We haven't decided... see my comments on the wiki
Reproduced here for ease of threading:
I'm torn about how to deal with RAM-inside-mappers. As far as I know, there are five ICs that have some: MMC5, MMC6, X1-005, X1-017, and Namco 163.
Some of these are easy: for the MMC6, X1-005, and X1-017, their internal RAM is at the exclusion of external RAM. The RAM size byte can just hold $40, $01, $10, or (per rounding up) $70 as appropriate.
For the MMC5, that RAM can be thought of as either of CPU or PPU memory, it's not clear whether it should be marked (and where it should be marked). Apparently no game ever battery-backed only the 1k internal RAM, so perhaps leaving the RAM size byte at 0 for MMC5s without external RAM is best, even if it's contradictory to the advice for the previous three. Only 2k, 8k, and 32k RAMs were available during the MMC5's commercial life, and they were only ever combined as (none, 8+0, 8+8, 32+0); clearly adding 1024 and rounding up would cause nonintuitive values here. (As an aside, there's also no way to express the 2+8 or 8+32 configurations in NES2.0; fortunately neither were used commercially)
The N163 came in 3 of the 4 possible variants: neither battery nor external RAM; battery but no external RAM; battery and external RAM. The first two would easily be $01 and $10, but the last worries me—adding internal+external and storing the rounded-up value in the "RAM size" means we'd have $80=16384 for 8192+128 battery-backed, or $60=4096 for 2048+128 battery-backed. This feels unintuitive to me. Simply always excluding internal RAM here (in addition to being contradictory to the advice given for the MMC6) means we'll have the nonsensical value 0 with the battery bit set in the rest of the header. Maybe the right answer is to only count the internal RAM when there's no external RAM? —13:43, 10 February 2013 (MST)
We can add the Datach system to the list. It provides a 256 byte EEPROM that is not strictly speaking part of the game, but rather the cartridge converter, and thus is shared between games. Perhaps here the byte should indicate whether the game uses the EEPROM, rather than whether it's available. —03:04, 14 August 2013 (MDT)