private function computeUnsignedChecksum( $bytestring )
	{
		$unsigned_chksum	= 0;
		for( $i=0; $i<512; $i++ )
			$unsigned_chksum += ord( $bytestring[$i] );
		for( $i=0; $i<8; $i++ )
			$unsigned_chksum -= ord( $bytestring[148 + $i]) ;
		$unsigned_chksum += ord( " " ) * 8;
		return $unsigned_chksum;
	}