[jan-jaap]

64bit is relative. Using the N32 ABI, the R4000 will process 64bits of data at a time, even when using 32bit pointers.

Using 64bit pointers on a system with a memory capacity limited to a couple of hundred MB is stupid. Don't forget, those double wide pointers eat up your RAM and CPU cache twice as fast. Binaries are larger and an N64 binary will run slower than an N32 binary.

FWIW: the Challenge/Onyx series run IRIX64, even with R4x00 CPUs.