HP·38E/38C - Slide Rule Museum

88/89 What Is a Program?
As you can see, program memory is separate from the four stack
registers, the LAST X register, and the financial registers. Notice that
instead of the original 20 storage registers (Ro through R9, R.o through
R. g) we now have seven storage registers, all of which will allow you to
perform storage register arithmetic. RI through R6 also accumulate
statistical data.
What happened to storage registers R7 through R9 and R.o through R'9?
They have been converted to program memory. Each storage register is
worth seven lines of programming. Rather than give you all 99 lines of
programming memory at once, thus depriving you of 13 storage registers
regardless of program length, the calculator converts storage registers
into program memory one at a time, as you need them.
Automatic Memory Allocation
After the initial eight lines of programming, the calculator converts
storage registers to program memory, seven lines at a time.
When you have P-lines of program memory, you have r-storage registers
left.
P-08 ,-20
P-15 ,-19
P-22 ,-18
P-29 ,-17
P-36 ,-16
P-43 ,-15
P-50 ,-14
P-57 ,-13
P-64 ,-12
P-71 ,-11
P-78 ,-10
P-85 ,-09
P-92 ,-08
P-99 ,-07