HP -15C Owner's Handbook Manual page 214

214 Appendix C: Memory Allocation
Total allocatable memory: 64 registers, numbered R through R .
2 65
[(dd – 1) + uu + pp + (matrix elements) + (imaginary stack) + (_
and f)] = 64. For memory allocation and indirect addressing, data
registers R through R are referred to as R through R .
.0 .9 10 19