What advantage do larger capacity PRG RAM chips offer?

[SatoshiMatrix]

I've got somewhat odd question.

I've noticed that some of my MMC5 Famicom koei games have various amounts of PRG RAM. Most have 8K PRG RAM, but a few have two banks of PRG RAM (for a total of 16K PRG RAM total) and one board I have even has 32K.


The question I have isn't about any specific game, but just in general:

What kinds of advantages would an Famicom/NES game have using 32K PRG RAM over an 8K PRG RAM version of the same game? MMC5 rom hacks are beginning to become a thing and I'm curious what the PRG RAM sizes mean.

[lidnariq]

What kinds of advantages would an Famicom/NES game have using 32K PRG RAM over an 8K PRG RAM version of the same game?

Same thing RAM ever means: ability to store more state. However, that meant a lot more money to manufacture a cartridge, though. (Koei published all of the NES/FC cartridges with more than 8 KiB of RAM, except for two Japanese exclusives). I have to assume that the US's comparatively lower game retail price meant that publishers weren't willing to take the chance on low margin or low sales.

MMC5 rom hacks are beginning to become a thing and I'm curious what the PRG RAM sizes mean.

I'm not certain what you're asking about? More RAM = more RAM. It's not really all that easy to retool an existing game to use more RAM unless it's permanently mapped, and using more than 8 KiB of RAM requires bankswitching it in, or (on the MMC5) bankswitching ROM out.

[Memblers]

In the case of adding more RAM to an existing game, it's only going to matter if the hacker makes some extensive changes to the game's code. If one wants to add features that require more variables, you're either going to have to find whatever parts of RAM happen to be unused (there will almost always be some amount, on the stack page at $01xx if nothing else), or expand the RAM. If the game didn't use WRAM to begin with, then that's simple to add.

In the case of the MMC5 in particular (and one of my WIP mappers, but nevermind, heheh), you can map the WRAM (aka PRG-RAM) into $6000-$DFFF, while all the more common mappers normally allow WRAM at $6000-$7FFF and only ROM above that. I'm assuming by MMC5 rom hacks, you mean converting games to use MMC5, because there just aren't many titles for the MMC5. In this case one advantage that would be easy to exploit, is being able to load the game's ROM banks into RAM, and basically you could create a "Game Genie on steroids" setup where you'd be able to modify as many bytes as you want, as well as being able to enable/disable those modifications during gameplay. This pretty much is what programmers call self-modifying code. The alternative is hand-assembling mods into the code, or disassembling and reassembling the code. But it involves some coding either way.

Other advantages would be a huge amount of RAM for variables, or storing stuff like level data which would be decompressed from ROM. Memory is really cheap today, but these older mappers are from the days when 512kB was huge.

[tepples]

A 32 KiB SRAM, as in SXROM and EWROM, might help with something complicated like SimCity or a social simulator. The version of mapper 69 (FME-7) on my PowerPak supports 32 KiB SRAM as well, and I have an idea for how it could be (compatibly) added to the definition of MMC3.