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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http:https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http:https://www.w3.org/1999/xhtml">
<head>
<title>Troubleshooting</title>
<link href="stylesheet.css" rel="Stylesheet" type="text/css" />
<script language="javascript" type="text/javascript" src="script.js"></script>
</head>
<body>
<div id="contentAll">
<div id="courseHead">
<h1>
Troubleshooting</h1>
</div>
<div id="pageAll">
<div id="pageBody">
<p style="color: red; font-weight: bold;">
This course has not yet been updated to work with the Raspberry Pi models B+
and A+. Some elements may not work, in particular the first few lessons about the
LED. It has also not been updated for Raspberry Pi v2.
</p>
<p>
There is little more satisfying than when an Operating System you've written works
perfectly, however this is, unfortunately, rarely the case. I lost count long ago
how many Operating Systems I've written that didn't work in making this course. This page contains advice
for what to do when things just aren't working. It is broken down into compile errors
that happen before you can even make the Operating System, load errors that prevent
your Operating System doing anything, and runtime errors, where your Operating System
doesn't do the correct thing. I've also added specific help on each of the tutorial
Operating Systems, such as things that can commonly go wrong. If you have a problem
that isn't explained here, and think others my be able to benefit from your experience,
send an email to me at <a href="mailto:[email protected]">[email protected]</a>, and
I will add it here.
It is also well worth taking a look at the <a href="http:https://www.raspberrypi.org/phpBB3/viewforum.php?f=72">Raspberry Pi forums</a> to see if anyone else has run into the same problem.</p>
<div class="ucampas-toc">
</div>
<h2 id="compile">
1 Compiler Errors</h2>
<p>
Compiler errors are errors that occur when the <span class="shellCodeInline">make</span>
command runs on your Operating System. I've also included some common warnings too.</p>
<h3 id="badinstruction">
1.1 Error: bad instruction</h3>
<pre class="sh"><var>source/file.s</var>:<var>8</var>: Error: bad instruction `<var>sdd r0,r1</var>'</pre>
<p>
This error occurs when you use a command that doesn't exist. First of all, check
that you haven't mistyped the command. If you're using condition codes as well as
other options such as an <span class="armCodeInline">str</span> command with a <span
class="armCodeInline">b</span> suffix to only store a byte, and a <span class="armCodeInline">
eq</span> suffix to only store if the last condition was equal, the correct
order is in fact <span class="armCodeInline">streqb</span> not <span class="armCodeInline">
strbeq</span>.</p>
<h3 id="immediate">
1.2 Error: immediate expression requires a # prefix</h3>
<pre class="sh"><var>source/file.s</var>:<var>32</var>: Error: immediate expression requires a # prefix -- `<var>add r0,1</var>'</pre>
<p>
This means that you're trying to use a constant number, such as adding the number
one, but forgot to put a # (e.g. <span class="armCodeInline">add r3,4</span> should
be <span class="armCodeInline">add r3,#4</span>. You must do so wherever you use
a constant on a command that normally uses registers. This is true even for calculated
constants such as <span class="armCodeInline">#3*4</span>.</p>
<h3 id="registerexpected">
1.3 Error: ARM register expected</h3>
<pre class="sh"><var>source/file.s</var>:<var>24</var>: Error: ARM register expected -- `<var>add 0,r1</var>'</pre>
<p>
This means that you typed something that was not a register, when a register was
expected. Double check your spelling, especially if you're using <span class="armCodeInline">
.req</span>. If you are, make sure you haven't used <span class="armCodeInline">.unreq</span>
between the <span class="armCodeInline">.req</span> and this command.</p>
<h3 id="pseudoop">
1.4 Error: unknown pseudo-op</h3>
<pre class="sh"><var>source/file.s</var>: Error: unknown pseudo-op: `<var>.suction</var>'</pre>
<p>
This error occurs when you use a pseudo operation that doesn't exit. Check your spelling.
</p>
<h3 id="invalidconstant">
1.5 Error: invalid constant (number) after fixup</h3>
<pre class="sh"><var>source/file.s</var>:<var>24</var>: Error: invalid constant (<var>c21</var>) after fixup</pre>
<p>
This error occurs when you use a constant which does not meet the requirements of the
function. The most common example of this is the <span class="armCodeInline">mov</span>
instruction, which only allows numbers which can be represented as an 8 bit number, shifted
left by an even number. For example c21<sub>16</sub> = 110000100001<sub>2</sub> and so cannot
be represented in a <span class="armCodeInline">mov</span>, but c20<sub>16</sub> =
110000100000<sub>2</sub> = 11000010<sub>2</sub> << 4, and so is valid in a <span class="armCodeInline">mov</span>.
Much the same rules apply to most constants in functions. Remember, to load in any constant,
use <span class="armCodeInline">ldr r0,=value</span>.
</p>
<h3 id="newline">
1.6 warning : end of file not at end of a line; newline inserted</h3>
<pre class="sh"><var>source/file.s</var>: warning : end of file not at end of a line; newline inserted</pre>
<p>
This means the last line in your file is not empty. You can ignore this, but to
fix it just add a new line at the end.
