$D3 = ( $num >> 0 ) & ( 1 << 4 ) - 1;  # 4 bits starting at bit 0
$D2 = ( $num >> 4 ) & ( 1 << 3 ) - 1;  # 3 bits starting at bit 4
$D1 = ( $num >> 7 ) & ( 1 << 1 ) - 1;  # 1 bit  starting at bit 7

##</code><code>##

$D3 = $num & ( 1 << 4 ) - 1;  # 4 bits starting at bit 0
$num = $num >> 4;

$D2 = $num & ( 1 << 3 ) - 1;  # 3 bits starting at bit 4
$num = $num >> 3;

$D1 = $num & ( 1 << 1 ) - 1;  # 1 bit  starting at bit 7
$num = $num >> 1;

##</code><code>##

@width = ( 4, 3, 1 );

$D3 = $num & ( 1 << $width[ 0 ] ) - 1;  # 4 bits starting at bit 0
$num = $num >> $width[0];

$D2 = $num & ( 1 << $width[ 1 ] ) - 1;  # 3 bits starting at bit 4
$num = $num >> $width[ 1 ];

$D1 = $num & ( 1 << $width[ 2 ] ) - 1;  # 1 bit  starting at bit 7
$num = $num >> $width[ 2 ];

##</code><code>##

sub bitfield {
    my ( $num, @field ) = @_;
    my @value;
    for my $width ( @field ) {
        push @value, $num & ( 1 << $width ) - 1;
        $num = $num >> $width;
    }
    return @value;
}