Debugging

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This page describes the process of obtaining diagnostic information from SeaBIOS and for reporting problems.

Diagnostic information

SeaBIOS has the ability to output diagnostic messages. This is implemented in the code via calls to the "dprintf()" C function.

On QEMU these messages are written to a special debug port. One can view these messages by adding '-chardev stdio,id=seabios -device isa-debugcon,iobase=0x402,chardev=seabios' to the QEMU command line. Once this is done, one should see status messages on the console.

On coreboot these messages are generally written to the "cbmem" console (CONFIG_DEBUG_COREBOOT). If SeaBIOS launches a Linux operating system, one can obtain the cbmem tool from the coreboot repository and run "cbmem -c" to view the SeaBIOS diagnostic messages.

Additionally, if a serial port is available, one may compile SeaBIOS to send the diagnostic messages to the serial port. See the SeaBIOS CONFIG_DEBUG_SERIAL option.

Trouble reporting

If you are experiencing problems with SeaBIOS, it's useful to increase the debugging level. This is done by running "make menuconfig" and setting CONFIG_DEBUG_LEVEL to a higher value. A debug level of 8 will show a lot of diagnostic information without flooding the serial port (levels above 8 will frequently cause too much data).

To report an issue, please collect the serial boot log with SeaBIOS set to a debug level of 8 and forward the full log along with a description of the problem to the SeaBIOS mailing list.

Debugging with gdb on QEMU

One can use gdb with QEMU to debug system images. To do this, add '-s -S' to the qemu command line. For example:

qemu -bios out/bios.bin -fda myfdimage.img -s -S

Then, in another session, run gdb with either out/rom16.o (to debug bios 16bit code) or out/rom.o (to debug bios 32bit code). For example:

gdb out/rom16.o

Once in gdb, use the command "target remote localhost:1234" to have gdb connect to QEMU. See the QEMU documentation for more information on using gdb and QEMU in this mode.

When debugging 16bit code, also run the following commands in gdb:

set architecture i8086
add-symbol-file out/rom16.o 0xf0000

The second command loads the 16bit symbols a second time at an offset of 0xf0000, which helps gdb set and catch breakpoints correctly.

To debug a VGA BIOS image, run "gdb out/vgarom.o" add use the gdb command "add-symbol-file out/vgarom.o 0xc0000" to load the 16bit VGA BIOS symbols twice.

If debugging the 32bit SeaBIOS initialization code with gdb, note that SeaBIOS does self relocation by default. This relocation will alter the location of initialization code symbols. Disable CONFIG_RELOCATE_INIT to prevent SeaBIOS from doing this.