CMPE 117 - Sample Midterm 2 Questions

These are sample questions for the second midterm. All problems relate to Esterel. The midterm is open books: in particular, you can take with you the Esterel primer. In the sample computations, ';' separates the signals received in different ticks (this is similar to the simulator syntax). Outputs are written in the line below the inputs, for clarity.

• Write a loop that emits O whenever either A or B is received.

• Write a loop that emits O when at least two of A1, A2, A2, A4, A5 have arrived. Example computation:
  

  A1 ; A2 ; ; A1 A4 ; ; ; A5 ; ; A2 ; ...
     ;  O ; ;     O ; ; ;    ; ;  O ; ...
  

• Write a module that, upon receipt of A, immediately switches between the following behaviors:
  • emit O every second T
  • emit O every third T
  Example computation:
  

  T ; T ; T ; T ; T ; A T ; T ; T ; T ; T ; T ; A T ; T ; ...
    ; O ;   ; O ;   ;     ;   ; O ;   ;   ; O ;     ; O ; ...
  

• Let n be an integer variable (in Esterel). Write some code that emits exactly n consecutive O, then terminates.

• Consider the following loop:

         loop
            [weak] abort 
               [immediate] await A
            when [immediate] B
         end loop 
  

  Here, the square brackets indicate optional keywords. There are eight cases, depending on the subset of the optional three keywords that are included in the program. For which of the eight cases is the loop non-immediate?

• There are three processes S, T, U, and a module R. When S begins, the event S_start is emitted; when S ends, the event S_end is emitted; similarly for T and U. S, T, U can start and stop at any time. Write a piece of code that controls whether R runs, such that:
  • when S is running, R should be suspended, and R should resume execution (from where it left off) once S terminates.
  • when T or U start running, R should be stopped; when both T and U stop running, R should be restarted from the beginning.

• Consider the following pieces of code. Do they satisfy constructive causality?
  • [ present O then emit A end] || [ present A then emit A ]

  • present A then emit A else emit A

  •       present A then 
             emit B
          end present 
      ||
          present B else 
             emit A 
          end present 
      ||
          emit A