*
*``````````````````````````````*
* MUL16 :: MULTIPLY WORDS      *
*-                            -*
* PERFORM MULTIPLICATION USING *
* THE SHIFT AND ADD ALGORITHM  *
*                              *
* THIS ALGORITHM PRODUCES AN   *
* UNSIGNED 32-BIT PRODUCT IN   *
* HIPROD AND MLIER WITH HIPROD *
* BEING THE HIGH WORD.         *
*                              *
* NOTE: WHILE THIS DOES PASS   *
* THE FULL 32BIT PRODUCT BACK  *
* VIA X AND Y, BUT THIS WOULD  *
* RETURN FALSE RESULTS IF ONE  *
* OF THE PARAMETERS HAD BEEN   *
* SIGNED. THIS, USUALLY ONLY   *
* THE LOW WORD IS USED.        *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*        LDA #>1000            *
*        PHA                   *
*        LDA #<1000 ; MCAND    *
*        PHA                   *
*        LDA #>3               *
*        PHA                   *
*        LDA #<3 ; MLIER       *
*        PHA                   *
*        JSR MUL16 ; = 3000    *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF MULTIPLIER       *
* HIGH BYTE OF MULTIPLIER      *
* LOW BYTE OF MULTIPLICAND     *
* HIGH BYTE OF MULTIPLICAND    *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LOW BYTE OF PRODUCT          *
* HIGH BYTE OF PRODUCT (16BIT) *
*                              *
*   Y = LOW BYTE OF HIGH WORD  *
*   X = HIGH BYTE OF HIGH WORD *
*   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.     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
MUL16
*
** GET RETURN ADDRESS
*
         PLA
         STA   :RETADR
         PLA
         STA   :RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :MLIER
         PLA
         STA   :MLIER+1
         PLA
         STA   :MCAND
         PLA
         STA   :MCAND+1
*
** PERFORM MULTIPLICATION USING THE SHIFT
** AND ADD ALGORITHM, WHICH PRODUCES AN
** UNSIGNED 32-BIT PRODUCT IN :HIPROD AND
** :MLIER WITH :HIPROD BEING THE HIGH WORD.
*
         LDA   #0
         STA   :HPROD     ; ZERO HIGH WORD
         STA   :HPROD+1
         LDX   #17        ; # OF BITS IN MPLIER
                          ; PLUS 1. EXTRA LOOP IS
                          ; TO MOVE LAST CARRY INTO
                          ; THE PRODUCT.
         CLC              ; CLEAR CARRY FOR 1ST TIME
                          ; THROUGH LOOP.
:MULLP
*
** IF NEXT BIT = 1 THEN
** HIPROD = HIPROD+MCAND
*
         ROR   :HPROD+1
         ROR   :HPROD
         ROR   :MLIER+1
         ROR   :MLIER
         BCC   :DECCNT    ; BR IF NEXT BIT OF
                          ; MULTIPLIER IS 0
         CLC              ; NEXT BIT=1 SO ADD MCAND
                          ; TO PRODUCT
         LDA   :MCAND
         ADC   :HPROD
         STA   :HPROD
         LDA   :MCAND+1
         ADC   :HPROD+1
         STA   :HPROD+1   ; CARRY = OVERFLOW
                          ; FROM ADD
:DECCNT
         DEX
         BNE   :MULLP     ; CONTINUE UNTIL DONE

         LDY   :HPROD     ; LOW BYTE OF HIGH WORD
         LDX   :HPROD+1   ; HIGH BYTE OF HIGH WORD
*
** PUSH LOW WORD OF PRODUCT ONTO THE STACK
*
         LDA   :MLIER+1
         TAX
         LDA   :MLIER
         TAY
*
** RESTORE RETURN ADDRESS
*
         LDA   :RETADR+1
         PHA
         LDA   :RETADR
         PHA
*
         LDA   :HPROD     ;24BIT
         RTS
*
** DATA
*
:MCAND   DS    3
:MLIER   DS    3
:HPROD   DS    2
:RETADR  DS    2