[ae68ff0] | 1 | @c |
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[4ffbc49] | 2 | @c COPYRIGHT (c) 1988-2007. |
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[ae68ff0] | 3 | @c On-Line Applications Research Corporation (OAR). |
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| 4 | @c All rights reserved. |
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| 5 | @c |
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[139b2e4a] | 6 | @c $Id$ |
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| 7 | @c |
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[ae68ff0] | 8 | |
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| 9 | @c |
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| 10 | @c The following figure was replaced with an ASCII equivalent. |
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| 11 | @c Figure 2-1 Object ID Composition |
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| 12 | @c |
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[20515fc] | 13 | |
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[ae68ff0] | 14 | @chapter Key Concepts |
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[20515fc] | 15 | |
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[ae68ff0] | 16 | @section Introduction |
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| 17 | |
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| 18 | The facilities provided by RTEMS are built upon a |
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| 19 | foundation of very powerful concepts. These concepts must be |
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| 20 | understood before the application developer can efficiently |
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| 21 | utilize RTEMS. The purpose of this chapter is to familiarize |
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| 22 | one with these concepts. |
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| 23 | |
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| 24 | @section Objects |
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| 25 | |
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[169502e] | 26 | @cindex objects |
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| 27 | |
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[ae68ff0] | 28 | RTEMS provides directives which can be used to |
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| 29 | dynamically create, delete, and manipulate a set of predefined |
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| 30 | object types. These types include tasks, message queues, |
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| 31 | semaphores, memory regions, memory partitions, timers, ports, |
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| 32 | and rate monotonic periods. The object-oriented nature of RTEMS |
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| 33 | encourages the creation of modular applications built upon |
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| 34 | re-usable "building block" routines. |
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| 35 | |
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| 36 | All objects are created on the local node as required |
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| 37 | by the application and have an RTEMS assigned ID. All objects |
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| 38 | have a user-assigned name. Although a relationship exists |
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| 39 | between an object's name and its RTEMS assigned ID, the name and |
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| 40 | ID are not identical. Object names are completely arbitrary and |
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| 41 | selected by the user as a meaningful "tag" which may commonly |
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| 42 | reflect the object's use in the application. Conversely, object |
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| 43 | IDs are designed to facilitate efficient object manipulation by |
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| 44 | the executive. |
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| 45 | |
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[adee5979] | 46 | @subsection Object Names |
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| 47 | |
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[169502e] | 48 | @cindex object name |
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[adee5979] | 49 | @findex rtems_object_name |
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[169502e] | 50 | |
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[ae68ff0] | 51 | An object name is an unsigned thirty-two bit entity |
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[adee5979] | 52 | associated with the object by the user. The data type |
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| 53 | @code{@value{DIRPREFIX}name} is used to store object names. |
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| 54 | |
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| 55 | @findex rtems_build_name |
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| 56 | |
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| 57 | Although not required by RTEMS, object names are often |
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| 58 | composed of four ASCII characters which help identify that object. |
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| 59 | For example, a task which causes a light to blink might be |
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| 60 | called "LITE". The @code{@value{DIRPREFIX}build_name} routine |
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| 61 | is provided to build an object name from four ASCII characters. |
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| 62 | The following example illustrates this: |
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| 63 | |
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[4ffbc49] | 64 | @ifset is-C |
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[adee5979] | 65 | @example |
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| 66 | rtems_object_name my_name; |
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| 67 | |
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| 68 | my_name = rtems_build_name( 'L', 'I', 'T', 'E' ); |
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| 69 | @end example |
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| 70 | @end ifset |
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| 71 | |
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[4ffbc49] | 72 | @ifset is-Ada |
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| 73 | @example |
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| 74 | My_Name : RTEMS.Name; |
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| 75 | |
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| 76 | My_Name = RTEMS.Build_Name( 'L', 'I', 'T', 'E' ); |
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| 77 | @end example |
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| 78 | @end ifset |
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| 79 | |
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[ae68ff0] | 80 | However, it is not required that the application use ASCII |
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| 81 | characters to build object names. For example, if an |
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| 82 | application requires one-hundred tasks, it would be difficult to |
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| 83 | assign meaningful ASCII names to each task. A more convenient |
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| 84 | approach would be to name them the binary values one through |
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| 85 | one-hundred, respectively. |
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| 86 | |
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[4ffbc49] | 87 | @ifset is-C |
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[35a81f8] | 88 | @findex rtems_get_object_name |
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| 89 | |
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| 90 | RTEMS provides a helper routine, @code{@value{DIRPREFIX}get_object_name}, |
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| 91 | which can be used to obtain the name of any RTEMS object using just |
