Answer from cs61c-aj (Kevin Li 15855812) for Question 1
Since data can be anything and data are represented by bits of 1's and 0's, someone can change some data and cause one of the bit to change and this can represent a totally different data now. For example, the instruction for withdraw at a teller can be changed so that instead of subtracting money during a withdraw money is added. A possible technique to prevent this might be to set up "guards" in the hardware to scan and see which bit is recenly change and see if it is an appropriate change. The trade off would be slower compute time.