1 | @c |
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2 | @c COPYRIGHT (c) 1988-2002. |
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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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6 | @c $Id$ |
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7 | @c |
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8 | |
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9 | @ifinfo |
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10 | @node Preface, Overview, Top, Top |
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11 | @end ifinfo |
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12 | @unnumbered Preface |
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13 | |
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14 | In recent years, the cost required to develop a |
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15 | software product has increased significantly while the target |
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16 | hardware costs have decreased. Now a larger portion of money is |
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17 | expended in developing, using, and maintaining software. The |
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18 | trend in computing costs is the complete dominance of software |
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19 | over hardware costs. Because of this, it is necessary that |
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20 | formal disciplines be established to increase the probability |
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21 | that software is characterized by a high degree of correctness, |
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22 | maintainability, and portability. In addition, these |
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23 | disciplines must promote practices that aid in the consistent |
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24 | and orderly development of a software system within schedule and |
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25 | budgetary constraints. To be effective, these disciplines must |
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26 | adopt standards which channel individual software efforts toward |
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27 | a common goal. |
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28 | |
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29 | The push for standards in the software development |
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30 | field has been met with various degrees of success. The |
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31 | Microprocessor Operating Systems Interfaces (MOSI) effort has |
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32 | experienced only limited success. As popular as the UNIX |
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33 | operating system has grown, the attempt to develop a standard |
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34 | interface definition to allow portable application development |
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35 | has only recently begun to produce the results needed in this |
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36 | area. Unfortunately, very little effort has been expended to |
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37 | provide standards addressing the needs of the real-time |
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38 | community. Several organizations have addressed this need |
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39 | during recent years. |
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40 | |
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41 | The Real Time Executive Interface Definition (RTEID) |
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42 | was developed by Motorola with technical input from Software |
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43 | Components Group. RTEID was adopted by the VMEbus International |
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44 | Trade Association (VITA) as a baseline draft for their proposed |
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45 | standard multiprocessor, real-time executive interface, Open |
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46 | Real-Time Kernel Interface Definition (ORKID). These two groups |
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47 | are currently working together with the IEEE P1003.4 committee |
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48 | to insure that the functionality of their proposed standards is |
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49 | adopted as the real-time extensions to POSIX. |
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50 | |
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51 | This emerging standard defines an interface for the |
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52 | development of real-time software to ease the writing of |
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53 | real-time application programs that are directly portable across |
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54 | multiple real-time executive implementations. This interface |
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55 | includes both the source code interfaces and run-time behavior |
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56 | as seen by a real-time application. It does not include the |
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57 | details of how a kernel implements these functions. The |
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58 | standard's goal is to serve as a complete definition of external |
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59 | interfaces so that application code that conforms to these |
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60 | interfaces will execute properly in all real-time executive |
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61 | environments. With the use of a standards compliant executive, |
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62 | routines that acquire memory blocks, create and manage message |
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63 | queues, establish and use semaphores, and send and receive |
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64 | signals need not be redeveloped for a different real-time |
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65 | environment as long as the new environment is compliant with the |
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66 | standard. Software developers need only concentrate on the |
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67 | hardware dependencies of the real-time system. Furthermore, |
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68 | most hardware dependencies for real-time applications can be |
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69 | localized to the device drivers. |
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70 | |
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71 | A compliant executive provides simple and flexible |
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72 | real-time multiprocessing. It easily lends itself to both |
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73 | tightly-coupled and loosely-coupled configurations (depending on |
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74 | the system hardware configuration). Objects such as tasks, |
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75 | queues, events, signals, semaphores, and memory blocks can be |
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76 | designated as global objects and accessed by any task regardless |
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77 | of which processor the object and the accessing task reside. |
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78 | |
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79 | The acceptance of a standard for real-time executives |
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80 | will produce the same advantages enjoyed from the push for UNIX |
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81 | standardization by AT&T's System V Interface Definition and |
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82 | IEEE's POSIX efforts. A compliant multiprocessing executive |
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83 | will allow close coupling between UNIX systems and real-time |
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84 | executives to provide the many benefits of the UNIX development |
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85 | environment to be applied to real-time software development. |
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86 | Together they provide the necessary laboratory environment to |
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87 | implement real-time, distributed, embedded systems using a wide |
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88 | variety of computer architectures. |
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89 | |
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90 | A study was completed in 1988, within the Research, |
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91 | Development, and Engineering Center, U.S. Army Missile Command, |
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92 | which compared the various aspects of the Ada programming |
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93 | language as they related to the application of Ada code in |
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94 | distributed and/or multiple processing systems. Several |
