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<ol class="chapter"><li class="affix"><a href="../index.html">Introduction</a></li><li class="affix"><a href="../background/index.html">Background</a></li><li class="affix"><a href="../requirements/index.html">Requirements</a></li><li><a href="../hardware/index.html"><strong aria-hidden="true">1.</strong> Meet your hardware</a></li><li><a href="../setup/index.html"><strong aria-hidden="true">2.</strong> Development environment setup</a></li><li><ol class="section"><li><a href="../setup/LINUX.html"><strong aria-hidden="true">2.1.</strong> Linux</a></li><li><a href="../setup/WINDOWS.html"><strong aria-hidden="true">2.2.</strong> Windows</a></li><li><a href="../setup/MACOS.html"><strong aria-hidden="true">2.3.</strong> macOS</a></li><li><a href="../setup/VERIFY.html"><strong aria-hidden="true">2.4.</strong> Verify the installation</a></li></ol></li><li><a href="../getting-started/00.00.README.html"><strong aria-hidden="true">3.</strong> Getting started</a></li><li><ol class="section"><li><a href="../getting-started/01.00.BUILD.html"><strong aria-hidden="true">3.1.</strong> Building</a></li><li><a href="../getting-started/02.00.FLASH.html"><strong aria-hidden="true">3.2.</strong> Flashing</a></li><li><a href="../getting-started/03.00.DEBUG.html"><strong aria-hidden="true">3.3.</strong> Debugging</a></li><li><a href="../getting-started/04.00.SOLUTION.html"><strong aria-hidden="true">3.4.</strong> Solution</a></li></ol></li><li><a href="../hello-world/00.00.README.html"><strong aria-hidden="true">4.</strong> Hello world</a></li><li><ol class="section"><li><a href="../hello-world/01.00.SEMIHOSTING.html" class="active"><strong aria-hidden="true">4.1.</strong> Semihosting</a></li><li><a href="../hello-world/02.00.UART.html"><strong aria-hidden="true">4.2.</strong> Serial communication</a></li><li><ol class="section"><li><a href="../hello-world/02.01.NIX.html"><strong aria-hidden="true">4.2.1.</strong> *nix</a></li><li><a href="../hello-world/02.02.WINDOWS.html"><strong aria-hidden="true">4.2.2.</strong> Windows</a></li></ol></li><li><a href="../hello-world/03.00.LED.html"><strong aria-hidden="true">4.3.</strong> GPIO and LEDs</a></li><li><ol class="section"><li><a href="../hello-world/03.01.SOLUTION.html"><strong aria-hidden="true">4.3.1.</strong> Solution</a></li></ol></li></ol></li><li><a href="../choice/00.00.README.html"><strong aria-hidden="true">5.</strong> Choose Your Own Adventure</a></li><li><a href="../microbit/00.00.README.html"><strong aria-hidden="true">6.</strong> micro:bit HAL basics</a></li><li><ol class="section"><li><a href="../microbit/01.00.BUTTONS.html"><strong aria-hidden="true">6.1.</strong> Buttons</a></li><li><a href="../microbit/02.00.DELAY.html"><strong aria-hidden="true">6.2.</strong> Delays</a></li><li><a href="../microbit/03.00.DISPLAY.html"><strong aria-hidden="true">6.3.</strong> Display</a></li></ol></li><li><a href="../serial/00.00.README.html"><strong aria-hidden="true">7.</strong> Serial UART - Blocking</a></li><li><ol class="section"><li><a href="../serial/01.00.ECHO.html"><strong aria-hidden="true">7.1.</strong> Echo Server</a></li><li><ol class="section"><li><a href="../serial/01.01.THEORY.html"><strong aria-hidden="true">7.1.1.</strong> Theory</a></li><li><a href="../serial/01.02.ECHO.html"><strong aria-hidden="true">7.1.2.</strong> Solution</a></li></ol></li><li><a href="../serial/02.00.html"><strong aria-hidden="true">7.2.</strong> Exercises</a></li><li><ol class="section"><li><a href="../serial/02.01.html"><strong aria-hidden="true">7.2.1.</strong> Reverse echo</a></li><li><ol class="section"><li><a href="../serial/02.01.SOLUTION.html"><strong aria-hidden="true">7.2.1.1.</strong> Solution</a></li></ol></li><li><a href="../serial/02.02.html"><strong aria-hidden="true">7.2.2.</strong> Countdown</a></li><li><ol class="section"><li><a href="../serial/02.02.SOLUTION.html"><strong aria-hidden="true">7.2.2.1.</strong> Solution</a></li></ol></li><li><a href="../serial/02.04.html"><strong aria-hidden="true">7.2.3.</strong> Quiz</a></li><li><ol class="section"><li><a href="../serial/02.04.SOLUTION.html"><strong aria-hidden="true">7.2.3.1.</strong> Solution</a></li></ol></li></ol></li></ol></li><li><a href="../display/00.00.README.html"><strong aria-hidden="true">8.</strong> LED display</a></li><li><ol class="section"><li><a href="../display/01.00.THEORY.html"><strong aria-hidden="true">8.1.</strong> Theory</a></li><li><a href="../display/02.00.PROBLEM.html"><strong aria-hidden="true">8.2.</strong> Problem</a></li><li><ol class="section"><li><a href="../display/02.01.LAYOUT.html"><strong aria-hidden="true">8.2.1.</strong> Layout</a></li><li><a href="../display/02.02.DELAY.html"><strong aria-hidden="true">8.2.2.</strong> Delays</a></li><li><a href="../display/02.03.MULT.html"><strong aria-hidden="true">8.2.3.</strong> Multiplexing</a></li></ol></li><li><a href="../display/03.00.SOLUTION.html"><strong aria-hidden="true">8.3.</strong> Solution</a></li><li><ol class="section"><li><a href="../display/03.01.LAYOUT.html"><strong aria-hidden="true">8.3.1.</strong> Layout</a></li><li><a href="../display/03.02.MULT.html"><strong aria-hidden="true">8.3.2.</strong> Multiplexing</a></li><li><a href="../display/03.03.FULL.html"><strong aria-hidden="true">8.3.3.</strong> Full Solution</a></li></ol></li></ol></li><li><a href="../sensors/00.00.README.html"><strong aria-hidden="true">9.</strong> WIP - Sensors and I²C</a></li><li><a href="../nb/00.00.README.html"><strong aria-hidden="true">10.</strong> WIP - Non-blocking</a></li><li><a href="../nb/00.00.README.html"><strong aria-hidden="true">11.</strong> WIP - Interrupts</a></li><li><a href="../rtfm/00.00.README.html"><strong aria-hidden="true">12.</strong> WIP - Real time</a></li><li><a href="../hal/00.00.README.html"><strong aria-hidden="true">13.</strong> WIP - Creating a HAL</a></li><li class="affix"><a href="../appendix/explore.html">Explore</a></li><li class="affix"><a href="../appendix/gdb.html">GDB cheatsheet</a></li><li class="affix"><a href="../appendix/troubleshooting.html">General troubleshooting</a></li></ol>
