@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 ];