HP 15c - Manual (Page 204)

204 Section 14: Numerical Integration
Memory Requirements
f requires 23 registers to operate. (Appendix C explains how they are
automatically allocated from memory.) If 23 unoccupied registers are not
available, f will not run and
Error 10 will be displayed.
A routine that combines f and _ also requires 23 registers of
space.
For Further Information
This section has given you the information you need to use f with
confidence over a wide range of applications. In appendix E, more esoteric
aspects of f are discussed. These include:

How f works.

Accuracy, uncertainty, and calculation time.

Uncertainty and the display format.

Conditions that could cause incorrect results.

Conditions that prolong calculation time.

Obtaining the current approximation to an integral.