Category: Uncategorized

This week was a non-amusing transfer from C to C++ and adding the GBA BIOS.

BIOS Map

The Nintendo BIOS contains the basic code needed to initalize and use the GBA. After that the BIOS hands over control to the game that’s plugged in. My initial thought was to load the file into the game, then I remembered the golden word which haunted me last week – mmap. It was used to create the memory which is mapped to the guest VM.

So, I mmap-ed the BIOS file into the memory, then added it in.

void* biosRom =
        mmap(0, BIOS_SIZE, PROT_READ | PROT_EXEC, MAP_SHARED, biosFd, 0);

Now its upto the kernel to manage it, and not my headache (for now).

Then I tried mapping the BIOS into the memory of the VM, and it segfaulted instantly. I changed some of the parameters to allow writes, and then it didn’t segfault, but the mmap didn’t go through.

After looking into the documentation, here’s the mistake I was mapping.

    struct kvm_userspace_memory_region memory_region = {
        .slot = 0,
        .userspace_addr = (unsigned long long)gbaMemory->onboardMemory,
        .guest_phys_addr = 0x02000000,
        .memory_size = ONBOARD_MEM_SIZE};

I was setting the slot to 0 for both the onboard memory (or where I was putting my code), and the BIOS page. They are actually different slots of memory, and need to be initialized as separate slots.

C++

I’m not used to C (and KVM and any of this) and it shows, so its probably a good idea to shift to C++ while I still can.

I wanted exceptions to handle unexpected failures, but I also didn’t want to model for releasing resources – a job better left for the compiler. C++ seems to be a better idea. I removed all the GOTOs, and got around to C++, then finally put an exception that helped me with my sanity.

class InitializationError : public std::exception {
    private:
    std::string message;
    public:
    InitializationError(std::string);
};

Being able to use exceptions along with constructors is useful. I’m aware that there’s a performance penalty, but it should be fine as long as I spend minimal time processing in my code (the kernel handles running the guest VM, not my code).

What’s next

The registers are still seemingly useful and garbage at the same time, but we shall see. Next week will mostly be travel and continued refactoring while I try to learn more, so next week will be week 5 essentially.