The HP67 internal flags S 9 and S 12 are used during keyboard data entry. The meanings for S9,S12 are:
0,0 – Overwrite X register.
e.g. CLx or ENTER was just pressed.
1,0 – Lift stack.
Digit (or “.” or EEX) causes Z to T, Y to Z and X to Y; then what you key in overwrites X. This is what happens after you have calculated something.
e.g. 4 ENTER 5 + (“9.00″ displayed) 6 (Y=9.00 X=”6.”).
1,1 – Input digit.
After you press the first digit (or “.” or EEX ?) S 12 gets set to 1. From then on, digits get added to the RHS as you key them in.
| CLx | “0.00” displayed, S 9 = 0, S 12 = 0 |
| 1 | “1.” displayed, S 9 = 1, S 12 = 1 |
| 2 | “12.” displayed, S 9 = 1, S 12 = 1 |
| 3 | “123.” displayed, S 9 = 1, S 12 = 1 |
| ENTER | Y=123.00, X=123.00, “123.00” displayed, S 9 = 0, S 12 = 0 |
| 4 | “4.” displayed, S 9 = 1, S 12 = 1 |
| + | “127.00” displayed, S 9 = 1, S 12 = 0 |
You can see this working in the hp67u emulator. Simply press a button then menu Advanced and look at the “S=…” item. you’ll see “.9..c.” if S 9 and S 12 are 1,
you’ll see “.9….” if just S 9 is 1, or
you’ll see “……” if both are 0.
