8.2 ACTIVATION OF THE PROCEDURE AND THE ACTIVATION RECORD

8.1 STORAGE ALLOCATION

One of the important tasks that a compiler must perform is to allocate the resources of the target machine to represent the data objects that are being manipulated by the source program. That is, a compiler must decide the run-time representation of the data objects in the source program. Source program run-time representations of the data objects, such as integers and real variables , usually take the form of equivalent data objects at the machine level; whereas data structures, such as arrays and strings, are represented by several words of machine memory.

The strategies that can be used to allocate storage to the data objects are determined by the rules defining the scope and duration of the names in the programming language. The simplest strategy is static allocation, which is used in languages like FORTRAN. With static allocation, it is possible to determine the run-time size and relative position of each data object during compilation. A more-complex strategy for dynamic memory allocation that involves stacks is required for languages that support recursion: an entry to a new block or procedure causes the allocation of space on a stack, which is freed on exit from the block or procedure. An even more-complex strategy is required for languages, which allows the allocation and freeing of memory for some data in a non-nested fashion. This storage space can be allocated and freed arbitrarily from an area called a "heap". Therefore, implementation of languages like PASCAL and C allow data to be allocated under program control. The run-time organization of the memory will be as shown in Figure 8.1.

Figure 8.1: Heap memory storage allows program-controlled data allocation.

The run-time storage has been subdivided to hold the generated target code and the data objects, which are allocated statically for the stack and heap. The sizes of the stack and heap can change as the program executes.