Debug: Null Pointer Check on PPU

Summary

Added a temporary diagnostic check in the methodrender_scanline()of the PPU to confirm whether the pointer to the MMU is null when the method is called. This verification usesprintfto issue a critical message confirming if the problem is at the Cython layer, specifically in how the pointer is passed from the Cython wrapper to the PPU constructor in C++.

Hardware Concept

In a hybrid Python/C++ emulator, communication between layers requires careful handling of the pointers. The data flow from Python to C++ goes through several layers:

1. Python:An instance is createdPyMMU, which contains a valid C++ pointer (MMU*).
2. Python:An instance is createdPyPPUpassing the objectPyMMU.
3. Cython:The builder ofPyPPUmust "unwrap" the objectPyMMUto extract the pointerMMU*raw file it contains and pass it to the PPU constructor in C++.
4. C++:The PPU constructor receives the pointerMMU*and stores it in a member of the class.

If at any point in this chain the pointer becomes corrupted or is not passed correctly, the PPU will attempt to accessing invalid memory, causing aSegmentation Fault. Checking for null pointer withprintfIt allows us to identify exactly where the chain fails.

Implementation

Added a temporary null pointer check to the start of the methodrender_scanline()that prints a critical message if the pointermmu_is null. This verification is binary and definitive:

Modified components

• PPU.cpp:was added#include <cstdio>to useprintf.
• PPU.cpp:Added verificationif (this->mmu_ == nullptr)at the beginning ofrender_scanline()which prints a message and returns early to avoid the crash.

Code added

#include <cstdio>

//...

void PPU::render_scanline() {
    // CRITICAL: Verify that mmu_ is not nullptr before accessing
    // ADD THIS CRITICAL CHECK TO THE BEGINNING OF THE METHOD
    if (this->mmu_ == nullptr) {
        printf("[PPU CRITICAL] The pointer to the MMU is NULL! The problem is in the Cython layer.\n");
        return; // We leave early to avoid the crash and see the message.
    }
    
    // ... rest of the render_scanline method ...
}

Design decisions

It is usedprintfratherstd::coutbecause it is safer for crash debugging (it does not depend on static objects from the C++ standard library that may not be initialized during a crash). The message is explicit and direct to quickly identify the problem if it occurs.

Note:This verification is temporary and will be removed once the problem is confirmed or ruled out. of the null pointer. If the message appears, we will confirm that the problem is in the Cython wrapper. If it does not appear and the crash persists, we will know that the problem is deeper.

Affected Files

• src/core/cpp/PPU.cpp- Added#include <cstdio>and null pointer checking withprintfinrender_scanline()

Tests and Verification

This verification is validated by actual emulator execution:

1. Recompilation:The C++ module must be recompiled with.\rebuild_cpp.ps1.
2. Execution:Run the emulator withpython main.py roms/tetris.gb.
3. Analysis of the result:
   • If the message appears:Confirm 100% that the problem is in the Cython wrapper (ppu.pyx) and we will know exactly where to correct it.
   • If the message does NOT appear and there is still a crash:Our hypothesis is incorrect and the problem is deeper (although this is highly unlikely).

Compiled C++ module validation:This check is executed directly in the code compiled from C++, so any message printed will confirm that the C++ code is running correctly and that the pointer is effectively null at that moment.

Sources consulted

• Cython documentation:https://cython.readthedocs.io/- C++ pointer management from Python
• Principles of Pointer Debugging: Based on general knowledge of debugging Python/C++ hybrid systems

Educational Integrity

Next Steps

• [ ] Recompile the C++ module:.\rebuild_cpp.ps1
• [ ] Run the emulator:python main.py roms/tetris.gb
• [ ] Analyze the result:
  • If the message appears: Review and correct the Cython wrapper (ppu.pyx)
  • If it does NOT appear: Investigate deeper causes of the Segmentation Fault
• [ ] Remove the temporary check after the problem is confirmed