collapseos/blk/606

( c n -- c )
( Computes n into crc c with polynomial 0x09
  Note that the result is "left aligned", that is, that 8th
  bit to the "right" is insignificant (will be stop bit). )
: _crc7
    XOR           ( c )
    8 0 DO
        2 *       ( <<1 )
        DUP 255 > IF
            ( MSB was set, apply polynomial )
            0xff AND
            0x12 XOR  ( 0x09 << 1, we apply CRC on high bits )
        THEN
    LOOP
;