*
*``````````````````````````````*
* SUB16 :: SUBTRACT WORDS      *
*-                            -*
* SUBTRACT ONE 16BIT NUMBER    *
* FROM ANOTHER, RETURNING THE  *
* RESULT IN X (HI) AND Y (LOW) *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*        LDA #>1000 ; MINU     *
*        PHA                   *
*        LDA #<1000            *
*        PHA                   *
*        LDA #>500 ; SUBT      *
*        PHA                   *
*        LDA #<500             *
*        PHA                   *
*        JSR SUB16             *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF SUBTRAHEND       *
* HIGH BYTE OF SUBTRAHEND      *
* LOW BYTE OF MINUEND          *
* HIGH BYTE OF MINUEND         *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
*   Y = RESULT LOW BYTE        *
*   X = RESULT HIGH BYTE       *
*   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.     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
SUB16
*
** GET RETURN ADDRESS
*
         PLA
         STA   :RETADR
         PLA
         STA   :RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :SUBT
         PLA
         STA   :SUBT+1    ; SUBTRAHEND
         PLA
         STA   :MINU
         PLA
         STA   :MINU+1    ; MINUEND
*
** MINUEND - SUBTRAHEND
*
         LDA   :MINU
         SEC
         SBC   :SUBT
         TAY              ; LOW BYTE
         LDA   :MINU+1
         SBC   :SUBT+1
         TAX              ; HIGH BYTE
*
** RESTORE REUTNR ADDRESS
*
         LDA   :RETADR+1
         PHA
         LDA   :RETADR
         PHA
*
         RTS
*
** DATA
*
:RETADR  DS    2
:MINU    DS    2
:SUBT    DS    2