megaduck-info

Mega Duck / Cougar Boy Console Info

A collection of technical information and resources for the Mega Duck console brought together in one place. Much of it researched by others, some by me (minor sound reg fixes, laptop model docs, some ports and patches, etc).

Web version

References

• Sean Riddle
• MAME: 1 2 3
• MiSTer

Credits / Thanks

Sean Riddle, Ruud van Falier, Nitro2k, Toxa, Youkal3, Zwenergy, Inside Gadgets, Kuddel, ManCloud

Software

Homebrew Games / Programs

• Browse releases (and find source code links) on the MegaDuck Homebrew and ROM Hack showcase.
  • There’s also a view for showing what’s new

ROM Patches

Patches for running Game Boy games on the MegaDuck

• See ROM patches in the MegaDuck Homebrew and ROM Hack showcase

Homebrew Music Tools

• Droneboy for MegaDuck

Emulators/FPGA Re-implementations

• Super Junior SameDuck:
  • Fork of SameBoy with support for MegaDuck Handheld and Laptop models (Development oriented)
  • Has correct audio emulation
• MiSTer Game Boy/MegaDuck core:
  • Has support for MegaDuck Handheld and Laptop models models
  • Has correct audio emulation
• MAME, Arcade DB Entry, Cart list xml
• SameDuck branch of SameBoy (modified version of SameBoy, not pre-built)
• Analogue Pocket OpenFPGA core

Development Tools

SDKs / Build related

C language

• GBDK-2020: C dev kit, supports Mega Duck as a build target
• Cate83 - A C-like programming language compiler for sm83 cpus (MegaDuck, Game Boy)
  • There are several example game programs for the MegaDuck

ASM

• hardware.inc for MegaDuck: Hardware constants and addresses for use with asm/RGBDS
  • Newer hardware.inc base version
• Asm83 - An Assembler, linker and librarian for retro CPUs including the sm83 (MegaDuck, Game Boy)

Music / SFX

• Mega Duck port of the hUGE Driver Game Boy music driver
• Mega Duck port of the CBTFX Game Boy Sound FX driver

  Peripherals

• Sample code for interfacing with the MegaDuck Laptop model Keyboard, RTC and speech module (Quique and Junior Computer). See related partial docs
  • GBDK examples
  • RGBDS asm examples
• Audio recordings of the 6 built-in speech phrases for the German and Spanish MegaDuck Laptop models

Hardware

Flash Cartridges

• picoDuck by zwenergy an Open-source RP2040-based flash cart
  • Alternate firmware which supports up to 1MB ROMs with the MD2 (only) type MBC (required for some homebrew and rom patches)
• A2 Heaven multi-flash cart
• Inside Gadgets 32K flash cart - Compatible with GBXCart flasher
• 3D Printable cartridge shell

Hardware Schematics / PCB Info

• Laptop Model Schematics

Adapters

• Adapter to plug Game Boy (flash) carts into the MegaDuck
  • Alternate sub board design which has the cart sticking up instead of down

Display mod / LCD Header info

• Display adapter board by zwenergy for Game Boy DMG style display kits
• Ruud van Falier’s DMG IPS adapter mod
  • 3D printable bracket for the DMG IPS display
• Game Boy Pocket (MGB) to Mega Duck display adapter

Laptop model Gamepad

• Info for building a replacement Gamepad / “Mouse” for the MegaDuck Laptop model

Serial Link Port

• The link port on the Mega Duck has the same pin order and signals as the Game Boy, but a different connector style (bare header).
• The serial control registers have the same address, control flags and behavior as a the Game Boy

With the use of a connector style adapter a Mega Duck and a Game Boy can exchange data over their link ports.

• Breakout board for Game Boy style link port cable receptacle

Diagram showing the link port receptacle*1 pinout when looking at the side of the console:

      Front of console
-----------------------------------
                                   |
--------      -----------------    |
 volume |    | CLK  SIN  VCC*2 |   |
 dial   |    | GND   -   SOUT  |   |
--------      -----------------    |
                                   |
-----------------------------------                                   
       Back of console

• 1: View and pinout is of the connector receptacle, not of the cable
• 2: For most uses the VCC pin should not be connected

Cartridge Pinout

• Mega Duck Cart Dumping
• Scans:
  • OEM (Pile Wonder) Front / Back
  • IG 32K Front / Back

AC Adapters

Handheld

• 3.5 mm exterior, 1.35mm interior
• 6 volts, centrer NEGATIVE polarity

Laptop

• 3.5 mm exterior, 1.35mm interior
• 9 volts, centrer POSITIVE polarity
• Example picture

USB Video Streaming

• MegaDuck Interceptor Modified firmware for the GB Interceptor which allows it to work with a MegaDuck. The device streams emulated video from the console over USB. Requires cart slot adapter for the device to connect to the MegaDuck.

