gearboy-debugging

Game Boy / Game Boy Color Debugging with Gearboy

Overview

Debug Game Boy, Game Boy Color, and Super Game Boy games using the Gearboy emulator as an MCP server. Control execution (pause, step, breakpoints), inspect the SM83 CPU and hardware (LCD, APU, SGB, sprites), read/write memory, disassemble code, trace instructions, and capture screenshots — all through MCP tool calls.

MCP Server Prerequisite

IMPORTANT — Check before installing: Before attempting any installation or configuration, you MUST first verify if the Gearboy MCP server is already connected in your current session. In the default mode, call debug_get_status directly. If Gearboy was intentionally started with --mcp-router, call get_tool_info with {"name":"debug_get_status"}, then call execute_tool with {"name":"debug_get_status","arguments":{}}. A valid response from either workflow means the server is active and ready.

Only if neither workflow is available or the call fails, you need to help install and configure the Gearboy MCP server:

Installing Gearboy

Run the bundled install script (macOS/Linux):

bash scripts/install.sh

This installs Gearboy via Homebrew on macOS or downloads the latest release on Linux. It prints the binary path on completion. You can also set INSTALL_DIR to control where the binary goes (default: ~/.local/bin).

Alternatively, download from GitHub Releases or install with brew install --cask drhelius/geardome/gearboy on macOS.

Connecting as MCP Server

Configure your AI client to run Gearboy as an MCP server via STDIO transport. Example for Claude Desktop (~/Library/Application Support/Claude/claude_desktop_config.json):

{
  "mcpServers": {
    "gearboy": {
      "command": "/path/to/gearboy",
      "args": ["--mcp-stdio"]
    }
  }
}

Replace /path/to/gearboy with the actual binary path from the install script. Add --headless before --mcp-stdio on headless machines.

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Debugging Workflow

1. Load and Orient

load_media → get_media_info → get_cpu_status → get_screenshot

Start every session by loading the ROM, confirming it loaded correctly (MBC type, ROM/RAM size, CGB/SGB flags), then checking CPU state and taking a screenshot to understand the current game state. If a .sym or .noi file exists alongside the ROM, symbols are loaded automatically.

Load additional symbols with load_symbols or add individual labels with add_symbol. Gearboy supports RGBDS, GBDK-2020, WLA-DX, no$gmb, SDCC/NoICE (.noi), EQU, and generic symbol formats.

2. Pause and Inspect

Always call debug_pause before inspecting state. While paused:

• CPU state: get_cpu_status — registers A, F, B, C, D, E, H, L, SP, PC, flags Z/N/H/C, IME, halt state, CGB double speed
• Disassembly: get_disassembly with a start/end address range
• Call stack: get_call_stack — current subroutine hierarchy
• Memory: read_memory with area name (ROM0, ROM1, VRAM, RAM, WRAM0, WRAM1, WRAM, OAM, IO, HIRAM) and address/length

3. Set Breakpoints

Use breakpoints to stop execution at points of interest:

Breakpoints support 3 memory area types: rom_ram, vram, and io.

Important: Read/write breakpoints stop with PC at the instruction after the memory access.

Manage breakpoints with list_breakpoints and remove_breakpoint.

4. Step Through Code

After hitting a breakpoint or pausing:

After each step, call get_cpu_status and get_disassembly to see where you are.

5. Trace Execution

The trace logger records CPU instructions interleaved with hardware events (LCD, APU, I/O, bank switching). Start the trace logger from the emulator's debugger window, then:

1. set_trace_log with enabled: true to start recording (optionally filter event types)
2. Let the game run or step through code
3. set_trace_log with enabled: false to stop (entries are preserved)
4. get_trace_log to read recorded entries

Tracing is essential for understanding timing-sensitive code, interrupt handlers, and hardware interaction sequences.

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Rewind (Time Travel Debugging)

• get_rewind_status reports whether rewind is enabled, how many snapshots are available, total capacity, and how many seconds are currently buffered.
• rewind_seek jumps to a specific buffered snapshot. The emulator must be paused first.
• Use rewind when you need to compare two nearby execution points without managing manual save states.

