# PICMicrocontollerMathMethod

## Multiply 16x16 bit from

```;malin@onspec.co.uk
;unsigned multiply of a2:a1 with b2:b1 leaving result in res4:res3:res2:res1
;	These 8 variables need to be defined
;
;	Program length	32 line
;	time 129 to 228 cycles
;	This program looks at the lsb of a1 to decide whether to add b1 to res2
;
;   and b2 to res3, with appropriate carrys
;	It then looks at the lsb of a2 to decide whether to add b1 to res3 and
;	b2 to res4, again with appropriate carrys.
;	The rotates then only have to be done 8 times
;
;	This is uses slightly more program but takes a little less time than
;	a routine that performs one 16 bit addition per rotate and 16 rotates
;
;	Multiple byte addition routine from Microchip AN617
;	Result registers used as loop counter from Bob Fehrenbach & Scott Dattalo
;

clrf    res4
clrf	res3
clrf    res2
movlw	0x80
movwf	res1

nextbit
rrf		a2,f
rrf		a1,f

btfss	status, carry
goto	nobit_l
movf	b1,w
addwf	res2,f

movf	b2, w
btfsc	status, carry
incfsz	b2, w
addwf	res3, f
btfsc	status, carry
incf	res4, f
bcf		status, carry

nobit_l
btfss	a1, 7
goto	nobit_h
movf	b1,w
addwf	res3,f
movf	b2, w
btfsc	status, carry
incfsz	b2, w
addwf	res4, f

nobit_h
rrf		res4,f
rrf		res3,f
rrf		res2,f
rrf		res1,f

btfss   status, carry
goto	nextbit

```

 file: /Techref/microchip/math/mul/16x16umalin.htm, 1KB, , updated: 2002/10/28 10:07, local time: 2024/2/28 11:33, TOP NEW HELP FIND:  44.200.77.92:LOG IN

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