tsmt
the PS1 port (from what i know) probably didnt need to be modified/adapted as much. even if the PS1 lacks a Z-buffer and only has 2MB of RAM, the N64/PS1 both rely on MIPS CPUs.
N64 doesn't even utilize the 64-bit feature much at all, it's inherited from the downscaled SGI-type hardware they based the system around. although it was actually 64-bit iirc, not some bullshit like the Jaguar's gorillion coprocessors.
worth noting that x86/Wintel took a lot longer in that department, hence why most high-end workstations were some RISC-type ISA UNIX boxes for a while