Green Light: Final Start

Summary

Opcode 0x08 (LD (nn), SP) fix unblocked the CPU stream, allowing the game to progress beyond address 0x2B10. The diagnostic log confirmed that the PC had advanced to 0x2B87, indicating that the CPU is executing valid code. All instrumentation was removed debug (Sniper, Stethoscope, Radioactive Markers) to allow the emulator run at native speed and the game completes its initialization.

Hardware Concept

This step is not about Game Boy hardware, but about thedevelopment and debugging process.

The Debugging Cycle:

During the development of an emulator, it is common to add debugging instrumentation (logs, traces, checkpoints) to diagnose problems. However, this instrumentation comes at a cost:

• Performance:Theprintfand additional memory accesses slow down performance significantly, especially when executed millions of times per second.
• Precision:The logs can change the timing of the execution, affecting the synchronization between components (CPU, PPU, Timer).
• Noise:The massive output of logs can hide relevant information and saturate system buffers.

The Release Phase:

Once a fix is confirmed to work (in this case, opcode 0x08), it is critical to remove all debug instrumentation to allow the emulator to run at real speed. This allows:

• Let the game complete its initialization sequence in real time.
• That the synchronization between components is precise.
• That performance is optimal for the user experience.

Implementation

Removed all debugging code blocks added in previous steps:

Cleanup in CPU.cpp

• Deleted#include <cstdio>:No longer needed for logs.
• Removed Sniper block (Step 0228):The block that monitored the range 0x2B10-0x2B20 and generated register status logs.
• Removed printf from opcode 0x08 (Step 0232):The radioactive tracer that confirmed the execution of the opcode.

Cleaning in viboy.py

• Removed Stethoscope block (Step 0230):The periodic diagnosis that It printed the vital status (PC, LCDC, VRAM) every 60 frames.

Result

The code is clean and ready to run at native speed. The emulator can now:

• Execute instructions without logging overhead.
• Maintain precise synchronization between CPU, PPU and Timer.
• Allow the game to complete its initialization in real time.

Affected Files

• src/core/cpp/CPU.cpp- Removed debug blocks (Sniper, printf del 0x08) and#include <cstdio>
• src/viboy.py- Removed Stethoscope block (periodic diagnosis)

Tests and Verification

The verification was carried out by observing the behavior of the emulator:

• Before cleaning:The Stethoscope log showed that the PC had advanced from 0x2B10 to 0x2B87, confirming that the opcode 0x08 fix worked.
• After cleaning:The emulator runs without logs, allowing speed native. The game is expected to complete its initialization and display the title screen.

Execute command:

python main.py roms/tetris.gb

Expected result:

• Clean console (no massive logs).
• Emulator running at real speed (60 FPS).
• Tetris title screen visible (if initialization completes successfully).

Sources consulted

• Bread Docs:Game Boy Pan Docs- General hardware reference

Note: This step is mainly about code cleanup and does not require documentation consultation specific hardware.

Educational Integrity

Next Steps

• [ ] Run the emulator and verify that it displays the Tetris title screen
• [ ] If the screen is still green/white, reactivate Sniper at the new PC address
• [ ] Continue implementing missing opcodes if necessary
• [ ] Optimize performance once the game is running properly