|To implement the trick using a zero reading register, i try to find a way
|not to use a file register, if i don?t have to.

|Some controllers have logic that make unimplemented registers read as '00'
|or 'FF'.  I could not find any info about PIC behaviour.
|Anybody knows?

PIC's are documented as having most of their "reserved" registers
read as zero.  There aren't any registers that are consistently
"reserved" among all PICs, though.

A fairly universal approach would be to use address 0x7F and make
sure to clear it at the start of code.  This will waste a byte of
RAM on those controllers that have lots, but not on the controllers
that have less than 96 bytes.

Alternatively, you may be able to take advantage of a register that
will always "happen" to contain zero when you need it for that (pro-
vided that it's initialized at the start of the code).  Iteration
counters are a very common case of this: many of them are zero out-
side the loop they control.

As another side note, if you have an iteration counter which is guar-
anteed to be zero when approaching the start of a loop, you may be
able to replace, e.g.

                movlw   8
                movwf   count

with

                bsf     count,3

or [if W holds a value which might be useful within the loop] you
could replace

                movlw   10
                movwf   count

with

                bsf     count,3
                bsf     count,1

Note that if /count/ is not initially zero, the above instructions may
set it to an undesired value.