This article was originally written in 2019 as part of a technical Game Boy series to be published as a bundle.

What is a game console without its games? A cafe without beer, according to Belgians. Carefully preserved game cartridges are the bread and butter of any retro game collection. As for the Game Boy family, there are four official different types of cartridges to collect: original gray Game Boy carts, black Game Boy carts that increased the color palette on the Game Boy Color, translucent Game Boy Color exclusive carts, and the smaller Game Boy Advance ones.

Inserting Game Boy Color games into an older Game Boy system is not impossible, but the notch on the top right of the cartridge prevents the power switch from sliding all the way to the left. Due to changes in aesthetics, owners of the Pocket could boot up these games, only to be greeted with a screen that states your Game Boy is incompatible with the games.

Provided the game did boot in the first place. Remember air-blowing into the cartridge to clear out any dust, because the boot-up sequence somehow got stuck at a badly scrambled Nintendo logo? These cartridge slots were not spring-loaded: there was no Zero Insertion Force (ZIF) connection smoothly accepting the cartridge that should avoid angry gamers from jamming the game in the handheld and potentially damaging things. Eventually, Nintendo did migrate towards a ZIF slot with the later Nintendo DS and Switch consoles.

Blowing the cartridge might never have fixed anything. Instead, simply removing the cartridge and reinserting it would have been enough for the connectors to correctly touch each other. In fact, according to Nintendo's NES Game Pak Troubleshooting webpage, overenthusiastic blowers might even damage the cart:

4. Do not blow into your Game Paks or systems. The moisture in your breath can corrode and contaminate the pin connectors.

The lesson here is to not spit on sensitive electronic contacts. Still, Blow The Cartridge became a popular meme, and even a retro video game comic. With the release of the Switch, Nintendo engineers came up with a clever solution to make sure game carts never came in contact with our mouth:

To avoid the possibility of accidental ingestion, keep the game card away from young children. A bittering agent (Denatonium Benzoate) has also been applied to the game card. This bittering agent is non-toxic.

Of course, the very first thing game reporters did was licking Switch cartridges to "verify these claims". Yuck.

As the fifth generation consoles advanced its technologies, with Sony's PlayStation on the front line, the rise of cheap mass-producible optical discs superseded the more expensive circuit boards with custom chips. Except that Nintendo's N64 reluctantly clung onto the cartridge system. CDs indeed increased storage capacity, making way for bigger texture maps and more intricate level design, but we tend to forget the biggest downside: loading times.

When comparing the Nintendo DS to the Sony PSP, this disadvantage is especially painful. This Gamespot article, bluntly subtitled "Let's face it. The PSP suffers from horrendous load times. But just how bad are they?", concludes that "PSP games take a long time to load. Hardly any PSP games took less than a minute to load and quite a few took two or more." The solution? Jailbraking your PSP and ripping games onto a Memory Stick, a horrendous proprietary flash memory card system. I'm sure it must have been a source of great jokes at Nintendo staff parties.

Memory-mapped IO is responsible for the near-instant load times, allowing developers to treat cartridge ROM and RAM as if they were soldered on the PCB of the device itself. Other advantages of cartridges include the freedom to install any sensor you could think of and less system RAM usage that is typically required to read CD-ROMs. While "blowing the cart" might fix a N64 cart, a damaged PS1 CD-ROM can only be tossed in the bin. Popular early home computers such as the Atari 400 and the Commodore 64 also made use of cartridges, next to magnetic media.

Game Boy cartridge design

While it is the iconic exterior plastic case that most people will fondly remember, it is the interior circuitry design of Game Boy games that is very compelling. Precisely because Game Boys only expect to load off something with 32 pins, there's a lot of creative margin for game developers to go wild.

Below are two examples of cartridge PCB designs: The Legend of Zelda: Oracle of Seasons (left) and Darkwing Duck (right). The Zelda PCB contains the 16 megabit game ROM (right), a battery for the 64K-SRAM (left) to save data, a MBC-5 controller chip above the RAM and a RAM protector on the top left. The noticeably simpler Darkwing Duck PCB only comes with a MBC-1 chip, without any way to save the game.

