HP 15c - Manual (Page 106)

106
Section 10
The Index Register
and Loop Control
The Index register (R
I
) is a powerful tool in advanced programming of the
HP-15C. In addition to storage and recall of data the Index register can use
an index number to:

Count and control loops.

Indirectly address storage registers, including those beyond R.
9
(R
19
).

Indirectly branch to program line numbers, as well as to labels.

Indirectly control the display format.

Indirectly control flag operations.
The V and % Keys
Direct Versus Indirect Data Storage With the Index Register
The Index register is a data storage register that can be used directly, with
V, or indirectly, with %.
*
The difference is important to note:
V
%
The V function uses the
number itself in the Index
register.
The % function uses the absolute
value of the integer portion of the
number in the Index register to
address
another data storage
register. This is called indirect
addressing.
* Note that the matrix functions and complex functions use the V and % keys also, but for different
purposes. Refer to sections 11 and 12 for their usage.