*
*``````````````````````````````*
*  ADD16 :: ADD 16-BIT VALUES  *
*-                            -*
* ADDS TWO SIXTEEN BIT VALUES  *
* AND RETURNS A 16 BIT VALUE   *
* IN Y (LOW) AND X (HIGH).     *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*        LDA  #>1000           *
*        PHA                   *
*        LDA  #<1000           *
*        PHA                   *
*        LDA  #>1200 ; ADD2    *
*        PHA                   *
*        LDA  #<1200           *
*        PHA                   *
*        JSR ADD16             *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* ADDEND 2 LOW BYTE            *
* ADDEND 2 HIGH BYTE           *
* ADDEND 1 LOW BYTE            *
* ADDEND 1 HIGH BYTE           *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
*   Y = LOW BYTE OF RESULT     *
*   X = HIGH BYTE OF RESULT    *
*   A = LOW BYTE OF RET ADDR   *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, THIS MAY NOT FALL   *
* UNDER THE APACHE 2.0 LICENSE *
* AGREEMENT, SINCE THE BOOK    *
* WAS WRITTEN BEFORE THE       *
* LICENSE!                     *
*                              *
* THAT SAID, LEVENTHAL WROTE   *
* THAT THE PURPOSE OF THE BOOK *
* WAS TO COPY THE SOURCE CODE, *
* AS REINVENTING THE WHEEL IS  *
* TEDIOUS, AND HE PLACED NO    *
* RESTRICTIONS ON ITS USE.     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
ADD16
*
** GET RETURN ADDR
*
         PLA
         STA   :RETADR
         PLA
         STA   :RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :ADD2
         PLA
         STA   :ADD2+1
         PLA
         STA   :ADD1
         PLA
         STA   :ADD1+1
*
         LDA   :ADD1
         CLC
         ADC   :ADD2
         TAY              ; LOW BYTE
         LDA   :ADD1+1
         ADC   :ADD2+1
         TAX              ; HIGH BYTE
*
** RESTORE RETURN ADDRESS
*
         LDA   :RETADR+1
         PHA
         LDA   :RETADR
         PHA
*
         RTS
*
** DATA
*
:RETADR  DS    2
:ADD1    DS    2
:ADD2    DS    2