list p=16f877
#include p16f877.inc

varx1 EQU .0
varx2 EQU .0
varx3 EQU .0

SCRATCH EQU 0x20
org 0x00
nop
goto start
org 0x8
start
bsf STATUS,RP0
movlw 0x00
movwf TRISB
bcf STATUS,RP0
; 2*(x1+x2+x3) or (x2+x3-1) or 12

movf varx1, W
addwf varx2, 0
addwf varx3, 0
addwf W,0
movwf varx1
movf varx2, W
addwf varx3,0
decf W,0
movwf varx2
movf varx1, W
iorwf varx2, 0
iorwf .12,0
movlw PORTB
End