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Call PIC Build Function

I am writing a fairly basic program in the PIC18 assembly. This requires me to write a routine to multiply two 16-bit numbers. This is what I have right now:

;***********************************************************************
; mul_16bit: subroutine that multiplies two 16 bit numbers stored in
;    addresses mul_16ptr1, mul_16ptr1+1 and mul_16ptr2,mul_16ptr2+1 and
;    returns the 32-bit result in addresses mul_16res1 to mul_16res1+3

;***********************************************************************
mul_16bit:
             movf    mul_16ptr2, W           ;multiply the lower bytes
             mulwf   mul_16ptr1, W
             movff   PRODH, mul_16res+1
             movff   PRODL, mul_16res
             movf    mul_16ptr2+1, W                 ;multiply upper bytes
             mulwf   mul_16ptr1+1, W
             movff   PRODH, mul_16res+3
             movff   PRODL, mul_16res+2
             movf    mul_16ptr2, W           ;multiply lower byte of num2
             mulwf   mul_16ptr1+1, W       ; and upper byte of num1
             movf    PRODL, W
             addwf   mul_16res+1, F
             movf    PRODH, W
             addwfc  mul_16res+2, F
             movlw   0                                       ; add carry
             addwfc  mul_16res+3, F
             movf    mul_16ptr2+1, W                 ;multiply upper byte
                                                     ;of num1 and lower
             mulwf   mul_16ptr1, W           ; byte of num2
             movf    PRODL, W                        ;add the result to mul_16res
             addwf   mul_16res+1, F          ;...
             movf    PRODH, W                        ;...
             addwfc  mul_16res+2, F          ;...
             movlw   0                                       ; add carry
             addwfc  mul_16res+3, F
             return

The way I wrote it now is that it multiplies the numbers stored in the registered one mentioned in the first comment and stores them in 4 registers in the comment. This works well if I only need to do this multiplication once or twice, i.e. I can just say something like:

mul_16ptr1   set    0x45
mul_16ptr2   set    0x47
mul_16res    set    0x50
call         mul_16bit

0x45 0x47 0x50. , , "" . ( LFSR1, LFSR2 LFSR0 ), POSTINC0 .. ?

+5
3

PIC18 normaly , RegA, RegB RegR. , :

RegA res 2    ;16bit var
ResB res 2    ;16bit var
ResR res 4    ;32bit var

:

;Constants declaration
    OperandA set 1234
    OperandB set 7777
;
;
;Prepare calling operand A   
    movlw low OperandA 
    movwf RegA 
    movlw high OperandA 
    movwf RegA + 1
;Prepare calling operand B         
    movlw low OperandB 
    movwf RegB + 0 
    movlw high OperandB 
    movwf RegB + 1
;Function call        
    call  MullAB_16bit
;Result is in RegR
+2

, PIC .

, - . Thomas McGahee, , , , "*" (BASIC, C, Pyastra, JAL, Forth ..).

GJ , , PIC16, FSR "PLUSW".

PIC18 "PLUSWx", . , " ", R. Reese ?

#include<18f4550>

OperandA res 2
OperandB res 2
Product res 4

clock_ticks res 2
useconds_per_clock_tick res 2
total_time res 4

    ; example of the "call" part of a possible 3-pointer calling convention.
    ; Public domain.
    ; To multiply by some number in Flash or EEPROM,
    ; first copy them (perhaps using TBLPTR/TABLAT)
    ; into some convenient temporary Operand buffer in RAM.
    ; Then:
    ; WARNING: untested code.
    ; put pointer to first (least-significant) byte of 16-bit operand A into FSR2
        BANKSEL FSR0
        lfsr2 OperandA 
    ; put pointer to first (least-significant) byte of 16-bit operand B into FSR1
        lfsr1 OperandB 
    ; put pointer to first (least-significant) byte of 32-bit product into FSR0
        lfsr0 Product
    ;Function call        
        call  mul16x16bit
    ;Result is in Product

    ; example of calling the same subroutine with different arguments.
        BANKSEL FSR0
        lfsr2 clock_ticks
        lfsr1 useconds_per_clock_tick
        lfsr0 total_time
        call mul16x16bit
    ; result is in total_time.
        return


    ;***********************************************************************
    ; mull16x16bit: subroutine that multiplies two 16 bit numbers
    ;    pointed to by the pointer FSR2, FSR2+1, FSR3, FSR3+1, and
    ;    returns the 32-bit result in addresses pointed to by
    ;    FSR0 to FSR0+3.
    ;***********************************************************************
    ; example of a function using a possible 3-pointer calling convention
    ; WARNING: untested code
    ; The pointers to operands are: FSR2, FSR1
    ; The pointer to the result is: FSR0.
    ; Mostly identical to code in the Microchip PIC18F2550 datasheet, page 98
    ; Public domain.

RESULT res 4 // temporary 4 byte register
TEMP EQU RESULT // temporary 1 byte register

mul_16bit:
         movlw   1                       ; multiply upper bytes
         movff   PLUSW2, TEMP
         movf    PLUSW1, W
         mulwf   TEMP
         movff   PRODH, RESULT+3
         movff   PRODL, RESULT+2

         movf    INDF2, W             ;multiply the lower bytes
         mulwf   INDF1, W
         movff   PRODH, RESULT+1
         movff   PRODL, RESULT+0

         movlw   1                   ; multiply the high byte of num2
         movf    PLUSW2
         mulwf   INDF1               ; and the low byte of num1
         movf    PRODL, W
         addwf   RESULT+1, F
         movf    PRODH, W
         addwfc  RESULT+2, F
         movlw   0                                       ; add carry
         addwfc  RESULT+3, F

         movlw   1                   ; multiply the high byte of num1
         movf    PLUSW1
         mulwf   INDF2               ; and the low byte of num2
         movf    PRODL, W
         addwf   RESULT+1, F
         movf    PRODH, W
         addwfc  RESULT+2, F
         movlw   0                                       ; add carry
         addwfc  RESULT+3, F

         movff   RESULT+0, POSTINC0   ; copy result to destination where FSR points.
         movff   RESULT+1, POSTINC0
         movff   RESULT+2, POSTINC0
         movff   RESULT+3, POSTINC0

         movlw   4
         subwf   FSR0  ; restore original value of FSR0.

         return
+1

, FSR0-FSR2, ? .

  movf   POSTINC0,w,c
  mulwf  POSTINC1,c     ; Op0L*Op1L (now both point at MSB)
  movff  PRODL,POSTINC2 ; Result0
  movff  PRODH,INDF2    ; Result1
  mulwf  POSTDEC1,c     ; Op0L*Op1H (now 0 points at MSB 1 at LSB)
  movf   PRODL,w,c
  addwf  POSTINC2,f,c   ; Result1 (now points at Result2)
  movlw  0
  addwfc PRODH,w,c
  movwf  POSTDEC2,c     ; Result2 (now points at Result1)
  movf   INDF0,w,c      ; Op0H
  mulwf  POSTINC1,c     ; Op1L
  movf   PRODL,w,c
  addwf  POSTINC2,f,c   ; Result1
  movf   PRODH,w,c
  addwfc POSTINC2,f,c   ; Result2 (carry may be outstanding)
  clrf   INDF2,f,c      ; Result3
  rlcf POSTDEC2, f, c; Store carry
  movf INDF0, w, c; Op0h
  mulwf POSTINC1, c; Op1H
  movf PRODL, w, c
  addwf POSTINC2, f, c
  movf PRODH, w, c
  addwfc INDF2, f, c

LFSR is cheaper than manually moving large amounts of data.

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