list p=16F84A
#include <p16F84a.inc>
__CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC

tmp EQU 0x0F

ORG 0x000 ; processor reset vector
bcf STATUS,RP1
bsf STATUS,RP0 ; switch to register bank 1
movlw 0x1C ; control word for port A
movwf TRISA ; RA4, RA3, RA2 - input, RA1, RA0 - output
movlw 0xFF ; control word for port B
movwf TRISB ; all are inputs for now
bcf STATUS,RP0 ; switch back to register bank 0

start
movf PORTA, w ; load data from port A into w
andlw 0x1C ; apply mask 00011100 - isolate A B C
movwf tmp ; save 000ABC00 in tmp
andlw 0x04 ; apply mask 00000100 - isolate C
xorlw 0x04 ; negate C in accumulator
btfsc STATUS,Z ; if !C=1 then
bsf tmp, 0x02 ; set 000ABC10 in tmp
movf tmp, w ; load 000ABC10 into accumulator
andlw 0x10 ; apply mask 00010000 - isolate A
rrf w ; rotate w to the right, so it lines up with B in tmp
xorwf tmp, w ; A xor B
andlw 0x08 ; isolate (A xor B) value
rrf w ; (A xor B) is now at 0x04
rrf w ; (A xor B) is now at 0x02, lines up with !C in tmp
iorwf tmp, w ; (A xor B) || !C
andlw 0x02 ; isolate result
movwf PORTA ; output result on port A
goto start

end