$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;