Educational Hack: Force Background Rendering for Visual Diagnosis

Summary

VICTORY! The deadlock is broken.The analysis ofHeartbeatreveals thatL.Y.is cycling correctly (LY=53, LY=107, LY=7), confirming that the native loop architecture in C++ has solved the root synchronization problem. However, the screen is still blank. The diagnosis ofHeartbeatshows thatLCDC=0x80, which means that the game has turned on the LCD (Bit 7=1) but keeps the background layer disabled (Bit 0=0) during initialization.

This Step implements a temporary "educational hack" on the C++ PPU to force the rendering of the background layer, ignoring the state of LCDC Bit 0. This allows us to check if the graphics data is already in VRAM before the game activates the background, visually confirming that our emulator is working correctly and that the problem is simply that the game has not yet reached the part where it activates the background.

Hardware Concept: Decoupling Game Logic

Game Boy games often turn on the LCD (LCDC Bit 7 = 1) but keep specific layers turned off (LCDC Bit 0 = 0for the background,Bit 1 = 0for sprites) while performing configuration tasks. This is a common technique during initialization:

1. The game turns on the LCD to start PPU synchronization.
2. Meanwhile, the game copies graphical data to VRAM (Nintendo logo tiles, sprites, etc.).
3. The game configures color palettes and other PPU registers.
4. OnlyafterOnce everything is ready, the game activates the graphic layers (LCDC Bit 0 = 1).

Our PPU is simulating this correctly, resulting in a blank screen because the game has explicitly told it not to draw the background. This is not an emulator bug; This is expected behavior based on hardware specifications.

According toBread Docs, the recordLCDC(0xFF40) controls the PPU with the following relevant bits:

• Bit 7:LCD Display Enable (1 = LCD on, 0 = LCD off)
• Bit 0:BG & Window Display Priority (1 = Background enabled, 0 = Background disabled)

The value0x80in hexadecimal it is1000 0000in binary:

• Bit 7 = 1:The LCD is on. The PPU is running and generating scan lines.
• Bit 0 = 0:The background is disabled. The PPU does not draw the background layer, resulting in a blank screen.

To debug and confirm that the CPU has successfully copied the Nintendo logo tiles to VRAM, we are going to temporarily "disobey" the game and force the PPU to draw the background layer whenever the LCD is on, regardless of the state of Bit 0.

Implementation

The educational hack consists of temporarily commenting on the LCDC Bit 0 check in the methodrender_scanline()of the PPU. This allows the PPU to read and render tile data from VRAM even when the game has the background disabled.

Modification in PPU.cpp

Updated the educational hack comment to reflect Step 0179 and added an explanation of the diagnosis based on theHeartbeat:

// --- EDUCATIONAL HACK (Step 0179) ---
// Comment this check to force the background to be rendered
// even if the game has it disabled (LCDC Bit 0 = 0).
// This allows us to see if the data is in VRAM during initialization.
// According to Pan Docs, Bit 0 of the LCDC controls whether Background is enabled:
// 0 = Background disabled (blank screen)
// 1 = Background enabled
//
// DIAGNOSIS: The Heartbeat displays LCDC=0x80 (Bit 7=1, Bit 0=0), which means
// that the game has turned on the LCD but keeps the background disabled for
// initialization. This temporary hack allows us to verify if the data
// graphics are already in VRAM before the game activates the background.
/*
if ((lcdc & 0x01) == 0) {
    // Background disabled, we could fill with white.
    return;
}
*/

Design Decisions

• Temporary Hack:This change is explicitly temporary and educational. Once we confirm that the data is in VRAM, we should revert this hack and wait for the game to activate the background correctly.
• Preservation of the Original Code:The original code is commented (not removed) to facilitate its later restoration.
• Diagnostic Documentation:A comment was added explaining the diagnosis based on theHeartbeatfor future reference.

Affected Files

• src/core/cpp/PPU.cpp- Updated the educational hack comment to reflect Step 0179 and added explanation of the diagnosis ofLCDC=0x80

Tests and Verification

This change does not require new unit tests, as it is a temporary debugging modification. The objective is visual verification:

1. Recompiling the C++ Module:

   .\rebuild_cpp.ps1

2. Running the Emulator:

   python main.py roms/tetris.gb

3. Expected Visual Verification:
   • If the graphics data is in VRAM, we should see the Nintendo logo scrolling down the screen.
   • If the screen is still blank, it means that the data has not yet been copied to VRAM or there is another problem in the rendering pipeline.

Note:This educational hack is a diagnostic tool. Once we confirm the VRAM status, we should revert this change and allow the game to control background rendering based on hardware specifications.

Sources consulted

• Bread Docs:Section on LCDC register (0xFF40) and PPU control
• Heartbeat Diagnosis:Analysis of LCDC register status during emulator execution

Educational Integrity

Next Steps

• [ ] Recompile the C++ module with the updated educational hack
• [ ] Run the emulator and visually check if graphics appear
• [ ] If graphics appear: Confirm that the emulator is working correctly and that the problem was simply the background activation timing
• [ ] If the screen is still blank: Investigate the rendering pipeline (reading VRAM, decoding tiles, writing to the framebuffer)
• [ ] Once the diagnosis is confirmed, revert the educational hack and allow the game to control the background rendering according to the hardware specifications