Milestone! First Graphics - Post-Victory Cleanup and Restoration of Accuracy

Summary

Milestone reached! The Joypad implementation in Step 0182 was the final piece. By running the emulator and pressing a key, the ROM's entropy loop was broken, the CPU proceeded to copy the graphics data to the VRAM, and, thanks to the "educational hack" of Step 0179, the Nintendo logo appeared on the screen. We have managed to render the first graphics. This Step performs "post-victory" cleanup: removes the forced rendering hack and debug logs to restore emulator accuracy and C++ core performance.

Hardware Concept: Restoring Precision

Debugging hacks are invaluable tools for diagnosing problems, but they are, by definition, inaccuracies. Our "educational hack" that forced the background to be rendered (LCDC Bit 0) allowed us to see the contents of the VRAM, but it went against the actual behavior of the hardware.

Depending on the hardware specifications, theLCDC register bit 0 (0xFF40)controls if the Background is enabled:

• Bit 0 = 0: Background disabled (blank screen)
• Bit 0 = 1: Background enabled (background is rendered)

The Cleaning Process:Now that we have confirmed that the system works end-to-end (Joypad works, CPU writes to VRAM, PPU renders), we need to remove this hack and trust that the game ROM will activate bit 0 of the LCDC at the correct time. If the logo still appears, it means that our emulation is accurate enough for the game to control the screen itself, just as it would on a real Game Boy.

Fountain:Pan Docs - LCD Control Register (LCDC), Bit 0: BG Display

Implementation

The cleanup consisted of three main tasks:

Task 1: Restore LCDC Bit 0 Verification

Insrc/core/cpp/PPU.cpp, the verification of Bit 0 of the LCDC that had been commented in Step 0179 was uncommented:

// --- RESTORING PRECISION (Step 0183) ---
// We reactivate the checking of Bit 0 of the LCDC.
// Now that the emulator is synced, the game should be able to
// to set this bit by itself at the right time.
if ((lcdc & 0x01) == 0) {
    // Background disabled, we do not render anything.
    return;
}

Task 2: Delete Debug Logs in PPU.cpp

All were removedprintfand static debug variables that had been added in Step 0180 to instrument the pixel pipeline:

• Static variable removeddebug_printed
• Removed the code block that printed detailed logs of the first pixels
• Deleted the code that markeddebug_printed = trueafter line 1
• Removed the include<cstdio>that is no longer needed

Task 3: Disable Triggered Plot System in CPU.cpp

The triggered trace system that had been implemented to diagnose logic loops was completely removed:

• Static variables removed:DEBUG_TRIGGER_PC, debug_trace_activated, debug_instruction_counter, DEBUG_INSTRUCTION_LIMIT
• Removed the code that activated the plot when the PC exceeded the trigger
• Removed code that printed logs for each instruction after activation
• Removed constructor code that reset debug counters
• Removed the include<cstdio>that is no longer needed

Design Decisions

Why delete the logs?Debug logs (especiallyprintf) within the critical emulation loop have a significant impact on performance. Every call toprintfrequires a kernel system call, which introduces latency and dramatically reduces execution speed. According to the project rules (cursorrules RULE.md, section 3.C), logging must be zero in the emulation loop except in explicit debug builds.

Why reset Bit 0?Precision is essential in emulation. Each hack reduces fidelity to the real hardware. If the emulator is accurate enough, the game should be able to control the screen itself without the need for hacks. If the logo still appears after this cleanup, we have proven that our emulation is accurate.

Affected Files

• src/core/cpp/PPU.cpp- LCDC Bit 0 verification restored, debug logs and cstdio include removed
• src/core/cpp/CPU.cpp- Removed trace system triggered and include from cstdio

Tests and Verification

The existing tests continue to pass, confirming that the cleanup did not break existing functionality:

$ pytest tests/test_core_ppu_rendering.py -v
======================== test session starts ========================
...
tests/test_core_ppu_rendering.py::TestPPURendering::test_render_scanline_with_tiles PASSED
tests/test_core_ppu_rendering.py::TestPPURendering::test_render_scanline_with_scroll PASSED
...
======================== 2 passed in 0.03s ========================

Visual Validation:When running the emulator withpython main.py roms/tetris.gband press a key, the Nintendo logo still appears. This confirms that:

1. The game correctly activates Bit 0 of the LCDC when ready to display graphics
2. Our emulation is accurate enough for the game to control the screen itself
3. The cleanup was successful: the code is free of hacks and performance improved

Performance Validation:The console is now clear of debug logs during normal execution, displaying only the "Heartbeat" if verbose mode is enabled. The emulation loop now runs faster without the overhead of theprintf.

Bottom line

After this cleanup, the emulator:

• ✅ Works correctly:The Nintendo logo continues to appear, confirming that the precision is enough for the game to control the screen
• ✅ It is free of hacks:The code respects the real behavior of the hardware, correctly verifying Bit 0 of the LCDC
• ✅ It has better performance:Without debug logs in the critical loop, the emulator runs faster
• ✅ You are ready for the next step:We can now implement the remaining hardware features (Window, Full Sprites, Audio, etc.) on a solid and precise basis

Milestone Achieved:We have managed to render the first graphics and demonstrate that the emulator is accurate enough for games to control the screen by themselves. This marks the end of the "get it started" phase and the beginning of the "implement the rest of the game's features" phase.

Next Steps

With the emulator running and rendering basic graphics, the next logical steps would be:

• WindowLayer:Implement the rendering of the Window layer (used for HUDs, menus, etc.)
• Complete Sprites:Fully implement the sprite system with all its features (priority, flip, palettes, etc.)
• Audio (APU):Implement the audio processor for the 4 channels (square 1 and 2, wave, noise)
• Optimizations:Optimize the rendering pipeline to further improve performance