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| 92 | its ID. This routine attempts to convert the name into a printable string. |
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| 93 | |
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| 94 | The following example illustrates the use of this method to print |
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| 95 | an object name: |
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| 96 | |
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| 97 | @example |
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| 98 | #include <rtems.h> |
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| 99 | #include <rtems/bspIo.h> |
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| 100 | |
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| 101 | void print_name(rtems_id the_object) |
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| 102 | @{ |
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| 103 | char buffer[10]; /* name assumed to be 10 characters or less */ |
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| 104 | char *result; |
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| 105 | |
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| 106 | result = rtems_get_object_name( id, sizeof(buffer), buffer ); |
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| 107 | printk( "ID=0x%08x name=%s\n", id, ((result) ? result : "no name") ); |
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| 108 | @} |
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| 109 | @end example |
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| 110 | @end ifset |
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| 111 | |
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| 112 | |
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[adee5979] | 113 | @subsection Object IDs |
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| 114 | |
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[169502e] | 115 | @cindex object ID |
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[71689d44] | 116 | @cindex object ID composition |
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[adee5979] | 117 | @findex rtems_id |
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[169502e] | 118 | |
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[ae68ff0] | 119 | @need 3000 |
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| 120 | |
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| 121 | An object ID is a unique unsigned thirty-two bit |
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[1fa78236] | 122 | entity composed of three parts: API, object class, node, and index. |
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[adee5979] | 123 | The data type @code{@value{DIRPREFIX}id} is used to store object IDs. |
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| 124 | |
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[ae68ff0] | 125 | |
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| 126 | @ifset use-ascii |
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| 127 | @example |
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| 128 | @group |
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[1fa78236] | 129 | 31 27 26 24 23 16 15 0 |
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| 130 | +---------+-------+--------------+-------------------------------+ |
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| 131 | | | | | | |
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| 132 | | Class | API | Node | Index | |
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| 133 | | | | | | |
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| 134 | +---------+-------+--------------+-------------------------------+ |
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[ae68ff0] | 135 | @end group |
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| 136 | @end example |
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| 137 | @end ifset |
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| 138 | |
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| 139 | @ifset use-tex |
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[1fa78236] | 140 | @sp1 |
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| 141 | @center{@image{ObjectId-32Bits,,2in}} |
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[ae68ff0] | 142 | @end ifset |
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| 143 | |
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| 144 | @ifset use-html |
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| 145 | @html |
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[1a34b8c] | 146 | <P ALIGN="center"><IMG SRC="ObjectId-32Bits.png" |
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| 147 | WIDTH=550 HEIGHT=400 ALT="32 Bit Object Id"></P> |
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[ae68ff0] | 148 | @end html |
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| 149 | @end ifset |
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| 150 | |
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[1fa78236] | 151 | The most significant five bits are the object class. The next |
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| 152 | three bits indicate the API to which the object class belongs. |
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| 153 | The next eight bits (16-23) are the number of the node on which |
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| 154 | this object was created. The node number is always one (1) in a single |
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[71689d44] | 155 | processor system. The least significant sixteen bits form an |
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| 156 | identifier within a particular object type. This identifier, |
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| 157 | called the object index, ranges in value from 1 to the maximum |
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| 158 | number of objects configured for this object type. |
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| 159 | |
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[1fa78236] | 160 | The four components of an object ID make it possible |
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[ae68ff0] | 161 | to quickly locate any object in even the most complicated |
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| 162 | multiprocessor system. Object ID's are associated with an |
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| 163 | object by RTEMS when the object is created and the corresponding |
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| 164 | ID is returned by the appropriate object create directive. The |
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| 165 | object ID is required as input to all directives involving |
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| 166 | objects, except those which create an object or obtain the ID of |
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| 167 | an object. |
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| 168 | |
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| 169 | The object identification directives can be used to |
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| 170 | dynamically obtain a particular object's ID given its name. |
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| 171 | This mapping is accomplished by searching the name table |
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| 172 | associated with this object type. If the name is non-unique, |
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| 173 | then the ID associated with the first occurrence of the name |
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| 174 | will be returned to the application. Since object IDs are |
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| 175 | returned when the object is created, the object identification |
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| 176 | directives are not necessary in a properly designed single |
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| 177 | processor application. |
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| 178 | |
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[71689d44] | 179 | In addition, services are provided to portably examine the |
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[d5671b1] | 180 | subcomponents of an RTEMS ID. These services are |
