2) For the program shown below, draw a picture of the run-time environment at th

Question

2) For the program shown below, draw a picture of the run-time environment at the point marked "HERE", and explain how variables x, y, and z, are accessed, for each of the following two techniques:

a) Static Links

b) Displays program

bar();

var b: integer;

procedure foo(var x: integer);

var x1: integer;

procedure baz(y: integer);

procedure zab(z: integer);

begin if z = 1 then zab(0) else x := y + z; // HERE

end:("zab") begin("baz") zab(1);

end("baz") begin("foo") z1 := 3;

baz(x1);

end:("foo") begin("main") foo(b);

end

Explanation / Answer

In this program if we consider the solution for static links at the run time environment can be described as:-

In statically-scoped languages variables are "non-local" in the nested sub-program that accesses them. These variables are stored in the activation record of the sub-program that declares them. When they are used as non-local variables, that activation record is found at runtime either using access static links or a display

bar();

var b: integer;

procedure foo(var x: integer);

var x1: integer;

procedure baz(y: integer);

procedure zab(z: integer);

begin if z = 1 then zab(0) else x := y + z; // HERE

end:("zab") begin("baz") zab(1);

end("baz") begin("foo") z1 := 3;

baz(x1);

end:("foo") begin("main") foo(b);

end

At the HERE instruction acivation record of zab procedure is checked for the value of z.

If the value is 1, zab(0) will be called with the value of z updated to 0.

else the activation records of foo, baz and zab are accessed to get the values of x, y and z and the value of x is updated as the sum of y and z in the activation record of foo.

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