history of the 4-level stack

[John R]

(09-11-2014 08:35 AM)axd1967 Wrote:  On the other hand, postfix allows to use computed register addresses (think of indirect addressing, but then on some machines that is also available).

That's true, and it would open some intriguing possibilities.

I just thought of another serious problem with postfix notation: it would not seem to admit unambiguous storage-register arithmetic operations without extra keystrokes, or new dedicated keys. In particular, suppose we want to add the contents of X to the contents of register 1, and store the result in register 1. In prefix notation, we simply key in "STO+ 1". However, we have a problem in postfix notation. The calculator can't use "1 STO+" to denote this operation because the previous contents of X (now lifted to Y as per the earlier comment) would be immediately stored in register 1 at the time that STO is pressed, so that the "+" would then be interpreted as a regular addition on the stack. The calculator also can't use "1 +STO", because the addition would similarly be applied to stack registers before any register storage would happen. It would seem that the only ways around this would be to require a trailing ENTER after a postfix STO (to tell the calculator no register-arithmetic operation is intended), or a shifted STO to indicate a pending trailing register-arithmetic operation, or dedicated keys for each possible register-arithmetic operation. None of these seems like a good option....