Answer from cs61c-es (Meng Chang Bruce Lin 16075852) for Question 2
First, subnormal is related to underflow threshold. Underflow threshold is defined when a calculation result in non-zero number whose magnitude would be less than the underflow threshold, than it is considered to have underflow. The result will then sometimes be "subnormal number", but it maybe round down or round up to the threshold value.
So, we can define subnormal as the the magnitude of a non-zero number that is less than the underflow threshold.
It is important because when result is too small and might not be able to be represented. Underflow is differnent than overflow issues. So, when we want to calculate smaller than predefined limites, underflow is useful. In another word, we can represent a very small value that fall into the gap between zero and the underflow thresholds. So, in this small range, we can present subnormal numbers.