</p>
<h3 id="undefinedref">
1.7 undefined reference</h3>
<pre class="sh"><var>.text(+0x18)</var>: undefined reference to `<var>label</var>'</pre>
<p>
This means you've used a label which the linker can't find. This is probably due
to a misspelling. Remember that labels are case sensitive and that labels in different
files require <span class="armCodeInline">.globl</span> commands before they're
accessible in other files.</p>
<h3 id="sections">
1.8 `section' referenced in section `section' of build/file.o: defined in discarded section `section' of build/file.o</h3>
<pre class="sh">`<var>.trxt</var>' referenced in section `<var>.init</var>' of <var>build/main.o</var>: defined in discarded section `<var>.trxt</var>' of <var>build/main.o</var></pre>
<p>
This means that you've used a <span class="armCodeInline">.section</span> command,
but you've specified a section other than .init, .text or .data. Only these sections
are copied into the kernel.img file, any others are discarded, hence the error is saying
that some of your code was discarded. Check your spelling on <span class="armCodeInline">.section</span> commands.
</p>
<h3 id="nofiles">1.9 arm-none-eabi-ld: no input files</h3>
<pre class="sh"><var>arm-none-eabi-ld</var>: no input files</pre>
<p>This error means that the linker hasn't found your code. Double check you've got a source directory
with .s files within it like main.s. Make sure you haven't got something like main.s.txt.</p>
<h3 id="nomake">1.10 make: *** No targets specified and no makefile found. Stop.</h3>
<pre class="sh">make: *** No targets specified and no makefile found. Stop.</pre>
<p>
This error is caused by running make in the wrong directory. The command line must
have the same working directory as the file makefile which is in the template. Use
the 'cd' command to change directories, then run make again.</p>
<h3 id="win64">1.11 Windows Only: make: Interrupt/Exception caught (code = 0xc00000fd, addr = 0x425073)</h3>
<pre class="sh">make: Interrupt/Exception caught (code = <var>0xc00000fd</var>, addr = <var>0x425073</var>)</pre>
<p>
This is an error that can occur on Windows when YARGTO has been installed in a directory
with a space in it's name, for example: C:\Program Files (x86)\YAGARTO\. To fix,
please reinstall YAGARTO in a directory with no spaces such as C:\YAGARTO\.</p>
<h3 id="linux64">1.12 Linux 64 bit Only: arm-none-eabi-as: No such file or directory</h3>
<pre class="sh">bash: <var>arm-none-eabi-as</var>: No such file or directory</pre>
<p>This error is caused by running the Linux version of the toolchain on a 64
bit machine without 32 bit compatibility libraries. These can be retrieved easily
using:</p>
<pre class="sh"><kbd>sudo apt-get install ia32-libs</kbd></pre>
<h2 id="load">
2 Load Errors</h2>
<p>
Load errors are errors that occur that prevent your Operating System from giving
any output. This can be the hardest to diagnose and fix. Unfortunately, by their
nature, they give off no indication of what is wrong.</p>
<p>
The first thing you should check is that the tutorial answer does work. This confirms
that you're installing things correctly, that your Raspberry Pi is not physically
damaged and that your SD card works. If the answer doesn't work, make sure Linux
still does. If it doesn't you may have a problem with your SD card or Raspberry
Pi physically. Reimage the SD card or get a new one. If Linux does work but the
tutorial does not, you may not be installing the Operating System correctly. Double
check you're replacing kernel.img in the FAT partition of the SD card.</p>
<p>
If the answer does work but your attempt does not, then we know it is something
in your code. On the later tutorials, try altering the start of your code to turn
on the OK LED, just so you know if it boots at all. If not, double check that you
have some code in the .init section which branches into your .text section. Try
enabling the OK LED from the .init section.</p>
<p>
Ultimately what we need is some output. If you can get the LED to turn on from your
early code, then this is just a runtime error. If placing that code in the .init
section still doesn't enable output, it may be worth going back to the template and
copying in your code bit by bit until it stops working. Sometimes you never do find
the error; In the past I've ended up copying the entire code back into the template
and suddenly it worked.</p>
<h2 id="runtime">
3 Runtime Errors</h2>
<p>
Behind load errors, runtime errors are the hardest to diagnose and fix. These occur
when your Operating System just doesn't do what you want.</p>
<p>
The most important thing is to get information out of the system. The OK LED is very
useful for this. If the Operating System seems to stop, or get stuck, try turning on the LED
just before and just after various commands. If it turns on when just before an instruction, but
won't on the instruction afterwards, then we know this is the problem. Please remember that turning
on the LED will generally alter r0 to r3, so use <span class="armCodeInline">push {r0,r1,r2,r3}</span>
and <span class="armCodeInline">pop {r0,r1,r2,r3}</span> to preserve these registers. If you're in looping code,
try flashing the LED in the loop to see how many loops actually happen.</p>
<p>
If you're in the later tutorials make sure to use the screen for information. Write out text
about the current status, values, etc, in order to learn what is going on. Once
you've got some idea, have a look at the following common problems to see
if you can spot what has happened.</p>
<p>
Remember, think outside the box. Perhaps a function you wrote ages ago had a bug
you never noticed.</p>
<h3 id="wordalign">
3.1 Alignment</h3>
<p>
One of the most subtle runtime errors is the ARM alignment constraint. Any <span
class="armCodeInline">str</span> or <span class="armCodeInline">ldr</span> instruction
will not function correctly unless the computed address is a multiple of the size
of the data being read. For example, if you're using <span class="armCodeInline">ldr
r0,[r1,#2]</span>, then the value of <span class="armCodeInline">r1,#2</span>
must be a multiple of 4. If not unpredictable results occur. You should always be
able to guarantee this is this case. If you're referring to a label, make sure you
have a <span class="armCodeInline">.align 2</span> command BEFORE the label. This
will ensure that the label's address is a multiple of 2<sup>2</sup> = 4. You can
use <span class="armCodeInline">.align 3</span> to align to a multiple of 2<sup>3</sup>
= 8, etc.</p>
<h3 id="hang">
3.2 Hanging</h3>
<p>
A processor 'hangs' (stops) if it encounters a bad instruction or a bad address.
