woshizyt: Is it going to be able to handle Populous as well? That includes 32k of ram on the card, right after the 4mbit range of the rom.
The SF2 mapper is dead simple. If you're familiar with Famicom, it's not unlike unrom and other such simple mappers in it's simplicity. The game uses the whole lower 8mbit external address range (typical hucard range). The bottom 4mbit range is fixed to the first 4mbit of the rom, the upper 4mbit of the 8mbit hucard range is used for swapping in 512k(4mbit) banks. So it's a 4Mbit+16Mbit format. Writing any value to 'external' address range of $1ff0 to 1ff3 selects which 4Mbit segment of the 16Mbit segment gets mapped into the upper 4Mbit hucard area. So $1ff0 = bank 4Mbit #0, $1ff1 = bank 4Mbit #1, etc.