Mouser have 64 megabyte PSRAMs.

I really want a Mac System 7 grade operating system for this chip...

I'd settle for Mac 512K ;)

https://github.com/evansm7/pico-mac/issues/7

No they do not. 64megabit

Did you bother to check? It's octal, not QSPI, so I don't know if it's compatible. (edit - and 1.8V, inconvenient)

Octal PSRAM usually cannot fallback to quad mode like some octal flash did.

Actually reading the datasheet[1] it doesn't look like it.

[1] https://www.mouser.dk/datasheet/2/1127/APM_PSRAM_OPI_Xccela_...

Did you? It doesnt do qspi mode

Of course I did. If you also did, you would know that they do in fact have 64 megabyte PSRAMs as I stated. So a helpful comment would have been "they're not compatible, though". Your reply as it stands just makes it sound like you maybe assumed that I don't know the difference between megabits and megabytes.