There had been register arithmetic features in other HP calculators before, but never with the depth and variety of the HP15c. We’ll see it in a moment.
What do we mean for register arithmetic? Let’s see how you would work with the contents of a register. Say that you want go get the contents of a register and add them to the X register. Without register arithmetic, you would need to call the contents of the register to X (RCL 5, for example), and then press “+” (by doing this, you would have raised the old X to Y, the old Y to Z, the old Z to T, and the old T would be lost – something to take care about when programming)
With register arithmetic, you would just do RCL + 5, and while the number of keys is the same, you don’t loss the T register!
I may agree that RCL + (or -, x or /) are interesting but nothing to write home about. However, you can do the same with STO: add the X contents to a register. This really saves strokes, and it is much more elegant: compare:
with register arithmetic:
STO + 5
It also makes for shorter programs. Be careful with the division and subtraction operations: you need to remember that the second operand is always the one in the X register.
There are other interesting operations, involving the I register: you can operate with the contents of the I register, (STO + I), or with the contents of the register adressed by the content of I: STO + (I). These are very useful when programming – really clarifying the program flow
(Not related to register arithmetic, but worthy to discuss here too, is the exchange of X with the contents of a register: either a direct number (0 to 9, .0 to .9), the I register, or the register indicated by the contents of register (I): you use the key X »«)