Low level qemu based debugging
I've to test some low level code on an ARM architecture. Typically experimentation is quite complicated on the real board, so I was thinking about QEMU.
What I'd like to get is some kind of debugging information like printfs or gdb. I know that this is simple with linux since it implements both the device driver for the QEMU Integrator and the gdb feature, but I'm not working with Linux. Also I suspect that extracting this kind of functionality from the Linux kernel source code would be complicated.
I'm searching from some simple operating system that already implements one of those features. Do you have some advice?
Answer
You don't need a target OS to debug code that's running inside QEMU -- QEMU already does that for you.
Specifically, QEMU supports remote debugging from GDB -- you can run QEMU with the appropriate command-line options and it will export an interface that a copy of GDB (running on the host machine) can connect to. At that point, you can debug the program in GDB pretty much just as if you were running it on the host machine.
http://wiki.osdev.org/GDB appears to have a bit more basic information; possibly not enough to completely get you started, but at least give you the basic idea and some terms to look for in the QEMU and GDB documentation. Skip over the bit about "Implementing GDB Stubs", which doesn't apply here since QEMU has one already, and start at the section on "Using Emulator Stubs". The short form is simply that you start QEMU with the -s option (export a GDB connection on localhost:1234) and the -S option (wait for a GDB "continue" command before starting execution), and then in GDB on your host you say target remote :1234 instead of run. Also, of course, you need to be using an ARM version of GDB rather than a native-x86 one.
(In addition, if you're willing to pay for a commercial solution, CodeSourcery's ARM toolchain has the IDE integration to set all of this up automatically, including support for "printf" to print into the debugger console. That works on a physical board, too, if you've got a hardware debugger. Usual disclaimer about me being a CodeSourcery employee applies -- but I do find it very easy to use.)
Update, 2012: CodeSourcery's toolchain is now called Mentor Graphics Sourcery CodeBench, but all the above still applies.