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<a class="header" href="#semihosting" id="semihosting"><h1>Semihosting</h1></a>
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<p>Semihosting is a feature which allows targets without I/O support to use the I/O of the host.
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When the special <code>BKPT</code> instruction is reached, the host reads the characters directly from the micro:bit's memory.</p>
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<a class="header" href="#semihosting-is-slow" id="semihosting-is-slow"><h2>Semihosting is slow</h2></a>
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<p>The most important thing to remember about semihosting is that it is slow.
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The processor halts entirely for each operation, making each operation take 107 milliseconds.
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This means that if you are doing any time sensitive work, you should not use it for logging.
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<a href="http://blog.japaric.io/itm/">Check out this blog post for more information.</a></p>
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<a class="header" href="#gdb" id="gdb"><h2>GDB</h2></a>
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<p>The first thing to do is to enable semihosting in GDB.
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As before, we will add this to <code>.gdbinit</code> to avoid typing it every time.</p>
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<p><code>.gdbinit</code></p>
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<pre><code class="language-gdb">target remote :3333
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monitor arm semihosting enable
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load
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</code></pre>
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<a class="header" href="#openocd" id="openocd"><h2>OpenOCD</h2></a>
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<p>You may have incorrectly assumed at this point that the outpust would appear in GDB.
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Remember that GDB simply connects to OpenOCD to interface with the micro:bit.
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OpenOCD is very loud currently,
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so it will be quite hard to see the output of our micro:bit in the noise.
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Fix this by stopping and restarting it with logging dumped to a file.</p>
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<pre><code class="language-console">openocd -f interface/cmsis-dap.cfg -f target/nrf51.cfg -l /tmp/openocd.log
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</code></pre>
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<a class="header" href="#panic" id="panic"><h2>Panic</h2></a>
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<p>The easiest way to use semihosting is to use it for the <code>panic!</code> macro.</p>
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<p><code>Cargo.toml</code></p>
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<pre><code class="language-toml">panic-semihosting = ""
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</code></pre>
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<p>You can then see what happens if you add a <code>panic!</code> to your code:</p>
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<pre><pre class="playpen"><code class="language-rust">fn main() -> ! {
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panic!("test-panic");
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}
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</code></pre></pre>
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<pre><code>Open On-Chip Debugger 0.10.0
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Licensed under GNU GPL v2
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For bug reports, read
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http://openocd.org/doc/doxygen/bugs.html
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panicked at 'test-panic', src/hello-world/src/main.rs:27:5
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</code></pre>
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<a class="header" href="#stdout" id="stdout"><h2>stdout</h2></a>
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<p>Finally, this is how to write to stdout.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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extern crate cortex_m_semihosting as sh;
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use core::fmt::Write;
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use sh::hio;
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// -- snip --
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let mut stdout = hio::hstdout().unwrap();
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stdout.write_str("semitest\n\r").unwrap();
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// or
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writeln!(hio::hstdout().unwrap(), "Init").unwrap();
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#}</code></pre></pre>
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<p>Writing to stderr is just as easy.</p>
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