The memory is volatile. It's the ti-57 lcd that had non-volatile memory, and it was a very different beast. In particular you had to partition between program steps and memory registers.
Thanks! The patent Claus references, 4125901, shows 13-bit ROM, but it sure looks like 16-bit to me.
It's been so long ago that I can't remember, but I can't believe that I had to re-enter programs every time I powered off the TI-57! The calculator itself disappeared somewhere along the way, but I found the manual.