I've been plowing through the source in Microchips App Note AN555 (asynch
serial i/o) trying to understand it, and can't quite figure out what a
common sequence is doing. I'm still pretty new at this, so please be
gentle. The code looks like this (comments are mine):

clrf      TMR0           ; clear both timers
clrwdt
bsf       STATUS,RP0     ; bank 1
movlw     07h
movwf     OPTION_REG     ; load 0x07 into option reg
bcf       STATUS,RP0     ; back to bank 0
clrf      TMR0           ; clear timer0
bsf       STATUS,RP0     ; back to bank 1
movlw     0Fh
movwf     OPTION_REG     ; load 0x0F into option reg
clrwdt                   ; clear wdt

So it's clearing timer0 and the wdt, loading the option register with
0x07, clearing timer0, then immediately re-loading option with 0x0F, then
clearing the wdt. To my untrained eye, it seems like a lot of thrashing
around that isn't actually accomplishing very much. This sequence is
executed near the start of each of the interrupt service routines
transmit bit, receive bit, etc.)

Check me here: ignoring the high nibble (which is the same in both cases)
0x07 in the option register means:

     prescaler assigned to tmr0, with "full" value (1:256)

And 0x0F in option means:

     prescaler assigned to wdt, with "full" value (1:128)

I guess my question is: why bother with the first option register load?
The end result, it seems, is to simply reset the wdt to it's longest
delay. Come to think of it, why bother with any of this? Usually just
after this code, timer0 is reloaded with a pre-calculated value
(depending on clock frequency and baud rate) and set running off the
internal clock, to generate the next interrupt (next bit time). It all
seems rather useless to me, but I'm quite sure I'm missing something
important, maybe side effects?

Any insight would be greatly appreciated.

Dave Johnson