244 Appendix B: Algebraic Mode (ALG)
Key in ALG Mode
LAST X in ALG mode is not used in this book. It is the one function that is quite
different in ALG and RPN modes. Refer to page 238 for the list of 23 different key
presses that cause the displayed X-register to be copied into the LAST X register in
RPN mode. In ALG mode, those key presses do not cause a refresh of the LAST X
register. Instead, the displayed number is copied into the LAST X register when a
new calculation commences either with digit entry (i.e., when one of the digits
0-9, the decimal point, or Æ is pressed) or with gF itself, but not when a
number is recalled using :.
When gF is actually executed in ALG mode, it simply swaps the value in the
X-register with the value in LAST X. Therefore, in ALG mode, it never lifts the stack
and in general, it changes the value in LAST X. However, in RPN mode,
gF always lifts the stack and leaves the value in LAST X unchanged.
In general, LAST X in ALG mode is only useful in programs, where it can often be
used instead of a numbered storage register, as it persists unchanged at least until
digit entry or gF. There is an example of its use in the Combinations program
in the hp 12c platinum Solutions Handbook.
The History Stack in ALG Mode
In ALG mode, the stack holds a "history" of 4 completed results. These results may
be rearranged using d and ~ in the same way as described on page 234, for
Note that once a two-numbered function (such as + , - , § , z or q) has had
the second argument entered, the two arguments cannot then be swapped using
~ because when the second argument is entered it replaces the first, which is
removed from the stack. So, if you wish to do 25.83 - 144.25, but then realize
you meant to do 144.25 - 25.83, the way to correct this is to proceed and do
}Þ. All ~ will do at this point is replace the 144.25 with whatever was
displayed before the 25.83 was entered, and the first argument remains
Clears any pending operations.