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Debugging with GDB

This is a short tutorial on how to use the cen64 debugger

Including Debugging Symbols

When compiling your C source files you need to include debug symobls. Using using GCC, you do this with the -g flag

mips64-elf-cc -c -g -o src/boot.o src/boot.c

This will include debugging symbols into src/boot.o. You can verify debugging symbols have been included in the .o by attempting to load it into gdb using gdb -q <path_to_.o>

This what it looks like when there are symbols

gdb -q src/boot.o
Reading symbols from src/boot.o...
(gdb)

This is what it looks like when there are no symbols

gdb -q src/boot.o
Reading symbols from src/boot.o...
(No debugging symbols found in src/boot.o)
(gdb)

Next you need to make sure the symbols are preserved in the linking process. This is the default behavoir of ln but if you find that the .o files that result from the linking process don't contain debugging symbols then they may be getting stripped out. This could either be from the -S flag used as in input to the ld program in they are getting removed as part of the linker script. If you are using spicy, it is configured to remove symbols by default. My fork of spicy it will not strip debug symbols.

The .o file generated from spicy or, if you aren't using spicy, the .o file used in objcopy to generate the rom file is the one you should load into gdb to get the symbols for the rom. objcopy should strip debugging symbols for you so there is no problem keeping them in during the final linker step.

Starting cen64 with the Debugger

To start cen64 with the debugger, simply run it with the -debug flag.

cen64 -debug localhost:2345 pifdata.bin game.n64

This will cause cen64 to pause and listen on port 2345 for an incoming connection from gdb. It will not run until gdb has connected.

Connecting with GDB

After you run cen64, you can then run and connect gdb to it. I found I had to use gdb-multiarch to get debugging with a MIPS processor to work. Before connecting to cen64 you need to start up gdb with the correct symbols

gdb-multiarch -q game.out

If everything worked you should see this

gdb-multiarch -q game.out
Reading symbols from game.out...
(gdb)

You can then type the following command into gdb

target remote localhost:2354

If everything is working it should look like this.

(gdb) target remote localhost:8080
Remote debugging using localhost:8080
0x80000000 in ?? ()
(gdb)

At this point gdb is working and ready to go. The debugger is paused in the boot code. You can set breakpoints using the break command.

This this example, main is the name of the entrypoint into the code.

break main

You can begin running your rom by using the command c.

c

This is not a tutorial on how to use gdb. If you need further help on how to debug you will have to look elsewhere. The recommened way to use this however is to find a gdb plugin for an IDE to handle the low level commands for you.