Now, we put in the rest of the keypress definitions, and adjust our associated data structures in support of these.  At this point, we notice that certain keys have different behaviours depending on the context in which they are pressed.  For instance, the EEX key either puts an exponent on a number that is being constructed, or it starts a new number of the form 1E+<…>.  Similarly, the CHS key either changes the sign of the number in the X register, or introduces a – sign in a number being constructed through the keypad.  The 5 buttons at the top of the calculator call functions in program space, but if there are no defined program steps, they call certain defined short-cuts.  There are also certain operations that don’t do anything in interactive mode, but have their effects only when inside a running program, such as the conditional instruction skip operators.

To cover these cases, we define several modes in which the calculator can find itself.  There’s NUMERIC-INPUT mode, in which keypresses construct a number.  RUN-MODE, the normal interactive mode when the calculator is responding to keypresses.  RUN-MODE-NO-PROG, a specialized form of RUN-MODE when there are no program steps defined.  PROGRAM-EXECUTION, when the calculator is running a program.  PROGRAMMING-MODE, when the calculator is recording a program from the keyboard.

Keypresses can return a normal exit code, or they can pass back directives to the calculator.  These include flow-control directives like GOTO, GOSUB, RETURN-FROM-SUBROUTINE, SKIP-NEXT-STEP, BACk-STEP, SINGLE-STEP, and RUN-STOP.  Also card reader/writer directives with CARD-OPERATION.  There are display directives like PAUSE-1-SECOND, REVIEW-REGISTERS, PAUSE-5-SECONDS, and DISPLAY-STACK.  There is the DELETE-CURRENT-STEP code that tells the calculator to delete a step in the program.  Then there’s TOKEN, which starts or continues the input of a number.  Any keypress that follows that is not TOKEN will implicitly end the input of the number and cause the calculator to behave as if ENTER had been pressed before the next non-TOKEN keypress is handled.

There is also an ERROR return code, which a keypress can send if an illegal operation occurs, such as division by zero or overflow.

With all this, we’re about ready to begin coding the state machine that will execute program steps.  That’s what we’ll begin doing next.

Meanwhile, the code at this point is found in the git repository under the tag v2014-11-12.