Musings and comments about our common interest
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 »«)