> I am trying to separate ASCII digits 0-9 to hex 0-9.

First thing you should do is start with a decent problem statement.  This
one is just gibberish.  What do you mean by separate?  What does it mean
to separate it "to hex"?

It looks like you really just want to determine whether a byte value
represents an ASCII character in the range of "0" to "9".  If so, two
compares is all you need:

;
;   Jump to BAD if value in W is not an ASCII character from "0"
;   to "9", otherwise fall thru with the 0-9 digit value in REG0.
;   REG0 is trashed if the character was out of range.
;
     addlw   -"0"   ;make 0-9 digit value if within range
     movwf   reg0   ;save digit value
     btfsc   reg0, 7 ;char was not below range ?
     goto    bad    ;was below range

     sublw   9      ;compare to max valid value
     skip_wle       ;within range ?
     goto    bad    ;no


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