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| 181 | described in detail later in this manual but are prototyped |
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| 182 | as follows: |
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[71689d44] | 183 | |
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| 184 | @cindex obtaining class from object ID |
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| 185 | @cindex obtaining node from object ID |
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| 186 | @cindex obtaining index from object ID |
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| 187 | @cindex get class from object ID |
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| 188 | @cindex get node from object ID |
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| 189 | @cindex get index from object ID |
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[d5671b1] | 190 | @findex rtems_object_id_get_api |
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| 191 | @findex rtems_object_id_get_class |
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| 192 | @findex rtems_object_id_get_node |
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| 193 | @findex rtems_object_id_get_index |
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[71689d44] | 194 | |
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| 195 | @example |
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[d5671b1] | 196 | uint32_t rtems_object_id_get_api( rtems_id ); |
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| 197 | uint32_t rtems_object_id_get_class( rtems_id ); |
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| 198 | uint32_t rtems_object_id_get_node( rtems_id ); |
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| 199 | uint32_t rtems_object_id_get_index( rtems_id ); |
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[71689d44] | 200 | @end example |
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| 201 | |
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[ae68ff0] | 202 | An object control block is a data structure defined |
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| 203 | by RTEMS which contains the information necessary to manage a |
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| 204 | particular object type. For efficiency reasons, the format of |
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| 205 | each object type's control block is different. However, many of |
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| 206 | the fields are similar in function. The number of each type of |
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| 207 | control block is application dependent and determined by the |
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| 208 | values specified in the user's Configuration Table. An object |
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| 209 | control block is allocated at object create time and freed when |
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| 210 | the object is deleted. With the exception of user extension |
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| 211 | routines, object control blocks are not directly manipulated by |
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| 212 | user applications. |
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| 213 | |
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| 214 | @section Communication and Synchronization |
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| 215 | |
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[169502e] | 216 | @cindex communication and synchronization |
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| 217 | |
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[ae68ff0] | 218 | In real-time multitasking applications, the ability |
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| 219 | for cooperating execution threads to communicate and synchronize |
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| 220 | with each other is imperative. A real-time executive should |
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| 221 | provide an application with the following capabilities: |
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| 222 | |
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| 223 | @itemize @bullet |
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| 224 | @item Data transfer between cooperating tasks |
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| 225 | @item Data transfer between tasks and ISRs |
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| 226 | @item Synchronization of cooperating tasks |
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| 227 | @item Synchronization of tasks and ISRs |
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| 228 | @end itemize |
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| 229 | |
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| 230 | Most RTEMS managers can be used to provide some form |
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| 231 | of communication and/or synchronization. However, managers |
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| 232 | dedicated specifically to communication and synchronization |
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| 233 | provide well established mechanisms which directly map to the |
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| 234 | application's varying needs. This level of flexibility allows |
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| 235 | the application designer to match the features of a particular |
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| 236 | manager with the complexity of communication and synchronization |
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| 237 | required. The following managers were specifically designed for |
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| 238 | communication and synchronization: |
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| 239 | |
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| 240 | @itemize @bullet |
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| 241 | @item Semaphore |
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| 242 | @item Message Queue |
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| 243 | @item Event |
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| 244 | @item Signal |
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| 245 | @end itemize |
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| 246 | |
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| 247 | The semaphore manager supports mutual exclusion |
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| 248 | involving the synchronization of access to one or more shared |
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| 249 | user resources. Binary semaphores may utilize the optional |
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| 250 | priority inheritance algorithm to avoid the problem of priority |
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| 251 | inversion. The message manager supports both communication and |
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| 252 | synchronization, while the event manager primarily provides a |
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| 253 | high performance synchronization mechanism. The signal manager |
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| 254 | supports only asynchronous communication and is typically used |
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| 255 | for exception handling. |
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| 256 | |
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| 257 | @section Time |
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| 258 | |
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[169502e] | 259 | @cindex time |
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| 260 | |
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[ae68ff0] | 261 | The development of responsive real-time applications |
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| 262 | requires an understanding of how RTEMS maintains and supports |
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| 263 | time-related operations. The basic unit of time in RTEMS is |
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| 264 | known as a tick. The frequency of clock ticks is completely |
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| 265 | application dependent and determines the granularity and |