Games bigger than 32 KB required a Memory Bank Controller to swap chunks of 16 KB on-the-fly. Almost all GBA games mapped the entire memory into address space. There was one exotic type of cartridge that did employ a complex MBC: GBA Video. Many emulators are unable to play the movie ROMs that eventually did get dumped, as the mystical MBC chip is still not completely reverse-engineered. More technical information is available at https://mgba.io/2015/10/20/dumping-the-undumped/.

Mapper chips like the Zelda one pictured above also evolved as the Game Boy technology was revised. MBC1 could work with a 2 MB ROM and 32 KB of RAM at most, while the MBC5 can map up to 8 MB and is compatible with the GBC's double clock speed mode.

Early games were frustrating not only because of their difficulty but also because of the lack of a save feature. Some developers intentionally left out the feature to artificially prolong the game, while others mostly cut it due to the added cost: anything extra soldered onto the PCB increased the manufacturing price. It sometimes breaks my heart to read in retro interviews with developers that the save feature didn't make it because the publisher wanted to cut costs¹.

Joonas Javanainen created an online Game Boy hardware database that contains photos of all possible cartridge circuit boards, available at https://gbhwdb.gekkio.fi. There, you can marvel at exotic MBC models, such as the MBC7 of Kirby Tilt 'n' Tumble which is capable of reading data from the 2-axis ADXL202E accelerometer that detects how you hold the GBC in order to roll Kirby to the appropriate direction.

Since we know everything is memory-mapped, something on the cartridge should be responsible for enabling access to installed sensors: the MBC. In order to read accelerometer data, you first have to latch it by writing AAH to location $A010. Then, X and Y values are accessible by reading two bytes from $A020 and $A040.

Kirby was not the only one to receive a special sensor: there are improved and more sensitive gyro sensors (Yoshi Topsy-Turvy and WarioWare: Twisted! for GBA), light sensors that help you kill vampires² (Boktai: The Sun Is in Your Hand for GBA), rumble packs that required an extra AA battery (Pokémon Pinball for GBC), extra infrared sensors that catch TV remote signals to reveal secrets in games (Robopon: Sun Version for GBC), and Real Time Clocks to catch 'em all or make sure your Tamagotchi is kept well-fed.

Certain labels on the sticker of the cartridge revealed the compatibility of special play modes:

• "Official Game Boy Video Link Game Pak": The game has multiplayer support via the Link Cables. Strangely enough, many later games with multiplayer support lacked this label.
• "Super Game Boy Game Pak": The game has enhanced Super Game Boy support that makes playing it on SNES more compelling.

Hey, there's a a cartridge in your cartridge?

The Super Game Boy (SGB) was as an inventive way to enlarge the SNES library with more than a thousand Game Boy Games. Nintendo engineers created a SNES cartridge that is able to play original Game Boy cartridges (or black GBC ones in GB mode).

Super Nintendo PCBs used the same mechanics as Game Boy PCBs: anything goes, as long as the pinout signal is what the machine expects it to be. Opening up a Super Game Boy reveals a Sharp SGB-CPU and two RAM chips, suspiciously familiar to those who inspected the Game Boy PCB. The Super Game Boy is essentially a Game Boy itself! However, a lack of a separate crystal means the clock frequency of the SNES is followed, which divides by five, giving 4.256 MHz on PAL systems instead of 4.194 MHz, resulting in a slightly too fast running game. The effect was even more pronounced on NTSC systems. This could be rectified by buying the Super Game Boy 2, only released in Japan, or by soldering on the correct crystal oscillator yourself.

Games that did not label "Super Game Boy Game Pak" could still be played on the SGB. A simple interface allowed players to change four colors of the GB palette and pick one of the few pre-defined borders that are present to adjust the SNES video signal (256x224) to the GB native resolution (160x144). The presence of the label indicated a few mostly unimpressive extras, such as more colors or fancy border images to choose from.

Some games did proudly wear the SGB label, managing to pull off very impressive tricks: from dramatically improved music (Animaniacs) and sound (Princess Peach actually yells "Help Me!" in Donkey Kong), and multiplayer support through up to four SNES controllers (the Bomberman GB and Wario Blast games), to even implementing an almost complete new 16-bit game (Space Invaders).

