sub queue {
    my ($self, $q) = @_;

    if (defined $q){
        if (! exists $queue{$q}){
            die "queue param requires a binary string.\n";
        }
        $self->{queue} = $queue{$q};
    }

    $self->{queue} = DEFAULT_QUEUE if ! defined $self->{queue};

    my $bits = $self->bits;

    # unset
    $bits &= ~MAX_QUEUE;

    # set
    $bits |= $self->{queue};

    my $lsb = $bits & 0xFF;
    my $msb = $bits >> 8;

    $self->register($msb, $lsb);

    return $self->{queue};
}