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| 266 | accuracy of all interval and calendar time operations. |
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| 267 | |
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[adee5979] | 268 | @findex rtems_interval |
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| 269 | |
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[ae68ff0] | 270 | By tracking time in units of ticks, RTEMS is capable |
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| 271 | of supporting interval timing functions such as task delays, |
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| 272 | timeouts, timeslicing, the delayed execution of timer service |
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| 273 | routines, and the rate monotonic scheduling of tasks. An |
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| 274 | interval is defined as a number of ticks relative to the current |
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| 275 | time. For example, when a task delays for an interval of ten |
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| 276 | ticks, it is implied that the task will not execute until ten |
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| 277 | clock ticks have occurred. |
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[adee5979] | 278 | All intervals are specified using data type |
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| 279 | @code{@value{DIRPREFIX}interval}. |
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[ae68ff0] | 280 | |
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| 281 | A characteristic of interval timing is that the |
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| 282 | actual interval period may be a fraction of a tick less than the |
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| 283 | interval requested. This occurs because the time at which the |
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| 284 | delay timer is set up occurs at some time between two clock |
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| 285 | ticks. Therefore, the first countdown tick occurs in less than |
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| 286 | the complete time interval for a tick. This can be a problem if |
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| 287 | the clock granularity is large. |
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| 288 | |
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| 289 | The rate monotonic scheduling algorithm is a hard |
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| 290 | real-time scheduling methodology. This methodology provides |
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| 291 | rules which allows one to guarantee that a set of independent |
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| 292 | periodic tasks will always meet their deadlines -- even under |
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| 293 | transient overload conditions. The rate monotonic manager |
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| 294 | provides directives built upon the Clock Manager's interval |
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| 295 | timer support routines. |
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| 296 | |
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| 297 | Interval timing is not sufficient for the many |
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| 298 | applications which require that time be kept in wall time or |
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| 299 | true calendar form. Consequently, RTEMS maintains the current |
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| 300 | date and time. This allows selected time operations to be |
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| 301 | scheduled at an actual calendar date and time. For example, a |
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| 302 | task could request to delay until midnight on New Year's Eve |
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| 303 | before lowering the ball at Times Square. |
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[adee5979] | 304 | The data type @code{@value{DIRPREFIX}time_of_day} is used to specify |
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| 305 | calendar time in RTEMS services. |
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| 306 | @xref{Clock Manager Time and Date Data Structures, , Time and Date Data Structures}. |
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| 307 | @findex rtems_time_of_day |
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[ae68ff0] | 308 | |
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| 309 | Obviously, the directives which use intervals or wall |
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| 310 | time cannot operate without some external mechanism which |
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| 311 | provides a periodic clock tick. This clock tick is typically |
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| 312 | provided by a real time clock or counter/timer device. |
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| 313 | |
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| 314 | @section Memory Management |
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| 315 | |
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[169502e] | 316 | @cindex memory management |
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| 317 | |
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[ae68ff0] | 318 | RTEMS memory management facilities can be grouped |
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| 319 | into two classes: dynamic memory allocation and address |
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| 320 | translation. Dynamic memory allocation is required by |
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| 321 | applications whose memory requirements vary through the |
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| 322 | application's course of execution. Address translation is |
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| 323 | needed by applications which share memory with another CPU or an |
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| 324 | intelligent Input/Output processor. The following RTEMS |
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| 325 | managers provide facilities to manage memory: |
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| 326 | |
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| 327 | @itemize @bullet |
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| 328 | @item Region |
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| 329 | |
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| 330 | @item Partition |
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| 331 | |
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| 332 | @item Dual Ported Memory |
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| 333 | @end itemize |
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| 334 | |
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| 335 | RTEMS memory management features allow an application |
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| 336 | to create simple memory pools of fixed size buffers and/or more |
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| 337 | complex memory pools of variable size segments. The partition |
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| 338 | manager provides directives to manage and maintain pools of |
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| 339 | fixed size entities such as resource control blocks. |
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| 340 | Alternatively, the region manager provides a more general |
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| 341 | purpose memory allocation scheme that supports variable size |
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| 342 | blocks of memory which are dynamically obtained and freed by the |
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| 343 | application. The dual-ported memory manager provides executive |
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| 344 | support for address translation between internal and external |
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| 345 | dual-ported RAM address space. |
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