HP 15c - Manual (Page 219)

Appendix C: Memory Allocation 219
For _ and f, allocation and deallocation of the required register
space takes place automatically.
*
Memory is thereby allocated only for the
duration of these operations.
Space for the imaginary stack is allocated whenever ´ V, ´
}, or | F 8 is pressed. The imaginary stack is deallocated when
" 8 is executed.
Space for matrix registers is not allocated until you dimension it (using
m). Reallocation takes place when you redimension a matrix. >
0 dimensions all matrices to 0

0.
* If you should interrupt a _ or f routine in progress by pressing a key, you could deallocate its
registers by pressing |n or ´ CLEAR M in Run mode.