1.	a.	Explain the structure of:

i. portable compilers;						4 marks
ii. high level optimising compilers.				4 marks

Why are their priorities different?				2 marks

b. Using the fragment of three address code below, show how a flow
graph can be obtained and how a loop can be identified. All statements
use the same notation as that used throughout the lectures.

(1) B1: I := 1
(2)   J := 1
(3)   K := 1
(4) B2: if I$>$N goto B5
(5) B3: if J$>$N goto B6
(6) B4: T1 := 0[SP]
(7)   T2 := T1 - 4
(8)   T3 := 4 * I
(9)   T4 := T2[T3]
(10)  T5 := 4[SP]
(11)  T6 := T5 -4
(12)  T7 := 4 * J
(13)  T8 := T6[T7]
(14)  if T4 $<=$ T8 goto B6						8 marks

c. What is the alternative to three address code, which is sometimes
used?                                      2 marks

2. a. Explain, with appropriate examples, four types of high level,
machine independent  optimisation.	8 marks

b. Consider the following attribute grammar, which defines a syntax
directed translation scheme. 

decls -> decl decls ¦  empty

decl -> 	type  list		list.inheritance:= type.type

type ->  	int			type.type:= integer

type ->  	real			type.type:= real

list ->  	list1 ,  id		list1.inheritance:= list.inheritance

				addtype(id.entry, list.inheritance)

list ->  	id			addtype(id.entry, list.inheritance)

id -> letter*

       and the following string

int a, b
real c, d

i.	Construct the parse tree, with the attributes of all nodes
shown.		8 marks 
ii.	Explain what is meant by a dependency graph for such a parse
tree.		4 marks