The question is: how did developers cram in all that extra information on a tiny ROM chip on a Game Boy cartridge? The answer lies in taking control of the SNES CPU by hacking the P14 and P14 output lines of the Joypad register at $FF00. A crude packet transfer system can be set up to transfer streams of 128 bits of data from the GB to the SNES CPU. Large 4KB data blocks can be transferred by temporarily dumping data into VRAM. There are commands to manipulate palette data, to transfer WRAM, and even to change SNES VRAM OBJ data³. The latter allows developers to replace 8-bit GB sprites with 16-bit SNES ones.

The SGB BIOS contains pre-defined sound effects (explosions, projectile hits, jumps, gate squeaks, ...) that can be played with the SOUND command. This saves developers some much-needed ROM space, although it is also possible to send data directly to the SNES APU via VRAM transfer.

We can identify the following chips on the above SGB PCB: two times 64K SRAM (top), the SGB BIOS ROM (right) and the GB CPU (left). The two smaller chips are the lockout chip (CIC, left) and an in-circuit debugger (ICD2-R, right). Since all Game Boy games are region free, the CIC does not do much here. Note the absence of a crystal clock. The SGB2, released only in Japan four years after the first SGB depicted here, also featured a Link Cable port. Why? Pokémon, that's why!

Sadly, GBC-exclusive games were not supported on the SGB. The 2003 GameCube accessory, confusingly called "Game Boy Player", is able to play all cartridges of the entire Game Boy family, including GBA games. The peripheral is installed via the high-speed port on the bottom of the GameCube. It bears more resemblance to the Sega CD extension than to the cartridge-in-cartridge SGB design. While the Game Boy Player enables another thousand games to be played on a bigger screen, the SGB-specific features of original GB games were lost. The good news is that the synchronization issue is fixed and some games supported the rumble feature of the GC controllers.

Memory-mapped cheating

As a kid, I never made it to the final boss of Teenage Mutant Ninja Turtles II: Back from the Sewers. No save slot and too much flying bullets to avoid in later areas eventually made me give up, until my dad bought me a Game Genie. It is another cartridge-in-cartridge device that fools the Game Boy into thinking it is running Turtles II, while in fact it is the Game Genie ROM that feeds the GB a modified version of the game.

The back of the bulky peripheral holds a tiny code book in which you can find cheats such as unlimited energy (001-B6B-3BE) or codes that start the game directly on act 5 (043-8BE-E66). Enabling one or more codes caused the Game Genie to hijack certain read requests from the CPU, and return other data instead of what the game ROM or RAM was supposed to do. This simple system was again made possible due to memory-mapped IO.

A Game Genie code encodes a memory address to look out for, a value to return instead, and an optional conditional operand. Since many games were memory banked, memory addresses locations could point to different portions of the ROM depending on the piece of memory currently being banked by the MBC. A comparison value (the last three hexadecimal values of the code) should in theory mitigate this, although it does not completely solve the issue. This means using Game Genie cheats could result in weird things happening.

The Turtles II invincibility Code 001-B6B-3BE can be decoded as 01000001101101100011010100000000 by moving and inverting bits according to a predetermined formula. The first four groups of bits represent the address: $41B6. the last two groups of two bits represent the condition and value: 35 and 00. Some bits serve as a simple integrity check, but other than that it is possible to mess with any ROM or RAM value as new codes can be easily made up⁴.

Gamers who prefer full control to dabble within memory space of Game Boy games should resort to the Pro Action Replay (in North America called GameShark) devices instead. First, you instruct the cheat device to take a snapshot of the entire WRAM. Next, you try to die - this shouldn't be the hard part. Then, another snapshot is taken, and the search for a needle in the haystack begins. Action Replay looks for differences in the values to try and find the memory location for the amount of lives. To speed up the search, you can repeat the method and tell it whether it should look for values that have increased, decreased, or stayed the same.

After multiple repeated attempts, you should have come up with a unique code (the address) on which a custom value can be set. Other similar cheat systems (X-Terminator Zie, KARAT Action Replay 4 MAX) come with a much needed RAM chip that enable storage of these codes. Imagine having to slog through this procedure every single time you want to play (and cheat).

