(Subject added)

> You can do this several ways,

Another way is XOR. When two bit patterns are the same the result is 0

         xorlw   "a"
         bz      is_a

If there are two or more characters to detect, eg "a" or "b" or "c"

         xorlw   "a"
         bz      is_a
         xorlw   "a" ^ "b"  ;restore W and re-test
         bz      is_b
         xorlw   "b" ^ "c"  ;restore W and re-test
         bz      is_c

Not quite what you were asking but it may come in handy one day

I have this routine to detect non-letter characters in a received string
and convert any lower case to upper case (to simplify testing in other
routines)

cloop    movlw   "^"          ;test for end-of-string character, "^"
         xorwf   indf0,w
         bz      eos          ;yes
         movfw   indf0        ;no, get char
         addlw   d255-"z"     ;test
         addlw   ("z"-"a")+1
         bnc     nextc        ;no
         movlw   0xe0         ;LC letter, subtract 0x20
         addwf   indf0        ;replace in string
nextc    incf    fsr0l        ;increment indices
         incf    str_ptr
         bra     cloop

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist