In article <32CFE892.5BEF@pacific.net>, Brooke <brooke@PACIFIC.NET>
writes
>> When saving status during interrupts on the 16c74,
>>   Microchip recommends the following:
>> movwf   W_Temp         ;Copy W to TEMP register, could be bank one or zero
>
>Note that W_Temp MUST be defined so that it appears in the same location
>independent
>of the RP0 (RP1, ...) bits. For example W_Temp equ h'20' AND W_Temp equ
>h'A0'.

I don't see anyone mentioning PCLATH context saving here! Essential if
you venture into page 1 of program memory..

To do it, you would add..

>
>> swapf   STATUS, w      ;Swap status to be saved into W
>> bcf     STATUS, RP0    ;Change to bank zero, regardless of current bank
>> movwf   STATUS_TEMP    ;Save status to bank zero STATUS_TEMP register
   movf    PCLATH,w
   movwf   temp_lath
   bcf     PCLATH,3       ;this line is crucial, otherwise your first
                          ;GOTO or CALL in the ISR could jump into the
                          ;other program page with terminal results..
                          ;(this is assuming your ISR resides in page 0)
>> :(ISR)

   movf    temp_lath,w
   movwf   PCLATH
>> swapf   STATUS_TEMP, w ;Swap STATUS_TEMP register into W
>>                       ;(sets bank to original state)
>> movwf   STATUS         ;Move W into STATUS register
>> swapf   W_TEMP, f      ;Swap W_TEMP
>> swapf   W_TEMP, w      ;Swap W_TEMP into W

--
Jim Main
jim@ewcomm.demon.co.uk
Visit Eastwood Communications at http://www.ewcomm.demon.co.uk
Broadcast Electronics & Custom Design