I had thought that my TI-92 and TI-86 handled 64 bits, but when I just tested them it's just that they display negative numbers with that many bits. It looks like they only calculate at 32 bits total. (I did fudge up my own 128-bit 'alu' calculator under DOS. It handles expressions, square root, and the basic operations and the division result provides both quotient and remainder and the multiplication provides 256 bit results. If it's of any interest, I'd be happy to provide it.)
Jon