I'd be surprised if the N64 Expansion Pak, which was required for the 64DD, was meant to be an analogue to the RAM Adapter cartridge on the FDS. You think Nintendo released it earlier than expected for third-parties to tinker with while the add-on was delayed?
Nah it's just that Nintendo decided to force it for 64DD.
The RAM Adapter cartridge for FDS was outright required to even load anything big enough in memory.
The N64 already does this to load from cartridge instead because you can't really run code directly from it without massive slowdowns, so 64DD really didn't need the Expansion Pak.
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