Bochs IRC Chat Transcripts The Bochs community held an IRC open discussion chat on Sunday, February 1, 2004. We talked about current and future developments.Here are some transcripts of earlier conversations: October 13, 2002, April 7, 2002, June 19, 2001, May 30, 2001. Bochs at ISCA-35. Bochs is a highly portable open source IA-32 (x86) PC emulator written in C, that runs on most popular platforms. It includes emulation of the Intel x86 CPU, common I/O devices, and a custom BIOS. Bochs can be compiled to emulate many different x86 CPUs, from early 386 to the most recent x86-64 In. Bochs was a genius inventor and developed a robot he nicknamed Box, that could act for him. He joined the Canadian superhero team Alpha Flight's training group Beta Flight. Roger Bochs was the.
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Bochs is a steadily improving emulator for the x86 platform. It greatly improves OS development because the virtual machine restarts much quicker than real hardware, plus it offers detailed debugging functionality that can help greatly during kernel development.
It is strongly suggested to get two installations of Bochs - in addition to the out-of-the-box version (which might well be one of the binary downloads), you should also compile a second instance with the internal debugger enabled - so when something strange occurs that your 'normal' debugging tools can't trace, you can cross-check it with the debug version. You may also wish to enable (and use) the debug IO ports.
You sent your code pointer (EIP) to some uninitialized memory area. This means you either followed a NULL (or uninitialized) pointer, or you damaged the return address of your stack frame. Make your code more clean, test pointers before you follow them, initialize every pointer (especially those who are on the stack) and enable *-Wall* in GCC.
The CPU didn't manage to invoke an exception handler and would normally triple fault. This is probably due to a bad 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' asm instructions like idiv 0, or
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:
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 32-bit value. Go back to your chip's data sheet and double-check that your sizes are correct.
Many of you have said 'but ... I do not have an LDT and I read it wasn't mandatory!?'. You're right. And so is Bochs. This message usually means that your program tried to load a selector with some garbage value, which happened to have the 3rd bit (Table Indicator) set. The CPU will try to look up the descriptor in the LDT, but there's no LDT registered! In most cases, the error comes from some mispairing of push and pop on the stack, which lead to a non-selector value to be loaded in a segment register.
If you're still stuck, download the Bochs source package and search for the message you received. Then, maybe you can add extra information to the message-printing code (like the faulty offset of a segfault, the segment limit, etc). But keep away from modifying Bochs' operations! Every time I suspected a bug in Bochs, I was just misunderstanding the Intel Manual...
will cause interrupts at real intervals, but may overload the emulated OS with PIT interrupts and overflow the stack (if it's pre-emptible) Using sync=slowdown will also set it to real-time speeds, but with a constant amount of virtual clock cycles between each virtual second. For both the slowdown and original clock, you need to set the amount of instructions executed per second to the desired value.
Newer versions of Bochs allow you to specify the vendor string in the CPU tag:
To use Bochs to boot your custom kernel, search for the 'floppya' and 'boot' lines in the bochsrc configuration file. The following configuration allows you to use, and boot from, the 'real' floppy drive of your computer:
For Windows users, that should read:
But this is awfully slow, and puts lots of stress on the floppy when you are in a tight patch - make - boot cycle. Bochs offers the use of image files, including an interactive tool to create an image file (bximage.exe). Note that Bochs will emulate the floppy's internals even when the image is a device rather than a regular file ...
Bochs has several features that eases debugging. Many of them must be enabled via an configure switch:
Bochs has a command-line internal debugger, and a graphical interface on top of it. You must compile Bochs with the configuration option and then edit your bochsrc file in order to enable the GUI debugger. I use this line in my bochsrc file to enable graphical debugging in X:
In a Windows environment, add this line to your bochsrc.bxrc
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.
Some useful macros when Bochs is compiled with the I/O debug ports enabled (port_e9_hack: enabled=1 if Bochs 2.4 or newer, configure --port-e9-hack if not):
When you're using Bochs with the internal debugger, you can trigger the debugger via a facility called magic breakpoints. To trigger a breakpoint, you can insert xchg bx, bx (in GAS syntax, xchgw %bx, %bx) anywhere in the code and Bochs will trap into the debugger as soon as it executes it. On real hardware this has no effect as it merely replaces the BX register with itself.
You should put the following line in your Bochs configuration file to have it listen to magic breakpoints:
On older versions, enabling the debugger alone doesn't compile in magic breakpoint support, you will also need to specify --enable-magic-breakpoint when configuring the build on those versions.
When using the internal debugger, you may use the following command to switch CPUs:
I also found this was necessary on occasion in the GUI debugger, even though it has buttons for each CPU.
You can pass a file containing debug commands to automatically run whenever you start bochs with the internal debugger. (bochs -rc <file>)
Bochs places an automatic breakpoint just before the BIOS loads, this can be automatically skipped by putting continue as the first command in the said file.
When using the internal debugger, you may change this line in your Bochs configuration file:
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).
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
and enter your password. On other linux distros, try the equivalent.
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:
A few notes:
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.