back to *64bit computers* planned before "RISC" ... in the 60s, the
IBM 7030 (also known as "
Stretch") was actually a 64 bit computer, with multi-processing units for floating point, integers and other operations like n bit character processing.
Don't you believe? Let's check it out!
Instructions were either 32-bit or 64-bit, and Fixed-point numbers were variable in length, stored in either binary, 1 to 64 bits
Surprisingly, IBM 7030's 0 register is called "$ Z", and it's a true 64-bit register that always reads as zero, can't be changed by writes, which is quite RISCish even if we're talking about a designed machine years before the term RISC was invented.
The problem is that in the 1960s looking ahead 30-40 years, well, it was quite limited by the technology of the time, and the results were much slower than expected and failed to hit any performance goals.