Suppose that the target virtual machine is stack-oriented and that the stack elements are integer values, and addresses can be stored as integers. Explain which other features are required in such a virtual machine. Invent a simple language of instructions for such a machine and show how it would be used to implement each of the expressions. [10 marks] (c) Suppose that the following rules are proposed as possible optimizations to be implemented in your compiler. Other details: Just please, dont answer if you are not sure

Then, expression simplifies to expression (fst e, snd e) e fst (e1, e2) e1 snd (e1, e2) e2 Describe how you could implement these rules so that the simplifications are made only when the program's semantics is correctly preserved. [5 marks] 5 (TURN OVER) CST.2016.3.6 5 Concepts in Programming Languages (a) Explain what is meant by a monad in a programming language, giving the two fundamental operations of a monad along with their types. [3 marks] (b) Consider the use of a monad for input-output. For the purposes of this question, take the IO monad as including two operations readint and writeint which respectively read integers from stdin and write integers to stdout. Give the types of these operators. [2 marks] (c) Assume MLreadint and MLwriteint are primitives with side effects for inputoutput and consider the ML expression add1 of type int: let val x = MLreadint() in MLwriteint(x+1); x end (i) Give an equivalent expression which uses the IO monad instead of side-effects, and state its type. [3 marks] (ii) Give a function run2diff which can be applied to your answer to part (c)(i). When so applied it should give a value in the IO monad which corresponds to ML code that runs add1 twice and returns the difference between the values read. [4 marks] (d) State what happens when attempting to compile and execute the following Java fragment (explaining the origin of any error messages or exceptions which might arise). Object n = new Integer(42), o = new String("DATA");

Maximum 5 marks in total for question part Maximum 3 marks for algorithm Description (max 3) - set variable called HighestScore to zero and variable called BestName to dummy value - loop 30 times to check each student's total score in turn - check student's score against HighestScore - if student's score > HighestScore then - replace value in HighestScore by student's score and store student's name in BestName - output BestName and HighestScore outside the loop Sample algorithm (max 3): HighestScore 0 BestName "xxxx" (1 mark) FOR Count 1 TO 30 IF StudentTotalScore[Count]

Explain the difference between 'y' and "xy" when used as constants in C. Describe the memory representation of both values. [4 marks] (b) Consider the following C program: void swap(int x, int y) { int temp = x; x = y; y = temp; } int main(int argc, char **argv) { int x = 0; int y = 1; swap(x, y); assert(x == 1); return 0; } Briefly explain the role of the assert statement and why this program will trigger an assert failure when executed. Supply two modified versions of the program that alter the swap function definition and, if necessary, its calls, to avoid this assert failure. One version should be in C, and the other should use C++ language features. [4 marks] (c) Describe the address-space layout (highlighting four areas of memory) of a typical compiled x86 C program, and how each of these areas are used by C constructs. [8 marks] (d) Briefly explain what undefined behaviour is in the (ii) Briefly explain why it is important to have unspecified behaviour in the definition of the C language. [1 mark] (b) Compare and contrast the struct and union keywords in C, supplying an example of a situation where it would be more appropriate to use a union rather than a struct. [4 marks] (c) Explain the following C or C++ language concepts. You may find it helpful to use short code fragments or diagrams to illustrate your answer. (i) The virtual keyword used to qualify a C++ member function and its impact on generated code. [4 marks] (ii) The role of the C preprocessor in the source-code compilation cycle, and why it is a useful tool for debugging. [4 marks] (iii) Templated functions in C++, giving one benefit and one drawback of using them compared with using a void* function in C.