Accessing Task Arguments
* All RTEMS tasks are invoked with a single argument which is specified when
* they are started or restarted. The argument is commonly used to communicate
* startup information to the task. The simplest manner in which to define a
* task which accesses it argument is:
* rtems_task user_task(
* rtems_task_argument argument
* Application tasks requiring more information may view this single argument
* as an index into an array of parameter blocks.
* @section ClassicTasksSecFloatingPointConsiderations Floating Point Considerations
* Creating a task with the @ref RTEMS_FLOATING_POINT attribute flag results in
* additional memory being allocated for the TCB to store the state of the
* numeric coprocessor during task switches. This additional memory is NOT
* allocated for @ref RTEMS_NO_FLOATING_POINT tasks. Saving and restoring the
* context of a @c RTEMS_FLOATING_POINT task takes longer than that of a @c
* RTEMS_NO_FLOATING_POINT task because of the relatively large amount of time
* required for the numeric coprocessor to save or restore its computational
* Since RTEMS was designed specifically for embedded military applications
* which are floating point intensive, the executive is optimized to avoid
* unnecessarily saving and restoring the state of the numeric coprocessor. The
* state of the numeric coprocessor is only saved when a @c
* RTEMS_FLOATING_POINT task is dispatched and that task was not the last task
* to utilize the coprocessor. In a system with only one @c
* RTEMS_FLOATING_POINT task, the state of the numeric coprocessor will never
* be saved or restored.
* Although the overhead imposed by @c RTEMS_FLOATING_POINT tasks is minimal,
* some applications may wish to completely avoid the overhead associated with
* @c RTEMS_FLOATING_POINT tasks and still utilize a numeric coprocessor. By
* preventing a task from being preempted while performing a sequence of
* floating point operations, a @c RTEMS_NO_FLOATING_POINT task can utilize
* the numeric coprocessor without incurring the overhead of a @c
* RTEMS_FLOATING_POINT context switch. This approach also avoids the
* allocation of a floating point context area. However, if this approach is
* taken by the application designer, NO tasks should be created as @c
* RTEMS_FLOATING_POINT tasks. Otherwise, the floating point context will not
* be correctly maintained because RTEMS assumes that the state of the numeric
* coprocessor will not be altered by @c RTEMS_NO_FLOATING_POINT tasks.
* If the supported processor type does not have hardware floating capabilities
* or a standard numeric coprocessor, RTEMS will not provide built-in support
* for hardware floating point on that processor. In this case, all tasks are
* considered @c RTEMS_NO_FLOATING_POINT whether created as @c
* RTEMS_FLOATING_POINT or @c RTEMS_NO_FLOATING_POINT tasks. A floating point
* emulation software library must be utilized for floating point operations.
* On some processors, it is possible to disable the floating point unit
* dynamically. If this capability is supported by the target processor, then
* RTEMS will utilize this capability to enable the floating point unit only
* for tasks which are created with the @c RTEMS_FLOATING_POINT attribute.
* The consequence of a @c RTEMS_NO_FLOATING_POINT task attempting to access
* the floating point unit is CPU dependent but will generally result in an
* exception condition.
* @section ClassicTasksSecPerTaskVariables Per Task Variables
* Per task variables are no longer available. In particular the
* rtems_task_variable_add(), rtems_task_variable_get() and
* rtems_task_variable_delete() functions are neither declared nor defined
* anymore. Use thread local storage or POSIX Keys instead.
* @section ClassicTasksSecBuildingTaskAttributeSet Building a Task Attribute Set
* In general, an attribute set is built by a bitwise OR of the desired
* components. The set of valid task attribute components is listed below:
* - @ref RTEMS_NO_FLOATING_POINT - does not use coprocessor (default)
* - @ref RTEMS_FLOATING_POINT - uses numeric coprocessor
* - @ref RTEMS_LOCAL - local task (default)
* - @ref RTEMS_GLOBAL - global task
* Attribute values are specifically designed to be mutually exclusive,
* therefore bitwise OR and addition operations are equivalent as long as each
* attribute appears exactly once in the component list. A component listed as
* a default is not required to appear in the component list, although it is a
* good programming practice to specify default components. If all defaults are
* desired, then @ref RTEMS_DEFAULT_ATTRIBUTES should be used. This example
* demonstrates the attribute_set parameter needed to create a local task which
* utilizes the numeric coprocessor. The attribute_set parameter could be @c
* RTEMS_FLOATING_POINT or @c RTEMS_LOCAL | @c RTEMS_FLOATING_POINT. The
* attribute_set parameter can be set to @c RTEMS_FLOATING_POINT because @c
* RTEMS_LOCAL is the default for all created tasks. If the task were global
* and used the numeric coprocessor, then the attribute_set parameter would be
* @c RTEMS_GLOBAL | @c RTEMS_FLOATING_POINT.
* @section ClassicTasksSecBuildingModeAndMask Building a Mode and Mask
* In general, a mode and its corresponding mask is built by a bitwise OR of
* the desired components. The set of valid mode constants and each mode's
* corresponding mask constant is listed below:
* <tr><th>Mode Constant</th><th>Mask Constant</th><th>Description</th></tr>
* <tr><td>@ref RTEMS_PREEMPT</td><td>@ref RTEMS_PREEMPT_MASK</td><td>enables preemption</td></tr>
* <tr><td>@ref RTEMS_NO_PREEMPT</td><td>@ref RTEMS_PREEMPT_MASK</td><td>disables preemption</td></tr>
* <tr><td>@ref RTEMS_NO_TIMESLICE</td><td>@ref RTEMS_TIMESLICE_MASK</td><td>disables timeslicing</td></tr>
* <tr><td>@ref RTEMS_TIMESLICE</td><td>@ref RTEMS_TIMESLICE_MASK</td><td>enables timeslicing</td></tr>
* <tr><td>@ref RTEMS_ASR</td><td>@ref RTEMS_ASR_MASK</td><td>enables ASR processing</td></tr>
* <tr><td>@ref RTEMS_NO_ASR</td><td>@ref RTEMS_ASR_MASK</td><td>disables ASR processing</td></tr>
* <tr><td>@ref RTEMS_INTERRUPT_LEVEL(0)</td><td>@ref RTEMS_INTERRUPT_MASK</td><td>enables all interrupts</td></tr>
* <tr><td>@ref RTEMS_INTERRUPT_LEVEL(n)</td><td>@ref RTEMS_INTERRUPT_MASK</td><td>sets interrupts level n</td></tr>
* Mode values are specifically designed to be mutually exclusive, therefore
* bitwise OR and addition operations are equivalent as long as each mode
* appears exactly once in the component list. A mode component listed as a
* default is not required to appear in the mode component list, although it is
* a good programming practice to specify default components. If all defaults
* are desired, the mode @ref RTEMS_DEFAULT_MODES and the mask @ref
* RTEMS_ALL_MODE_MASKS should be used.
* The following example demonstrates the mode and mask parameters used with
* the rtems_task_mode() directive to place a task at interrupt level 3 and
* make it non-preemptible. The mode should be set to @c
* RTEMS_INTERRUPT_LEVEL(3) | @c RTEMS_NO_PREEMPT to indicate the desired
* preemption mode and interrupt level, while the mask parameter should be set
* to @c RTEMS_INTERRUPT_MASK | @c RTEMS_PREEMPT_MASK to indicate that
* the calling task's interrupt level and preemption mode are being altered.
* @defgroup LocalPackages Local Packages
* @brief Local packages.