Eric Johnson wrote:
>
> 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'.

> 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
> :(ISR)
> 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
>
>   Why not:
> swapf   W_Temp         ;save w with nibbles reversed, could be either bank

This is not the proper syntax for swapf.
The assembler will convert the above instruction to swapf W_Temp,F
ie. swap the nibbles in file W_Temp and put the result into file F
(W_Temp).

The syntax is: swapf f,d where f=a file(RAM) location and d=destination
(W or F)
Note that in the Harvard architecture there are no instructions that
move data
from one file (RAM) location to another file (RAM) location.  Data
always moves
to/from the W register.  This is because there are not enough bits in a
single
command word to contain 2 file (RAM) addresses.  Von Neumann machines
use a
variable number of bytes/instruction to handle this.

> swapf   STATUS, w      ;get status and reverse the nibbles
> bcf     STATUS, RP0    ;set bank 0
> movwf   STATUS_TEMP    ;save old status with nibbles reversed
> :(ISR)                ;interrupt service routine
> swapf   STATUS_TEMP, w ;get old status
> movwf   STATUS         ;restore old status (and bank register)
> swapf   W_TEMP, w      ;get back w with correct nibble order
>
> By pre-swapping w, it looks like I can save a step.
>
>   I also like to:
> movf    FSR, w        ;get indirect file select register
> movwf   Temp_Fsr, f   ;save indirect files select register
> :(ISR)                  ;interrupt routine
> movf    Temp_Fsr, w   ;get saved indirect file select register
> movwf   FSR, f        ;restore indirect files select register
> clrwdt               ;clear watchdog timer

Required if you are using FSR.  DO after STATUS & W are saved.

Thanks for asking, I think I now understand this better myself,
Brooke

PS I wonder if Harvard architecture machines would be easier to
understand
if the instructions that worked with RAM were different from
instructions
that worked with the program space in some way that made it clear which
space was being used?