If your code gets stuck on a branch, load or store command, this is likely to be
the problem. You can use a condition around turning on the OK LED to check this.</p>
<h3 id="infinity">
3.3 Infinite Loops</h3>
<p>
Similarly to hanging, a processor can easily get stuck in loops. If the
processor reaches one of your loops, but never leaves, this could well be the
problem. Double check the conditions for leaving the loop will be satisfied.</p>
<h2 id="advice">
4 General Advice</h2>
<p>To help you, every time you compile a kernel, two extra files are compiled. kernel.list
contains a direct listing of all the assembly code, and kernel.map contains a map
of all your labels. You can use these files to mentally simulate the processor and
check it will do the correct thing. The processor starts at address 0 with all registers
in an undefined state. Try mentally checking that the processor will do what you
want. For things like alignment issues, you can double check everything is as it
should be with kernel.map.</p>
<h2 id="tutorials">
5 Tutorial Specific Advice</h2>
<h3 id="ok05stuck">
5.1 OK05 Doesn't Flash; Light Stays On</h3>
<p>
If you followed a previous version of this tutorial, a common problem on OK05 is
to have the OK LED stay on continuously rather than flashing a pattern. This is caused
by a change in the way the modern bootloaders load the kernel; they load it at address 0x8000 not 0.
Either replace makefile and kernel.ld with the ones currently in the template. Or alternatively
add the line <span class="shellCodeInline">kernel_old=1</span> to the file config.txt on the SD card, or create the file with this line in it.</p>
<h3 id="screen01blank">
5.2 Screen01 Displays Nothing</h3>
<p>
If you followed a previous version of this tutorial, this was a common problem.
The code in framebuffer.s has been altered to fix this problem. Specifically, it is
necessary to add 0x40000000 to the address of FrameBufferInfo before writing it to the mailbox.
</p>
<h3 id="screen02blank">
5.3 Screen02 Displays Nothing</h3>
<p>
See <a href="#ok05stuck">OK05 Doesn't Flash; Light Stays On</a>.</p>
<h3 id="input01blank">
5.4 Input01 Displays Nothing</h3>
<p>
First of all, check if this is an issue with the screen or the keyboard by running
the solution to Input02. It prints a message to the screen before receiving keyboard
input. If nothing shows on Input02, then see the help for <a href="#screen01blank">Screen01</a>.
If it does display, but you still can't type, then your keyboard may be incompatible with my USB driver.
Unfortunately, due to it's basic code, the driver doesn't support every keyboard.
Try to find other USB keyboards to use. I've personally tested 11 brands of keyboard,
of which 6 worked.</p>
</div>
<div id="pageFooter">
<hr />
<p>Spot a mistake? You can help improve this tutorial on <a href="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/chadderz121/bakingpi-www">GitHub</a>.</p>
<p><a rel="license" href="http:https://creativecommons.org/licenses/by-sa/3.0/deed.en_GB"><img alt="Creative Commons Licence" style="border-width:0" src="http:https://i.creativecommons.org/l/by-sa/3.0/88x31.png" /></a><br /><span xmlns:dct="http:https://purl.org/dc/terms/" property="dct:title">Baking Pi: Operating Systems Development</span> by <span xmlns:cc="http:https://creativecommons.org/ns#" property="cc:attributionName">Alex Chadwick</span> is licensed under a <a rel="license" href="http:https://creativecommons.org/licenses/by-sa/3.0/deed.en_GB">Creative Commons Attribution-ShareAlike 3.0 Unported License</a>.</p>
<p>Based on contributions at <a href="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/chadderz121/bakingpi-www">https://github.com/chadderz121/bakingpi-www</a>.</p>
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