HP 15c - Manual (Page 213)

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Appendix C
Memory Allocation
The Memory Space
Storage registers, program lines, and advanced function execution
*
all draw
on a common memory space in the HP-15C. The availability of memory for
a specific purpose depends on the current allocation of memory, as well as
on the total memory capacity of the calculator.
Registers
Memory space in the HP-15C is allocated on the basis of registers. This
space is partitioned into two pools, which strictly define how a register may
be used. There is always a combined total of 67 registers in these two pools.

The data storage pool contains registers which may be used only
for data storage. At power-up (Continuous Memory reset) this
equals 21 registers. This pool contains at least three registers at all
times: R
I
, R
0
, and R
1
.

The common pool contains uncommitted registers available for
allocation to programming, matrices, the imaginary stack, and
_ and f operation. At power-up there are 46 uncommitted
registers in the common pool.
* The use of _, f, Complex mode, or matrices temporarily requires extra memory space, as
explained later in this appendix.