Sega Master System / Game Gear / SG-1000 ROM Hacking with Gearsystem

Overview

Hack, modify, and translate Sega Master System, Game Gear, and SG-1000 ROMs using the Gearsystem emulator as an MCP server. Search memory for game variables, create cheats, find text strings for translation, locate sprite data, inspect VRAM/CRAM, and reverse engineer data structures — all through MCP tool calls. Use save states or rewind as checkpoints and fast forward to reach specific game states. The emulator also supports Game Genie and Pro Action Replay codes natively. Hardware documentation is available in the references/ directory and also at SMS Power! Development Documents.

MCP Server Prerequisite

IMPORTANT — Check before installing: Before attempting any installation or configuration, you MUST first verify if the Gearsystem MCP server is already connected in your current session. Call debug_get_status — if it returns a valid response, the server is active and ready.

Only if the tool is not available or the call fails, you need to help install and configure the Gearsystem MCP server:

Installing Gearsystem

Run the bundled install script (macOS/Linux):

bash scripts/install.sh

This installs Gearsystem 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/gearsystem on macOS.

Connecting as MCP Server

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

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

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

Hardware Documentation (References)

SMS/GG/SG-1000 hardware documentation is available in the references/ directory. Load them into your context when investigating specific hardware. Additional community documentation is available at SMS Power! Development Documents.

Core Technique: Memory Search

Memory search is the primary tool for ROM hacking. It uses a capture → change → compare cycle to isolate memory addresses holding game values.

The Search Loop

1. memory_search_capture    → snapshot current memory state
2. (change the value in-game using controller_button, fast forward, etc.)
3. memory_search            → compare against snapshot to find changed addresses
4. Repeat 2-3 until only a few candidates remain
5. read_memory / write_memory → verify and modify the found addresses

Search Operators and Types

memory_search supports these operators: <, >, ==, !=, <=, >=

Compare types:

• previous — compare current value to last captured snapshot (most common)
• value — compare current value to a specific number
• address — compare current value to value at another address

Data types: hex, signed, unsigned

Example: Finding the Lives Counter

1. memory_search_capture                         → snapshot with 3 lives
2. Lose a life in-game (play or use controller_button)
3. memory_search (operator: <, compare: previous) → values that decreased
4. memory_search_capture                         → snapshot with 2 lives
5. Lose another life
6. memory_search (operator: <, compare: previous) → narrow further
7. Or use: memory_search (operator: ==, compare: value, value: 1)
   → find addresses holding exactly 1
8. write_memory on the candidate address to set lives to 99
9. get_screenshot to verify the change took effect

Example: Finding a Score Counter

Score values are often stored as multi-byte (16-bit little-endian on Z80):

1. memory_search_capture                                → snapshot at score 0
2. Score some points in-game
3. memory_search (operator: >, compare: previous)       → values that increased
4. memory_search_capture
5. Score more points
6. memory_search (operator: >, compare: previous)       → narrow down
7. read_memory on candidates — look for values matching current score
8. write_memory to set a custom score

For 16-bit values: the low byte is at address N, high byte at N+1 (Z80 is little-endian). Many SMS games use BCD (Binary Coded Decimal) for score display — each nibble holds a single digit (e.g., score 1234 stored as $12 $34).

Fast Forward for Efficiency

Use fast forward to speed through gameplay when you need to trigger in-game changes:

set_fast_forward_speed (4 = unlimited)
toggle_fast_forward              → enable
(play through the game section)
toggle_fast_forward              → disable

This is essential when you need to reach specific game states without waiting in real-time.

Save States as Checkpoints

Save states are critical for ROM hacking — they let you save your position and retry modifications:

select_save_state_slot (1-5)     → pick a slot
save_state                       → save current state
(try modifications)
load_state                       → revert if something breaks

Use different slots for different game states (e.g., slot 1 = start, slot 2 = boss fight, slot 3 = specific level).

list_save_state_slots shows all slots with ROM name, timestamp, and validity.

Rewind as an Alternative

The emulator also records continuous snapshots into a rewind ring buffer. Use get_rewind_status to check availability, then rewind_seek to jump to any recorded point without manual save/load. This is especially useful for quickly reverting after a failed memory write — pause, seek back a few snapshots, and retry.