MBCs / Bank Switching

MBC Controllers

Laptop model System ROM MBC (CEFA Super Quique, Hartung Super Junior Computer):

• Informal MBC Number: 0xE0 (SuperJuniorSameDuck emulator)
• Informal extension: .md0
• Register: 0x1000
  • ROM Bank
    • Selected by writing (0 - 15) in Lower Nibble (mask 0x0F)
    • Bank Size/Region: Switches the full 32K ROM region-
  • SRAM Bank (on secondary memory cart plugged into memory cart slot)
    • Selected by writing (0 - 3) in Upper Nibble (mask 0x30)
    • Bank Size/Region: 8k mapped at 0xA000 - 0xBFFF
• Laptop Games: MegaDuck_Laptop_BilderLexikon.md0, MegaDuck_Laptop_DataBank.md0 (requires SRAM cart)
• System ROMs: MegaDuck_Laptop_SystemROM_German.md0, MegaDuck_Laptop_SystemROM_Spanish.md0
• Note: Uses a delay of ~41 M-Cycles (executed from WRAM) after writing the bank switch before resuming execution from ROM. Unclear if required.

OEM Games:

• 32K with NO switchable banks
  • Sometimes with extension: .bin, but that may also be used for banked ROMs with some emulators
  • Handheld Games: Arctic Zone, Bomb Disposer, Magic Maze, Pile Wonder, Street Rider, The Brick Wall, Trap and Turn, Vex
• 32K switchable banks
  • Sometimes with extension: .md1
  • Informal MBC Number: 0xE1 (SuperJuniorSameDuck emulator)
  • Register: Bank selected by writing 0 - 1 to 0xB000
  • Bank Size/Region: Switches the full 32K ROM region
  • Handheld Games: Puppet Knight, Suleiman’s Treasure
• Upper switchable 16K banks
  • Sometimes with extension: .md2
    • Variant of MD2 + SRAM: extension is .md2s
  • Informal MBC Number: 0xE2 (SuperJuniorSameDuck emulator)
  • Register: Bank selected by writing 1 - 3 or 1-7 to 0x0001 depending on total ROM size (64K or 128K)
  • Bank Size/Region: 16K in the Upper ROM region 0x4000 - 0x7FFF (lower 16K at 0x0000 - 0x3FFF is fixed bank 0)
  • Laptop Games: MegaDuck_Laptop_MusikModul.md2
  • Handheld Games: 2nd Space, Ant Soldiers, Armour Force, Beast Fighter, Black Forest Tale, Captain Knick Knack, Commin Five in One, Duck Adventures, Four in One, Magic Tower, Railway, Snake Roy, Worm Visitor, Zipball

MBC type per game is according to Reddit

Implementation in emulators

Game Boy ROMs contain an embedded meta-data header which (usually) indicates the type of MBC (Memory Bank Controller) used. This allows emulators to automatically emulate the correct cart hardware (the Game Boy hardware itself ignores this and doesn’t need it since the cart hardware is present).

OEM Mega Duck ROMs lack this header which means that emulators have to use other methods to infer what type of cart MBC is required by a given ROM.

The main methods for selecting the right MegaDuck cart MBC:

• By File Extension (.md1 = 32K banks, .md2 = Upper 16K Banks, .bin = 32k no mbc)
  • Analogue Pocket OpenFPGA MegaDuck Core
• By hashes of the ROMs
  • MAME (partially)
• By emulating both MBC styles at the same time in case a game uses either. This works since the two MBCs use a different register address.
  • MAME (partially), MiSTer

System ROMs

• Handheld Model: No System/Boot ROM
• CEFA Toys Super Quique Laptop Model (Spanish): Partial disassembly of the System ROM
• Hartung Super Junior Computer (German)

Porting from Game Boy to Mega Duck

Developing for the Mega Duck is mostly identical (cpu & integrated peripherals) to the Original Game Boy though it has a couple changes listed below.

Register and flag names will mostly follow those in the Game Boy dev hardware.inc, but may have some GBDK-isms.

Most of this research was done by others, I’ve added a small amount.

Summary of Hardware changes versus the Game Boy:

Physical / Hardware:

• Different cartridge pinout / connector
• Different serial link port connector shape

Programming:

• No Boot ROM
• Cartridge Boot Logo: not present on Mega Duck
• Cartridge Header data: not present on Mega Duck
  • Checksum header: not present on Mega Duck. Do not apply checksum to ROM after building as on the Game Boy
• Program Entry Point: 0x0000 (on Game Boy: 0x0100 )
• Initial registers
  • Handheld Models: (AF, BC, DE, HL, SP) are random at startup (due to lack of boot rom) with some affinity toward 0xFFFF / 0x0000
  • Laptop Model: Based on System ROM design (no stack init before first call/ret) it appears SP is NOT 0x0000 at power-on
    • Possibly it’s something like 0xFFFE or lower which would allow pushing and popping the stack before init without a resulting crash
• Serial control
  • Laptop Mode: Some Game Boy emulators do not support triggering SC to send before loading the send value into SB, however this is a design pattern that the Laptop System ROM uses to interface with the hardware. So it needs to be implemented to more closely follow the hardware (same for Game Boy).
  • For Super Junior SameDuck it is implemented as follows: Writing 0x81 to SC -> Resets serial master clock=low, Does NOT clock the first bit immediately. Instead waits for serial_master_clock to complete a full low->hi->low transition before the first bit gets clocked on negative edge.
• Display registers address and flag definitions: Some changed (details below)
• VRAM data:
  • The user program should clear it before use.
  • For Handhelds: since a boot ROM does not run on startup it means Tile Map and Pattern data are not cleared and will have (consistent) random data.
  • For Laptop models: The System ROM will not clear VRAM before launching a user program from the system menu. The font patterns left in VRAM on program startup can be used to tell the difference between the Super Quique (Spanish) and Super Junior (German) laptop models. See example code.
• Audio registers address and flag definitions: Some changed (details below)
• Different MBC bank switching register addresses than Game Boy MBCs

