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Bochs: Difference between revisions
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=== 3rd exception with no resolution===
The CPU didn't manage to invoke an exception handler and would normally [[Triple Fault|triple fault]]. This is probably due to a bad [[Interrupt Descriptor Table|IDT]] register content, or a bad IDT descriptor. Sometimes (but less likely), it can also be due to a severe bug in your exception handler code. Check your exception works with "illegal"
<syntaxhighlight lang="asm">
push 0xf001
pop ds
mov ax, ~[ds:0x12345678]
</syntaxhighlight>
In several cases, there are other error messages prior to this one which can provide more details in the error. Some common messages that might be displayed:
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===I/O Operand Size===
Bochs performs some rather paranoid checks on I/O operand size. Reading a byte from port 0x1234 is usually not the same thing as reading a
=== fetch_raw_descriptor: LDTR.valid=0 ===
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display_library: x, options="gui_debug"
In
display_library: win32,
It seems that on Windows, the "option" flag (what the above line used to read) will be accepted, but the GUI window will not appear.
===I/O debugger macros===
Some useful macros when Bochs is compiled with the I/O debug ports enabled (<tt>port_e9_hack: enabled=1</tt> if Bochs 2.4 or newer, <tt>configure --port-e9-hack</tt> if not):
<syntaxhighlight lang="c">
//outputs a character to the debug console
#define BochsConsolePrintChar(c) outportb(0xe9, c)
//stops simulation and breaks into the debug console
#define BochsBreak() outportw(0x8A00,0x8A00); outportw(0x8A00,0x08AE0);
</syntaxhighlight>
===Magic Breakpoint===
When you're using
You should put the following line in your
magic_break: enabled=1
On older versions, enabling the debugger alone doesn't compile in magic breakpoint support, you will also need to
===Debugging SMP===
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Bochs places an automatic breakpoint just before the BIOS loads, this can be automatically skipped by putting <tt>continue</tt> as the first command in the said file.
===Debugging Triple Faults===
When using the internal debugger, you may change this line in your Bochs configuration file:
reset_on_triple_fault 0
This line disables the emulator reset on a Triple fault, enabling you to debug the code after a Triple fault occured (Very useful while implementing paging).
==Compiling Bochs from Source==
Bochs has many compile-time configuration options, some of which conflict, and therefore a binary distribution of Bochs may not be suitable for your purposes. I found it was best to compile my own copy of Bochs to be sure I had the features that I needed. Also, you should consider using the CVS snapshot version of Bochs if the released version is old and not working for you. I found this was necessary up until version 2.4 was released, for example. On Ubuntu, you may have to run
<syntaxhighlight lang="bash">
sudo apt-get install libgtk2.0-dev
</syntaxhighlight>
and enter your password. On other linux distros, try the equivalent.
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The array of Bochs configuration options can be confusing, and you cannot assume the defaults are going to be sensible. These are the options I use, this can get you started:
<syntaxhighlight lang="bash">
./configure --enable-smp \
--enable-cpu-level=6 \
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--disable-plugins \
--disable-docbook \
--with-x --with-x11 --with-term --with-sdl2
</syntaxhighlight>
A few notes:
* If you are on Windows, that last line should probably read "--with-win32".
* On Linux, using SDL as the display library over X11 is preferable as the performance appears to increase greatly on some setups
* Bochs has GDB stub support, and its own internal debugger. These cannot be compiled into the same Bochs binary. The internal debugger is very useful, its flag is --enable-debugger
* The GDB stub in Bochs does not support SMP, last time I checked.
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* Post-2.4.2 several of the CPU specific options were folded into the CPU-level specification and are therefore deprecated. They have been removed from the example above.
* The default compile does not support x86-64, --enable-x86-64 will turn it on
* On many Linux distributions it is possible to install Bochs via a package manager. For example, on distributions that use apt-get we can do
<syntaxhighlight lang="bash">
sudo apt-get install bochs
sudo apt-get install bochs-x
</syntaxhighlight>
to install Bochs and the X11 plugin (which may crash on ubuntu/linux mint: install the sdl plugin and use sdl instead of x as the display library in this case). Note that there is a big chance that the graphical debugger is not enabled in the binaries from the package manager.
==See Also==
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===External Links===
*[http://bochs.sourceforge.net Bochs Homepage]
*[http://bochs.sourceforge.net/doc/docbook/user/internal-debugger.html Bochs internal debugger commands documentation]
*[http://www.codeproject.com/system/MakingOS_2.asp CodeProject article on using Bochs]
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