Finding and Modifying Game Data

Text and String Discovery

To find text strings for translation or modification:

1. Determine the character encoding — SMS/GG games often use custom character maps stored as tile patterns in VRAM, not ASCII
2. read_memory across ROM areas scanning for known byte patterns
3. Use memory_find_bytes to search for specific byte sequences
4. Set read breakpoints on suspected text addresses with set_breakpoint (type: read) to confirm they're read by the text rendering routine
5. get_screenshot to correlate displayed text with memory contents
6. read_memory (area: vram) to examine the tile patterns used for font rendering

Sprite and Graphics Data

1. list_sprites — view all 64 sprites with positions, sizes, and pattern indices
2. get_sprite_image — capture individual sprite images as PNG
3. read_memory (area: vram) — examine tile pattern data and sprite attribute table
4. read_memory (area: cram) — inspect palette colors
5. get_vdp_registers — check pattern generator and sprite attribute table base addresses
6. Set read breakpoints (area: vram) on sprite data to find the rendering code
7. get_screenshot before/after modifications to see visual changes

VRAM and Tile Data

The VDP uses VRAM for all graphics:

• Tile patterns: 8x8 pixel tiles, 4 bits per pixel (16 colors per palette)
• Nametable: Background tile map referencing tile patterns
• Sprite Attribute Table (SAT): Y position, X position, tile index for each sprite

Use read_memory (area: vram) with the base addresses from get_vdp_registers to access these.

Data Tables and Structures

1. debug_pause → get_disassembly around code that loads data
2. Look for LD instructions with indexed or indirect addressing — these point to data tables
3. read_memory at the target addresses to dump the table contents
4. add_memory_bookmark to mark discovered data regions
5. add_symbol to label data table entry points for future reference

Creating Cheats

Infinite Lives / Health

1. Find the address using the search loop (above)
2. Set a write breakpoint: set_breakpoint (type: write) on the address
3. debug_continue → when it hits, get_disassembly to see the decrement code
4. Note the instruction (e.g., DEC (HL) or LD (addr),A)
5. Option A: Periodically write_memory to reset the value (simple poke cheat)
6. Option B: Identify the decrement routine for a NOP patch

Watching Values in Real-Time

Use add_memory_watch on discovered addresses. Watches appear in the emulator's GUI memory editor, letting you monitor values as the game runs — useful for verifying cheats work across different game situations.

Write Breakpoint Technique

The most powerful cheat-finding technique:

1. Find the variable address via memory search
2. set_breakpoint (type: write) on that address
3. debug_continue — the emulator stops when the game writes to that address
4. get_z80_status + get_disassembly reveals the exact code modifying the value
5. get_call_stack shows what triggered the write
6. You now know exactly where and how the game manages that variable

Translation Workflow

1. Identify the Font System

1. get_screenshot of a screen with text
2. read_memory (area: vram) — find tile patterns used for font characters
3. Set read breakpoints on tile data addresses to trace back to the rendering code
4. get_disassembly to find the character mapping table (byte value → tile index)
5. add_symbol to label the font table and rendering routine

2. Find String Data

1. Look for sequential text bytes in ROM using read_memory with large ranges
2. Use memory_find_bytes to search for known byte patterns
3. Cross-reference with the character table to decode strings
4. add_memory_bookmark to mark each string location

3. Measure Space Constraints

ROM hacking translations must fit within existing space:

1. read_memory to determine how much space each string occupies
2. Check for string terminators (commonly $00, $FF, or length-prefixed)
3. If the translation is longer, look for unused ROM space or abbreviate

4. Apply and Test

1. write_memory to patch translated strings into memory
2. get_screenshot to verify rendering
3. save_state before each change so you can load_state if it breaks
4. Test all screens that display modified text

Memory Map Quick Reference

Use list_memory_areas to get the full list. Key areas:

SMS Memory Layout (rom_ram)

RAM ($C000-$DFFF) is the most common location for game variables (lives, health, score, position).

Bookmarks and Organization

Keep your hacking session organized:

• add_memory_bookmark — mark discovered data regions, variable locations, string tables
• add_memory_watch — track values that change during gameplay
• add_symbol — label addresses in disassembly for readability
• add_disassembler_bookmark — mark code routines you've identified

Use list_memory_bookmarks, list_memory_watches, list_symbols, list_disassembler_bookmarks to review.

Persisting Changes

Changes made via write_memory to ROM areas are applied to the emulator's in-memory copy only — they are not persisted to the ROM file on disk. To create a permanent patch, use command-line tools (e.g., a binary patch script) to apply the discovered modifications to the actual ROM file.