A branch predictor with a high prediction accuracy is often employed to enable deeply pipelined processors to be exploited. What limits the complexity and size of such a branch predictor? (c) In the best case, how can a branch predictor and branch target buffer enable a branch instruction and the instruction at the branch's target address to be fetched in consecutive clock cycles? (d) Loop unrolling and predicated execution are two techniques that may be used to improve the performance of loops. (i) How do these techniques improve performance? (ii) What costs or disadvantages are associated with each technique? 

(a) Explain in general how the actions that a user takes are related to the user's goals. Your answer should make reference to the function of perception, and to the nature of the cognitive processing that must occur. (b) Describe a class of problems for which it is not possible to formulate goals. Give a specific example of a problem in this class, and with reference to that example, explain how it illustrates two significant attributes of the class. [6 marks] (c) If an interactive system has several alternative models to describe the user's goal, how can Bayes' theorem be used to improve the system usability?