Or a much more efficient routine would be (in parallax mnemonics):
ChkEncoder  mov new,encoder ;move the ra port the new var
                and     new,#00000011   ;clear all but encoder bits
                mov temp, new
                xor     temp,old        ;is old and new same?
                jz      :Return         ;if so quit
                clc                     ;clear carry
                rl      old             ;shift old to align old.0 with new.1 
bit
                xor     old,new         ;xor bits to determine direction
                jb      old.1,:up       ;if bit is 1 then up (CW,etc), if bit 
is 0 then down (ccw,etc)


In a message dated 10/1/99 4:59:48 PM Eastern Daylight Time, 
morgans.rt@TELIA.COM writes:

> Generally,
>  
>  Get the two phase bits
>  
>  Together with the two bits from the last read form a 4-bit word
>  
>  Use this for a 16 entry goto table
>  
>  The table will contain 4 each of gotos for count up, count down, no change 
(
> glitch) and error (both phase changed, probably overspeed).
>  
>  An easy way of getting the bits right is to left rotate them into a 
register,
>  then mask using andlw 1111b before addwf PCL.  Save the register until 
next 
> call to this routine.
>  
>  Interrupt on change is excellent for getting an interrupt on any change.
>  
>  But the routine can also be called anytime as it will handle no change too.
>  
>  I did this on a F877 this week, using interrupt on change  :)
>  
>  It did reach 13kHz+ count and at the same time 4kHz timer 0 int driving 
some 
> software timers etc, using 4,096 MHz xtal and 2-level interrupt structure.
>  (Interrupt get what happened / Interrupt post service with interrupt re-
> enabled (soft timers, safety checks, buffers, etc) / Main prog)
>  
>  ...wait I'll dump the 2-phase decoder right here... there is some macros, 
i.
> e ifc=if clear then execute next instruction = Mchip btfss.  there are also 
> some debug macros DEBUGsomething, 16-bit count variable is named 
W_something..
> .
>  
>  But I think you will get the idea.
>  I«ll threw in a few english words...
>  Enjoy!
>  
>  ----8<----
>  
>           ifc             INTCON,RBIF             ;Har porttillstŒndet 
> Šndrats?
>             goto  isrP_NoRBIF             ;Om inte: hoppa till nŠsta 
> kollrutin
>  
>  ;LŠs in nya tillstŒndet 
>           movf    PORTB,w                 ; *** Ingen annanstans fŒr PORTB 
> lŠsas dierkt! ***
>           c               INTCON,RBIF             ;Kvittera att vi har lŠst
>           movwf   B_PORTBIN               ;lagra fšr denna samt andra 
rutiner 
> pΠport B
>  
>  ;Bearbeta tvŒfassignal genast!
>  ;HŠmta fasbitarna frŒn B_PORTBIN till B_2phState 1:0, de fšrra i 3:2...
>           cc                              ;Rotate the new bits in...
>           ifs             B_PORTBIN,5
>             sc
>           rlf             B_2phState,f
>           cc
>           ifs             B_PORTBIN,4
>             sc
>           rlf             B_2phState,f
>           movf    B_2phState,w
>           andlw   b'1111'
>           addwf   PCL,F           ;Before Now     don«t care this below
>  
>           goto    isrBchange_NO   ; 0 0   0 0             00      00
>           goto    isrBchange_UP   ; 0 0   0 1             01      10
>           goto    isrBchange_DN   ; 0 0   1 0             10      01
>           goto    isrBchange_ERR  ; 0 0   1 1             11      11
>  
>           goto    isrBchange_DN   ; 0 1   0 0             01      01
>           goto    isrBchange_NO   ; 0 1   0 1             00      11
>           goto    isrBchange_ERR  ; 0 1   1 0             11      00
>           goto    isrBchange_UP   ; 0 1   1 1             10      10
>  
>           goto    isrBchange_UP   ; 1 0   0 0             10      10
>           goto    isrBchange_ERR  ; 1 0   0 1             11      00
>           goto    isrBchange_NO   ; 1 0   1 0             00      11
>           goto    isrBchange_DN   ; 1 0   1 1             01      01
>  
>           goto    isrBchange_ERR  ; 1 1   0 0             11      11
>           goto    isrBchange_DN   ; 1 1   0 1             10      01
>           goto    isrBchange_UP   ; 1 1   1 0             01      10
>  ;       goto    isrBchange_NO   ; 1 1   1 1             00      00
>  isrBchange_NO
>  ;       DEBUGIND_const 4        ; TEST 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> <
>           s               F_2phGlitch     ;Indikera "glitch"      GLITCH?
>           ;ev godkŠnna glitch?  NŠe; lšs ut = prioritera sŠkerhet
>  isrBchange_ERR
>           DEBUGIND_const 5        ; TEST 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> <<
>           s               F_2phSpeed      ;Indikera "šverfart"  OVERSPEED
>           goto    isrP_2phEnd
>           
>  isrBchange_UP
>           INC2    W_2phcnt
>           ifzc
>             goto isrP_2phEnd
>           s               F_2phRange      ;Indikera "utanfšr skala"
>           comf    W_2phcnt,f      ;€ndra 0000h till FFFFh
>           comf    W_2phcnt+1,f
>  ;       DEBUGIND_const 7        ; TEST 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> <
>           goto    isrP_2phEnd
>  
>  isrBchange_DN
>           FDEC2   W_2phcnt
>           comf    W_2phcnt+1,W    ;€r hšgbyten annat Šn FF sŒ Šr det OK
>           ifzc                                    ;AlltsŒ: Šr 
ettkomplementet 
> annat Šn 0?
>             goto isrP_2phEnd              ;I sΠfall OK!
>           comf    W_2phcnt,W      ;Annars kolla lŒgbyten likadant...
>           ifzc
>             goto isrP_2phEnd
>                                                   ;Om wrappade till FFFF:
>           s               F_2phRange      ;Indikera "utanfšr skala"
>           clrf    W_2phcnt        ;nollstŠll rŠknaren..
>           clrf    W_2phcnt+1      ;
>  
>  ;       DEBUGIND_const 6        ; TEST 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> <
>  
>  isrP_2phEnd
>  
>  ;       DEBUGIND_low4 W_2phcnt  ; TEST 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> <
>           
>  isrP_NoRBIF
>  
>           DEBUGIND_const 4;Int on change klar