The following code does this
;pic16f84a question
;A de-multiplexer has an input line, pinRB0, and four output
;lines, output0 to output3, which are, respectively, the pins RA0-RA3. the
;demultiplexer copies the digital signal appearing on the input line on to one of
;the four output lines. The output line is selected by a two bit
;selection code input on the pins RB1 and RB2.

PLEASE LOOK AT THE CODE BECAUSE ITS NOT EXACTLY DOING WHA IT IS MENTIONED ABOVE ANYONE CAN HELP TO FIX THIS..............PLEASE


#DEFINE PAGE0 BCF 0x03,5
#DEFINE PAGE1 BSF 0x03,5

PCL: EQU 0x02
STATUS: EQU 0x03
PORTA: EQU 0x05
TRISA: EQU 0x05
PORTB: EQU 0x06
TRISB: EQU 0x06

W: EQU 0
F: EQU 1

ORG 0x0
goto MAIN
org 5

MAIN:
clrf PORTA
clrf PORTNB
PAGE1
movlw B'00000111'
iorwf TRISA,F ;RA0-RA2 as input
clrf TRISB ;Port B0-B7 as output
PAGE0
loop:

;get the output selector
movf PORTB,w
rrf WREG
andlw B'00000011'

;convert 0-3 to bit-mask
call table
movwf Mask

;switch all zeroes to ones in mask and vice versa
xorlw B'11111111',w

;read all the bits of PORTA but the selected one (leave it clear)
andwf PORTA,w

;if PORTB.0 is set, set also selected output in PORTA
btfsc PORTB,0
xorwf Mask,w

;write modified value back to PORTA
movwf PORTA
goto loop

table:

addwf PCL,F
retlw B'00000001'
retlw B'00000010'
retlw B'00000100'
retlw B'00001000'

 

You are setting PORTA as input and PORTB as output. It should be the other way around.