NES games are mainly written in the 6502 assembly language and reside in the Program ROM, while the game's graphics (tiles) are stored in Character memory.
Furthermore, games were sold (or rented out) at retail stores under the approval of Nintendo.
The alternative medium
Even though it was only released in Japan, I thought this would be a good opportunity to introduce a short-lived but peculiar add-on that, just like the mappers, brought in more capabilities to this console. This peripheral, called the Famicom Disk System (FDS), shipped in 1986 (~3 years after the Famicom). It resembled an external floppy drive and came bundled with an oddly shaped cartridge called the 'RAM adapter'.
The drive , where floppies are inserted (the photo shows a cardboard floppy inserted for protection). It either runs on six C batteries (1.5 V each) or a 3.6 W AC adapter.
The RAM adapter , fitted on the cartridge slot of the Famicom and connected to the drive via a cable.
The Famicom Disk System added the following functionality to the console:
- A new medium of distribution for games called Famicom Disk . Based on the 'Quick Disk' by Mitsumi, it offers ~64 KB of data per side and is re-writable.
- An extra audio channel that uses wavetable synthesis .
FDS equipment mounted onto the Famicom .
Unlike multi-chip cartridges, the floppy is a simple medium. Hence, all game data needs to be squashed inside, though it is organised with the use of a proprietary file system.
Nonetheless, the architecture of the Famicom/NES strictly enforces segregated Program and Character memory. The 'RAM adapter', which came with the box, addresses this by housing 32 KB of Program RAM and 8 KB of Character RAM to buffer the game data read from the floppy disk. In doing so, this setup enables the console to read data as if it were reading from a cartridge-based game.
To operate the drive, the RAM adapter embeds an additional 8 KB ROM to store a Basic Input/Output System (BIOS) . This program performs the following tasks:
- Displays a splash animation.
- Bootstraps the game from the floppy disk. Behind the scenes, the BIOS contains routines to copy the floppy's contents to the appropriate memory chip, allowing the console to read them.
- Provides I/O APIs for games to use, such as traversing the disk's file system.
Example of two retail games for the FDS. The blue 'flavour' of the disc is dust proof.
Video - The FDS splash animation, waiting for the user to... uhm... insert a game.
During that era, Nintendo deployed some 'kiosks' at retail stores, so users could bring their floppies and overwrite them with a new game at a reduced price.
Unfortunately, after a few years of lifespan, the Famicom Disk System was discontinued. Consequently, subsequent games reverted to the cartridge medium. On the bright side, new mappers were introduced with similar (or better) capabilities compared to the FDS' functions.