Chapter 10 - Implementing Subprogram

Question 1

what is subprogram linkage

Answer 1

the subprogram call and return operations of a language

Question 2

5 things that a PL needs to do during a subprogram call

Answer 2

1) follow whatever parameter passing method is used
2) create local variables by allocating storage and binding those variables to storage
3) save the execution status of the calling program unit
4) arrange to transfer control to the subprogram and ensure that control can return to the proper place when the subprogram execution is complete
5) if nested subprograms are allowed, make non-local variables visible to the called subprogram

Question 3

4 things a subprogram must do upon termination

Answer 3

1) move the local values of the associated formal parameters to the actual parameters for the out or inout mode semantics
2) deallocate storage used for local variables
3) restore the execution status of the calling program unit
4) return control to the calling program unit

Question 4

What is a dynamic link?

Answer 4

a pointer variable that points to the top of the activation record (same as a stack frame) instance of the caller

Question 5

what is a dynamic chain or call chain?

Answer 5

a collection of dynamic links in the stack at a given time

Question 6

What is local_offset?

Answer 6

the location of local variables of a subprogram relative to the beginning of the activation record

Question 7

8 characteristics of an Algol60-like program stack

Answer 7

• parameters often passed by two methods
• local variables are often dynamically allocated
• recursion must be supported
• static scoping must be supported
• activation record format is static but size is dynamic
• the dynamic link points to the top of an instance of the activation record of the caller
• an activation record instance is created dynamically when a subprogram is called

Question 8

what is a static link?

Answer 8

a pointer variable that points to the bottom of the activation record of the static parent

only used when nested sub-programs are allowed in order to access non-local variables

Question 9

what is a static chain?

Answer 9

a chain of static links that connect certain activation records

Question 10

what is static depth?

Answer 10

an integer associated with a static scope whose value is the depth of nesting of that scope. (main starts at 0, and each nested sub-program is +1)

Question 11

what is chain offset or nesting depth?

Answer 11

the static depth of reference - static depth of declaration

Question 12

Two issues with static chain method

Answer 12

• nonlocal references are slow if # of scopes between reference and declaration variable (i.e. chain offset) is large
• cost of non-local references can change with code upgrades

Question 13

How are blocks represented on the stack?

Answer 13

as parameterless subprograms using the static chain process