📄 ex12_math.htm
字号:
<html><head><title>EXAMPLE 12: FLOATING POINT IN REAL-TIME TASKS</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex11_jitter.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ack.htm">[next]</a><h1>Example 12: Floating Point in Real-Time Tasks</h1><p>This example demonstrates how to use floating-point math in real-timetasks. Aside from loading the RT Linux math library, which is donein the usual "insrtl" script along with support for scheduling, FIFOs,etc., you need to signify that you want the floating point unit (FPU)registers to be saved and restored by RT Linux. Otherwise, these areleft alone, and the use of the FPU by an RT task will clobber thefloating point calculations of any Linux program. This is neverdesirable behavior, so always make sure to enable FPU save/restore inyour RT task if you use floating point.<p>Refer to the <ahref="../ex12_math/math_task.c">commented real-timesource code</a> and the <ahref="../ex12_math/math_app.c">commented applicationsource code</a> for the details.<h2>Principle of Operation</h2><ul><li>When the RT Linux scheduler interrupt service routine runs, itsaves the context of the currently executing Linux process, includingmicroprocessor registers and the stack frame. When it's done with theRT tasks, it restores them so the interrupted Linux process can resumecorrectly. <li>However, by default RT Linux does not save/restore the context ofthe floating-point unit (FPU). This is because it takes more time todo so, and many RT tasks don't use the FPU.<li>To tell RT Linux that normal Linux applications will use the FPU(e.g., the calculator, Matlab, etc.), call<pre>rt_linux_use_fpu(1); /* tell RT Linux that Linux wants the FPU saved */</pre>This can be done once during the initialization of your RTapplication. If you know in advance that no Linux process will beusing floating point, you can omit this.<li>Each task that uses floating point must also set a flag indicatingthat FPU context must be saved and restored when it runs. This can bedone by passing an argument to 'rt_task_init()', e.g.,<pre>rt_task_init(&rt_task, task_code, initial_arg, STACK_SIZE, RT_LOWEST_PRIORITY, 1, /* uses floating point unit */ signal_handler);</pre>Alternatively, the task can selectively turn FPU context handling offand on during execution, for example to speed things up in sectionswhere no floating point will be done, by calling 'rt_task_use_fpu()', e.g.,<pre>rt_task_use_fpu(1); /* tell RT Linux that this task will use the FPU */</pre><li>This demo sets up a single periodic task that increments afloating point value by some small amount, computes the sine andcosine, and increments a cumulative count by sin^2 + cos^2 = 1, thusflexing the floating point unit (FPU).<li>In this task, every second the FPUsave/restore flag is alternately set and cleared. When cleared, theFPU registers are not saved, thus rendering all Linux processfloating-point calculations useless.<li>A companion process to thistask runs a big sum computation and shows what the effect of bad FPUcontext looks like: terrible floating-point results for one second,good ones for the next second, etc.:<pre>...50000005000000.00000050000005000000.000000...nannannan...50000005000000.00000050000005000000.000000...</pre>The 'nan' here means "not a number," that is, the floating point valueis junk that does not conform to the IEEE floating pointspecification.<li>Note that when you run the demo, you may get good output all thetime. It depends on the vagaries of when the Linux and RT tasksexecute. Don't assume that if this demo shows good output, you don'tneed to call the aforementioned functions.</ul><h2>Running the Demo</h2>To run the demo, change to the 'ex12_math' subdirectory of thetop-level tutorial directory, and run the 'run' script by typing<pre>./run</pre>Alternatively, change to the top-level tutorial directory and run the'runall' script there by typing<pre>./runall</pre>and selecting the "Floating Point" button.<p>You'll see diagnostics messages printing out.<p><a href="../ex12_math/math_task.c">See the Real-Time Task Code</a><p><a href="../ex12_math/math_app.c">See the Linux Application Code</a><hr><a href="./ack.htm">Next: Acknowledgements</a><p><a href="./ex11_jitter.htm">Back: Example 11, Measuring TimingJitter</a> </body></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -