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$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

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

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

@width = ( 4, 3, 1 );

...

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

sub bitfield {
    my ( $num, @field ) = @_;
...
    }
    return @value;
}