;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; MULTIPLY multiplier, multiplicand
; Multiply two 16-bit numbers into a 16-bit product, permitting overflow. 

; Device data and reset vector
	P = pic16c55
	#include <16c55.inc>   ; processor assembler definitions
	_CONFIG _xt_osc & _wdt_off & _protect_off
        reset   start

        org     8
mulcH   Res      d'1'       ; MSB of multiplicand. 
mulcL   Res      d'1'       ; LSB of multiplicand. 
mulpH   Res      d'1'       ; MSB of multiplier.
mulpL   Res      d'1'       ; LSB of multiplier.
prodH   Res      d'1'       ; MSB of product. 
prodL   Res      d'1'       ; LSB of product. 
index   Res      d'1'       ; temporary counter

        org     0

start        MOVLW d'0'                 ; Outputs for LEDs to 
             TRIS 6h
             MOVLW d'0'                 ; display 16-bit result.
             TRIS 7h
             MOVLW d'0'                 ; Problem: multiply
             MOVWF mulcH
             MOVLW 0x0B                 ; 0Bh by 1016h
             MOVWF mulcL
             MOVLW 0x10                 
             MOVWF mulpH
             MOVLW 0x16                 
             MOVWF mulpL
             CALL multiply              
             MOVF prodL,w               ; Show result in binary on 
             MOVWF 6h
             MOVF prodH,w               ; LEDs connected to ports.
             MOVWF 7h
             GOTO $                     ; Endless loop

Multiply
             CLRF prodH                 ; Clear product to make
             CLRF prodL                 ; way for new calculation.
             MOVLW d'16'                ; Number of bits to calc. 
             MOVWF index
Multiply_loop
             BCF status,c               
             RLF prodL                  ; Shift product left.
             RLF prodH                  
             BCF status,c               
             RLF mulcL                  ; Shift multiplicand left.
             RLF mulcH                  
             BTFSS status,c             ; If carry, add multiplier
             GOTO Multiply_skip
             MOVF mulpL,w               ; to the product. Else skip.
             ADDWF prodL
             BTFSC status,c             ; 16-bit addition: prod+mulp
             INCF prodH                 
             MOVF mulpH,w               
             ADDWF prodH
Multiply_skip
             DECFSZ index               
             GOTO Multiply_loop
             RETLW 0h                   
             
             
             end