For more playful "memory-mapped cheating", see my Password Hacking Sylvester & Tweety: Breakfast On The Run attempt.

Never lose your saves

Another weird cartridge-in-cartridge contraption is the Mega Memory Card by InterAct⁵. It looks exactly like an Action Replay, but instead of cheating, it simply dumps raw cartridge RAM data into its own much bigger RAM. Now your Game Boy Camera pictures or caught Pokémon are safe and you don't have to worry about losing your savegame due to a dead a battery. "Never Lose Your Saves!" it states. But what happens when the Mega Memory card battery itself dies? Well, it can't because it uses a more modern form of flash memory and thus does not require a power supply. Recent adaptations of this idea (The GB USB Smart Card) come with a USB port that allows you to transfer the savegame to a PC.

Next to these specialized solutions, there are also cartridges that allow you to run your own games. The most popular carts are EverDrive and EZ-Flash, both with revisions for GB(C) or GBA. These usually accept some form of SD card that make transferring save or ROM data a piece of cake. The EZ-Flash Omega version even comes with two plastic housings: one that fits regular GBA slots and one that cozily snuggles into the shorter Nintendo DS Lite GBA slot.

Flash carts support multiple game ROMs, making it a popular choice among gamers who don't like hauling lots of cartridges. Instead, they extract the game ROM from the cartridge and upload it to the SD card. And with that, we have entered a gray area: it is also possible to illegally download ROMs from shady websites.

The sad tale of counterfeit cartridges

Precisely because of the nature of Game Boy cartridges, it is very easy to create fake ones. All that is required is knowledge of electronics and the inner workings of the GB. Once a ROM image has been ripped from an official cartridge, it could be duplicated and flashed into unofficial chips. See the following photo comparing a real and fake cart and their PCBs:

Top: an official Castlevania: Harmony of Dissonance board. Bottom: a fake Castlevania: Aria of Sorrow board. Note the difference in color, the cheap looking material, and the sloppy Glob-top epoxy chip coating. Most games with save states come with a battery, so flash RAM is also a warning sign. Granted, compared to cloning and burning a CD-ROM, the circuit board required more effort. That did not stop the black market from thriving. With good fakes, trying to discern real from fake without opening up the plastic housing is near impossible, even for the trained collector⁶. Here are a few guidelines for GBA carts:

• The "Game Boy Advance" words on top are not thick enough or not misplaced.
• The art on the sticker does not match the official art. Watch out: this is region-specific! Note the subtle shade on the Castlevania logo on the lower cartridge, that is missing in the fake one.
• The logos do not match or are misplaced. The Konami logo seems to have disappeared.
• A stamp with two digits should be present on the sticker, which is hard to see in photos.
• The arrow on the bottom does not have the correct width.
• The back of the cartridge is screwed with the wrong type of screw.

Many counterfeit all-in-one Game Boy cartridges like the one above were sold in the nineties. It does not even contain 88 games, since many titles that can be selected promptly boot the same game. Note again the usage of cheaper PCB materials, and the absent "Nintendo" words on top of the plastic casing.

If you are unsure, your best bet is to demand a picture of the PCB. Trustworthy sellers are aware of piracy issues and will happily comply. I should have taken better precautions when buying GBA games via the internet. I knew there were fake Aria of Sorrow carts floating around and I double-checked the art, without realizing that a logo and subtle shadows were missing.

Counterfeit original Game Boy cartridges also exist, albeit to a lesser extend compared to the vast amount of GBA forgeries. Fakes are easily spotted because the "Nintendo" logo is absent on top of the cartridge itself. Also, the back of the case does not feature Nintendo's proprietary screws. To this day, I have yet to encounter a fake Game Boy Color cartridge. I'd dare to say the translucent design worked against creating duplicates: mistakes are easily spotted without the need of a screwdriver.

If you're interested in the technicalities of one of the chips on the Game Boy PCB, the Memory Bank Controller or MBC, see Historical Usage of Memory Bank Controllers in Game Boy Carts to continue the Game Boy cart adventure.