HP 15c - Manual (Page 214)

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