Laptop:

• Built-in System ROM
• Additional Cart Slot for add-on Memory Card (access not yet documented)
• Peripherals attached via Serial/Link port
  • Keyboard + Piano Keys
  • RTC
  • Cart
  • Detached Gamepad
  • Mechanism to launch cartridges in the Cart Slot
  • External communication port for the printer (type not yet documented)

Sound Register Value/Data Changes

These changes should be kept in mind when porting Sound Effects and Music Drivers written for the Game Boy.

1. Registers NR12, NR22, NR42, and NR43 have their contents nybble swapped.
   • To maintain compatibility with the Game Boy the value to write (or the value read) can be converted this way: ((uint8_t)(value << 4) | (uint8_t)(value >> 4))
2. Register NR32 has the volume bit values changed.
   • Game Boy: Bits:6..5 : 00 = mute, 01 = 100%, 10 = 50%, 11 = 25%
   • Mega Duck: Bits:6..5 : 00 = mute, 01 = 25%, 10 = 50%, 11 = 100%
   • To maintain compatibility witht he Game Boy the value to write (or the value read) can be converted this way: (((~(uint8_t)value) + (uint8_t)0x20u) & (uint8_t)0x60u)

Sound Register Changes Table

• Addr Change means deviation from the expected linear incrementing register address order per sound channel that the Game Boy registers follow. The shuffled order on the Mega Duck can break assumptions in Game Boy sound drivers which attempt to write each channel as a sequential block using an incrementing register pointer.

Reg	Alt Name	Game Boy	Mega Duck	Data Change	Addr Change
NR10	rAUD1SWEEP	0xFF10	0xFF20
NR11	rAUD1LEN	0xFF11	0xFF22		Addr +1
NR12	rAUD1ENV	0xFF12	0xFF21	nybble swap	Addr -1
NR13	rAUD1LOW	0xFF13	0xFF23
NR14	rAUD1HIGH	0xFF14	0xFF24
NR21	rAUD2LEN	0xFF16	0xFF25		Addr -1
NR22	rAUD2ENV	0xFF17	0xFF27	nybble swap
NR23	rAUD2LOW	0xFF18	0xFF28
NR24	rAUD2HIGH	0xFF19	0xFF29
NR30	rAUD3ENA	0xFF1A	0xFF2A
NR31	rAUD3LEN	0xFF1B	0xFF2B
NR32	rAUD3LEVEL	0xFF1C	0xFF2C	volume bit swizzle
NR33	rAUD3LOW	0xFF1D	0xFF2E		Addr +1
NR34	rAUD3HIGH	0xFF1E	0xFF2D		Addr -1
NR41	rAUD4LEN	0xFF20	0xFF40
NR42	rAUD4ENV	0xFF21	0xFF42	nybble swap	Addr +1
NR43	rAUD4POLY	0xFF22	0xFF41	nybble swap	Addr -1
NR44	rAUD4GO	0xFF23	0xFF43
NR50	rAUDVOL	0xFF24	0xFF44
NR51	rAUDTERM	0xFF25	0xFF46		Addr +1
NR52	rAUDENA	0xFF26	0xFF45		Addr +1

Graphics Register Bit Flag Changes

LCDC Flag	Game Boy	Mega Duck		Purpose
LCDCF_B_ON	.7	.7	(same)	Bit for LCD On/Off Select
LCDCF_B_WIN9C00	.6	.3		Bit for Window Tile Map Region Select
LCDCF_B_WINON	.5	.5	(same)	Bit for Window Display On/Off Control
LCDCF_B_BG8000	.4	.4	(same)	Bit for BG & Window Tile Data Region Select
LCDCF_B_BG9C00	.3	.2		Bit for BG Tile Map Region Select
LCDCF_B_OBJ16	.2	.1		Bit for Sprites Size Select
LCDCF_B_OBJON	.1	.0		Bit for Sprites Display Visible/Hidden Select
LCDCF_B_BGON	.0	.6		Bit for Background Display Visible Hidden Select

Graphics Register Address Changes

Register	Game Boy	Mega Duck
LCDC	0xFF40	0xFF10
STAT	0xFF41	0xFF11
SCY	0xFF42	0xFF12
SCX	0xFF43	0xFF13
LY	0xFF44	0xFF18
LYC	0xFF45	0xFF19
DMA	0xFF46	0xFF1A
BGP	0xFF47	0xFF1B
OBP0	0xFF48	0xFF14
OBP1	0xFF49	0xFF15
WY	0xFF4A	0xFF16
WX	0xFF4B	0xFF17

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