Tutorial 13 - Debugger
Let's rock by implementing an interactive debugger in our exception handler! :-) Now that we have printf(), it shouldn't be hard.
$ qemu-system-aarch64 -M raspi3 -kernel kernel8.img -serial stdio Synchronous: Breakpoint instruction > x 0007FFF0: 13 60 09 00 00 00 00 00 24 10 20 3F 00 00 00 00 .`......$. ?.... > i x30 x30+64 00080804: D2800000 movz x0, #0x0 00080808: 94003D1C bl 0x8FC78 0008080C: 94003ECF bl 0x90348 00080810: D69F03E0 eret 00080814: D503201F 27 x nop >
A very minimal and simple debugger (~300 lines in C).
breakpoint a newly defined keyword. We can use this anywhere in our code where we want to invoke the debugger
dbg_decodeexc() similar to exc_handler in tutorial 11, decodes the cause of the exception and prints it
dbg_getline() yep, another low level library we're missing. We need a way to allow the user to edit command line
and return it as a string when he/she presses Enter. A minimal implementation, as usual
dbg_getoffs() this function parses the command line for arguments. Accepts hex, decimal number in
dbg_main() the main loop of the debugger.
Because it's small (~64k), yet supports all ARMv8.2 instructions, only depends on sprintf() (which we have now), I decided to use the Universal Disassembler for this tutorial. If you don't want to compile a disassembler into your debugger, simply set the DISASSEMBLER define 0 in top of dbg.c.
_vector table looks different. We have to save registers in memory with
dbg_saveregs, print out
the cause of the exception, and call our mini-debugger's main loop.
We'll test our shiny new
breakpoint keyword in C.