====== ASCII Char to 4 bit integer ======


This is a very simple routine that converts an ASCII char contained in 'a' to a 4 bit number returned in 'a'. 
It can deal with upper/lowercase hex letters 'a'..'f' as well as 'A'..'F' and returns the error code 255 for
nonvalid input like 'g'..'z', 'G'...'Z' or other non hex digits like interpunction, spaces, math symbols etc...
 
<code 6502acme>
  sec
  sbc #'0'
  bcc error
  
  cmp #10
  bcc return

nondigit
  and #$3f
  cmp #('f'-'0'+1)
  bcs error
  cmp #('a'-'0')
  bcc error
  
  sbc #('a'-'0'-10) 
  rts
error
  lda #255
return
  rts
</code>