# Geargrafx Romhacking

> Hack, modify, and translate TurboGrafx-16 / PC Engine / SuperGrafx ROMs using the Geargrafx emulator MCP server. Provides workflows for memory searching, value discovery, cheat creation, data modification, sprite/text finding, and translation patching. Use when the user wants to create cheats, find game values in memory, modify ROM data, translate a PC Engine game, patch game behavior, create ROM hacks, discover hidden content, change sprites or graphics, find text strings, do infinite lives or health hacks, search for score or item counters, or reverse engineer data structures in TurboGrafx-16 or PC Engine games. Also use for any ROM hacking, memory poking, or game modification task involving Geargrafx.

- Skill: `drhelius/geargrafx-romhacking` (Agent Skill, multi-file: 9 files)
- Install (CLI): `npx skillmds@latest add drhelius/geargrafx-romhacking`
- Raw SKILL.md: https://api.skillmd.com/api/skills/drhelius/geargrafx-romhacking/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Product & Planning
- Author: drhelius (https://skillmd.com/u/drhelius)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/drhelius/geargrafx-romhacking

---


# TurboGrafx-16 / PC Engine ROM Hacking with Geargrafx

## Overview

Hack, modify, and translate TurboGrafx-16, PC Engine, and SuperGrafx ROMs using the Geargrafx emulator as an MCP server. Search memory for game variables, create cheats, find text strings for translation, locate sprite data, and reverse engineer data structures — all through MCP tool calls. Use save states as checkpoints and fast forward to reach specific game states.

## MCP Server Prerequisite

**IMPORTANT — Check before installing:** Before attempting any installation or configuration, you MUST first verify if the Geargrafx MCP server is already connected in your current session. In the default mode, call `debug_get_status` directly. If Geargrafx 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 Geargrafx MCP server:

### Installing Geargrafx

Run the bundled install script (macOS/Linux):

```bash
bash scripts/install.sh
```

This installs Geargrafx 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](https://github.com/drhelius/Geargrafx/releases/latest) or install with `brew install --cask drhelius/geardome/geargrafx` on macOS.

### Connecting as MCP Server

Configure your AI client to run Geargrafx as an MCP server via STDIO transport. Example for Claude Desktop (`~/Library/Application Support/Claude/claude_desktop_config.json`):
```json
{
  "mcpServers": {
    "geargrafx": {
      "command": "/path/to/geargrafx",
      "args": ["--mcp-stdio"]
    }
  }
}
```
Replace `/path/to/geargrafx` with the actual binary path from the install script. Add `--headless` before `--mcp-stdio` on headless machines.

### Hardware Documentation (References)

PC Engine hardware documentation is available in the [references/](references/) directory. Load them into your context when you need data formats, memory layout, or hardware details.

| Reference | File | Load when... |
|---|---|---|
| HuC6280 CPU | [references/huc6280_cpu.md](references/huc6280_cpu.md) | CPU registers, MPR mapping, timer, I/O, opcode reference |
| Instruction Set | [references/huc6280_instructions.md](references/huc6280_instructions.md) | Opcode reference, addressing modes, cycle counts |
| PSG | [references/huc6280_psg.md](references/huc6280_psg.md) | 6-channel sound: waveform, noise, LFO, DDA |
| HuC6270 VDC | [references/huc6270_vdc.md](references/huc6270_vdc.md) | BAT, sprites, scroll, DMA, VRAM layout |
| HuC6260 VCE | [references/huc6260_vce.md](references/huc6260_vce.md) | Palette, dot clock, color format |
| HuC6202 VPC | [references/huc6202_vpc.md](references/huc6202_vpc.md) | Video Priority Controller (SuperGrafx): window, priority |
| Memory Map | [references/memory_map.md](references/memory_map.md) | Full memory map: MPR pages, I/O, WRAM, VRAM, ROM banking |

---

## 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 HuC6280):

```
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 (HuC6280 is little-endian).

---

## 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 screenshot availability.

### 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 — PC Engine games often use custom character maps stored in VRAM tiles, not ASCII
2. `read_memory` across ROM scanning for known byte patterns
3. Use `memory_find` with `text` for literal strings or `hex_bytes` for encoded byte sequences
4. Set read breakpoints on suspected text addresses with `set_breakpoint` (type: read) to confirm they're used for rendering
5. `get_screenshot` to correlate displayed text with memory contents

### Sprite and Graphics Data

1. `list_sprites` to see all 64 sprites with position, size, pattern, and palette info
2. `get_sprite_image` to capture individual sprite images as PNG
3. `get_huc6270_registers` to find BAT address, sprite attribute table settings
4. `read_memory` on VRAM and SAT areas to analyze tile and sprite data
5. Set read breakpoints on sprite data addresses to find the rendering code
6. `get_screenshot` before/after modifications to see visual changes

Reference the VDC hardware docs ([references/huc6270_vdc.md](references/huc6270_vdc.md)) for BAT and sprite format details.

### Data Tables and Structures

1. `debug_pause` → `get_disassembly` around code that loads data
2. Look for LDA/LDX/LDY instructions with absolute or indexed 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 $0042 or STA $0042)
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_huc6280_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. Find text rendering code by setting read breakpoints on VRAM areas
3. Trace back to find the character mapping table
4. `read_memory` to dump the font/character table from VRAM tiles
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` with `text` for literal strings or `hex_bytes` 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 with IDs and sizes. Common areas:

| Area | Description | Use |
|---|---|---|
| WRAM | Working RAM (8KB, 32KB on SGX) | Game variables, state, stack |
| ZP | Zero Page ($0000-$00FF) | Fast variables: lives, health, score, position |
| ROM | Game ROM | Code and static data |
| CARD RAM | HuCard RAM | Extra RAM on some cards |
| BRAM | Backup RAM (2KB) | Save data, high scores |
| PALETTES | VCE color table (512 bytes) | Color palette data |
| VRAM | VDC video RAM (64KB) | Tiles, BAT, sprite patterns |
| SAT | Sprite Attribute Table (512 bytes) | Sprite positions, sizes, patterns |
| CDROM RAM | CD-ROM working RAM | CD game data |
| ADPCM | ADPCM sample RAM (64KB) | Voice/sample playback data |
| ARCADE | Arcade Card RAM | Arcade Card game data |

Zero Page ($0000-$00FF) is the most common location for game variables (lives, health, score, position) because HuC6280 zero page addressing is faster.

---

## 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.