Gameboy Emulator Development - Part 15

Summary of the video (Part 15: Window + Gamepad for a Game Boy Emulator)

1) PPU update: implement the Window rendering

• Added a window line variable to the PPU context (tracking the currently rendered horizontal line within the window).
• During PPU rendering, the emulator increments this window line when LY falls within the window’s vertical range, using LCD/window coordinate logic such as:
  • LY >= WY and LY < WY + 144
• Reset window_y to 0 at VBlank, so the counter restarts every frame.

2) Window visibility logic

• Implemented a helper (e.g., “window visible”) to quickly determine whether the window should affect the current pixel.
• This checks LCD/window enable and screen bounds using X/Y comparisons (including checks like x within limits and y < y_res).

3) Pipeline rendering: load window tiles instead of background tiles

• In the PPU pipeline’s background/window fetch area, added a function such as pipeline load window tile.
• When pixels fall inside the window region and the window is enabled:
  • Compute the tile row via window_y / 8
  • Compute the tile column via logic like ((x + 7) - WX), including bounds checks
  • Fetch the tile id from the window tile map address:
    • LCDC win map area
  • Select the correct tile data bank/ID offset:
    • If using the second tile set, increment the tile index by 128
• Added a missing guard condition (return when the window is not visible).
• Result: the test ROM matched expectations.

4) Game input: implement the Joypad / gamepad register (0xFF00)

• Implemented Game Boy joypad behavior using the FF00 I/O register:
  • Selection bits choose whether reads refer to:
    • Direction buttons (D-pad)
    • Action buttons (A/B/Start/Select)
• Added gamepad.h and gamepad.c with:
  • A gamepad_state structure storing button states: start, select, a, b, up, down, left, right
  • Selection/flag fields controlling whether the register is targeting direction or buttons
• gamepad_get_output() builds the FF00 return value:
  • Starts from a default like 0xCF
  • Uses the Game Boy convention: 0 means pressed, 1 means not pressed
  • Clears specific bits depending on which keys are pressed and the current selection mode.

5) Wire gamepad into the emulator bus / IO

• Updated emulator IO/MMIO handling:
  • Read at 0xFF00 returns gamepad_get_output()
  • Write to 0xFF00 updates selection bits via gamepad_set_cell(...)

6) UI event handling: map SDL keys to joypad inputs

• Updated ui.c to process SDL key down/up events and update the gamepad state.
• Mapped inputs:
  • A button → Z
  • B button → X
  • Start → RETURN
  • Select → TAB
  • D-pad → arrow keys
• Added a function like ui_on_key(key, down) to update the gamepad struct accordingly.
• Verified with games:
  • Dr. Mario: player switching works; arrow keys move and play correctly.
  • Tetris: game plays, confirming PPU + input integration.
  • Super Mario Land: reported stopping due to an unresolved issue (to be fixed later).

7) Next planned work

• Next video: implement MBC (Memory Bank Controller) to support larger ROMs and cartridge types requiring banking/RAM (e.g., Metroid, Zelda).

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Key speakers / sources

• Main speaker: “Low Level Devil” (host narrating the emulator development series)
• Sources referenced: Game Boy hardware documentation for the 0xFF00 joypad register, and emulator GitHub/test ROMs used for validation.