Gameboy Development Forum

kay_kidoutai wrote:

As far as I know, custom silicon development and fabrication is extremely expensive and it seems weird to me that these mappers are only in a couple games. Can anyone expand on this list of games that uses the following mappers?

Not as extremely expensive as you might think. You might be thinking of the more glamorous leading edge technology of the industry, for example using EUV lithography to produce smaller feature sizes to take the latest big development. This kind of leading edge tech requires billions and billions of dollars in R&D to get to a working state at all. When those mapper chips were made in the 1990s they would have been using a stable trailing edge technology node. Basically 1980s/early 1990s technology at that point. It would definitely make sense for an expected run of a few hundred thousand up to a few million chips.

All of the examples implemented functionality that simply wasn't possible with a more common MBC, so a custom chip was the only practical solution.

MBC6: mapping additional flash memory.

MMM01: remapping bank 0 for multiple game collections.

TAMA5: Tamagotchi functionality that can run in the background without external power.

MBC7: Accessing an accelerometer for special gameplay features.

HuC1: From memory, IR functionality for communication. (Probably just really one digital IO pin that can control an IR LED, and one that can read an IR receiver.)

HuC3: From memory, IR functionality for communication and RTC.

8MB ROM MBC5: This is as far as I know present on all MBC5 chips, just that the extra address bit is not hooked up to anything, so it makes no practical difference.

One glaring omission from your list is the Gameboy Camera. While it was used in only one game it was significant, and it's obvious why it required a custom chip. For a GB cartridge dumper, it's also significant because it requires a clock signal on pin 2 of the cartridge bus (aka PHI) to be able access SRAM and the camera image. If you didn't add that to your hardware, you're SOL unless you add a mod wire. (Unlike anything else on the list which I think is just different special writes/reads.)

Another one is the  Nintendo Power cartridge which has a MegaChips MX15002 chip which provides multi ROM capability (again, remapping bank 0) as well as flash programming. It was am officially released flash cartridge that was used with special kiosks where you could buy games.

Then there are unofficially released games and flash cartridges. Flash cartridge released during the late 1990s and going forward to today often had multi ROM capability. Modern SD card based cartridge obviously use extra logic to access the SD card.

One other notable example of a custom "MBC" is the Wisdom Tree mapper which is using a single standard 74 series logic chip as a mapper. This has some limitations in that...
- Writes are done using the lower address bits (!) instead of data bits. This was required because the timings didn't work out with using the data bits.
- It can only swap the full 32 kB ROM space, ie, it permanently controls the upper address bits going to the ROM chip instead of having a "bank 0".
- It can start at any random bank, meaning the ROM header and some bootstrap code needs to be duplicated across all banks.

I guess I also forgot about the MBC30 in the JP copy of Pokemon Crystal and the MBC1M