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95 | critical conclusions were derived from the study. These |
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96 | conclusions have a major impact on the way the Army develops |
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97 | application software for embedded applications. These impacts |
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98 | apply to both in-house software development and contractor |
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99 | developed software. |
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100 | |
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101 | A conclusion of the analysis, which has been |
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102 | previously recognized by other agencies attempting to utilize |
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103 | Ada in a distributed or multiprocessing environment, is that the |
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104 | Ada programming language does not adequately support |
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105 | multiprocessing. Ada does provide a mechanism for |
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106 | multi-tasking, however, this capability exists only for a single |
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107 | processor system. The language also does not have inherent |
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108 | capabilities to access global named variables, flags or program |
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109 | code. These critical features are essential in order for data |
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110 | to be shared between processors. However, these drawbacks do |
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111 | have workarounds which are sometimes awkward and defeat the |
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112 | intent of software maintainability and portability goals. |
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113 | |
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114 | Another conclusion drawn from the analysis, was that |
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115 | the run time executives being delivered with the Ada compilers |
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116 | were too slow and inefficient to be used in modern missile |
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117 | systems. A run time executive is the core part of the run time |
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118 | system code, or operating system code, that controls task |
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119 | scheduling, input/output management and memory management. |
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120 | Traditionally, whenever efficient executive (also known as |
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121 | kernel) code was required by the application, the user developed |
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122 | in-house software. This software was usually written in |
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123 | assembly language for optimization. |
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124 | |
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125 | Because of this shortcoming in the Ada programming |
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126 | language, software developers in research and development and |
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127 | contractors for project managed systems, are mandated by |
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128 | technology to purchase and utilize off-the-shelf third party |
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129 | kernel code. The contractor, and eventually the Government, |
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130 | must pay a licensing fee for every copy of the kernel code used |
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131 | in an embedded system. |
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132 | |
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133 | The main drawback to this development environment is |
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134 | that the Government does not own, nor has the right to modify |
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135 | code contained within the kernel. V&V techniques in this |
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136 | situation are more difficult than if the complete source code |
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137 | were available. Responsibility for system failures due to faulty |
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138 | software is yet another area to be resolved under this |
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139 | environment. |
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140 | |
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141 | The Guidance and Control Directorate began a software |
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142 | development effort to address these problems. A project to |
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143 | develop an experimental run time kernel was begun that will |
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144 | eliminate the major drawbacks of the Ada programming language |
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145 | mentioned above. The Real Time Executive for Multiprocessor Systems |
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146 | (RTEMS) provides full capabilities for management of tasks, |
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147 | interrupts, time, and multiple processors in addition to those |
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148 | features typical of generic operating systems. The code is |
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149 | Government owned, so no licensing fees are necessary. RTEMS has |
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150 | been implemented in both the Ada and C programming languages. |
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151 | It has been ported to the following processor families: |
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152 | |
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153 | @itemize @bullet |
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154 | @item Intel i80386 and above |
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155 | @item Intel i80960 |
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156 | @item Motorola MC68xxx |
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157 | @item Motorola MC683xx |
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158 | @item MIPS |
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159 | @item PowerPC |
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160 | @item SPARC |
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161 | @item Hewlett Packard PA-RISC |
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162 | @item Hitachi SH |
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163 | @item AMD A29K |
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164 | @item UNIX |
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165 | @end itemize |
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166 | |
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167 | Support for other processor families, including RISC, CISC, and DSP, is |
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168 | planned. Since almost all of RTEMS is written in a high level language, |
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169 | ports to additional processor families require minimal effort. |
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170 | |
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171 | RTEMS multiprocessor support is capable of handling |
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172 | either homogeneous or heterogeneous systems. The kernel |
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173 | automatically compensates for architectural differences (byte |
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174 | swapping, etc.) between processors. This allows a much easier |
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175 | transition from one processor family to another without a major |
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176 | system redesign. |
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177 | |
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178 | Since the proposed standards are still in draft form, |
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179 | RTEMS cannot and does not claim compliance. However, the status |
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180 | of the standard is being carefully monitored to guarantee that |
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181 | RTEMS provides the functionality specified in the standard. |
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182 | Once approved, RTEMS will be made compliant. |
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183 | |
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184 | This document is a detailed users guide for a |
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185 | functionally compliant real-time multiprocessor executive. It |
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186 | describes the user interface and run-time behavior of Release |
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187 | @value{VERSION} of the @value{LANGUAGE} interface |
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188 | to RTEMS. |
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189 | |
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