Typical flow:

1. debug_pause
2. get_rewind_status
3. rewind_seek to an earlier snapshot
4. get_cpu_status and get_disassembly to inspect the restored point
5. debug_continue or keep stepping from there

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Hardware Inspection

LCD Controller

• get_lcd_registers — all LCD registers: LCDC, STAT, SCY, SCX, LY, LYC, DMA, BGP, OBP0, OBP1, WY, WX with decoded bit fields. CGB registers: KEY1, VBK, HDMA, BCPS, BCPD, OCPS, OCPD, SVBK
• get_lcd_status — current LCD mode (0-3), screen enabled, LY, LYC match, CGB info

APU (Audio)

• get_apu_status — all 4 audio channels: Square 1 (with sweep), Square 2, Wave, Noise: volume, frequency, envelope, duty cycle, wave RAM, panning, master volume

SGB (Super Game Boy)

• get_sgb_status — SGB active flag, mask mode, multiplayer state (player count/current), last command code/data, transfer countdown/destination, border animation state, effective palettes (4×4 colors as hex), and attribute map (20×18 palette assignments)
• SGB memory areas available via read_memory: SGB_TILES (border tile patterns), SGB_MAP (border tilemap), SGB_BPAL (border palettes), SGB_SPAL (system palettes), SGB_ATF (attribute files), SGB_AMAP (attribute map), SGB_EPAL (effective palettes)

Sprites (OAM)

• list_sprites — all 40 OAM sprites: position, tile index, attributes (priority, X/Y flip, palette, CGB bank)
• get_sprite_image — individual sprite rendered as PNG

Screen Capture

• get_screenshot — current rendered frame as PNG

Use screenshots after stepping or continuing to see the visual impact of changes.

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Memory Areas

Use list_memory_areas to get the full list:

When SGB mode is active, additional areas are available:

WRAM and HIRAM are the most common locations for game variables (lives, health, score, position).

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Common Debugging Scenarios

Finding an Interrupt Handler

1. toggle_irq_breakpoints to enable breaking on the target IRQ (VBlank, LCD STAT, Timer, Serial, Joypad)
2. debug_continue to run until the IRQ fires
3. get_cpu_status + get_disassembly to see the handler code
4. get_call_stack to see how deep you are
5. add_symbol to label the handler address and any subroutines it calls

The Game Boy interrupt vectors are at fixed addresses: VBlank=$0040, LCD STAT=$0048, Timer=$0050, Serial=$0058, Joypad=$0060.

Diagnosing Graphics Corruption

1. debug_pause → get_lcd_registers — check LCDC (enable bits), SCX/SCY (scroll), WX/WY (window)
2. get_lcd_status — verify LCD mode, LY position
3. read_memory on VRAM to inspect tile data and tile maps
4. list_sprites — check OAM for incorrect positions, tiles, or attributes
5. Set read/write breakpoints on VRAM addresses to catch corruption source
6. get_screenshot to see the current visual state

Analyzing a Subroutine

1. set_breakpoint at the subroutine entry point
2. debug_continue → when hit, get_cpu_status
3. Step through with debug_step_into / debug_step_over
4. After each step: check registers, read relevant memory
5. add_symbol for the routine and any called subroutines
6. add_disassembler_bookmark to mark interesting locations

Tracking a Variable

1. add_memory_watch on the variable's address — watches are visible in the emulator GUI
2. Set a write breakpoint with set_breakpoint (type: write) on that address
3. When hit, get_disassembly reveals what code is modifying it
4. get_call_stack shows the call chain leading to the write

Timing Analysis

1. toggle_irq_breakpoints to break on Timer or VBlank
2. Start the trace logger from the debugger window
3. get_trace_log to see the interleaved CPU + hardware events
4. get_lcd_registers to check timer and LCD timing configuration
5. Correlate interrupt fires with code execution in the trace

Investigating CGB Features

For Game Boy Color games:

1. get_cpu_status — check if CGB double speed mode is active
2. get_lcd_registers — inspect CGB-specific registers (KEY1, VBK, HDMA, BCPS/BCPD, OCPS/OCPD, SVBK)
3. read_memory on VRAM with bank selection to inspect both VRAM banks
4. list_sprites — check CGB-specific attributes (VRAM bank, CGB palette)

Debugging SGB Features

For Super Game Boy games:

1. get_sgb_status — check if SGB is active, inspect mask mode, command state, transfer status
2. get_media_info — verify is_sgb flag in ROM header
3. read_memory on SGB_TILES / SGB_MAP / SGB_BPAL to inspect border tile data, tilemap, and palettes
4. read_memory on SGB_SPAL to inspect system palettes loaded via PAL_TRN
5. read_memory on SGB_ATF to inspect attribute files loaded via ATTR_TRN
6. get_sgb_status → check effective_palettes and attribute_map for game colorization
7. get_screenshot to see the rendered SGB output (256×224 with border)

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Organizing Your Debug Session

• Symbols: Use add_symbol liberally to label addresses you've identified — makes disassembly readable
• Bookmarks: Use add_disassembler_bookmark for code locations and add_memory_bookmark for data regions
• Watches: Use add_memory_watch for variables you're tracking across steps
• Save states: Use save_state / load_state to snapshot and restore emulator state at interesting points
• Screenshots: Capture visual state with get_screenshot after significant changes