Every time a program creates a new object some space must be allocated for it in working memory. Thereafter it is referred to by a pointer. The exceptions are small integers and single precision decimals which need no more space than a pointer, so they are simply copied to wherever they are needed. They are called `simple' objects, whereas the objects that are referred to via pointers are referred to as `compound' objects.
The Pop-11 memory manager (in Poplog it's the Poplog memory manager, shared by all the Poplog languages) maintains a region of storage in virtual memory space, in which compound objects are created, including large numbers, strings, words, lists, vectors, and even procedures. This area is known as the `heap'.
Every now and again the attempt to create a new object is hindered by lack of space in the heap. At that point an automatic garbage collection program is invoked which works out which objects are no longer accessible by any portion of the current program. The garbage collector then rearranges the heap so that all the accessible objects are compacted together (and all pointers to them are correspondingly changed). This leaves additional space in the heap free for new objects to be created. If there is no free space even after a garbage collection, the storage manager will try to get more space from the operating system in order to enlarge the heap (the limit to such enlargement is set by the user-assignable variable popmemlim).
It is possible that eventually there is no more space in the machine, and attempts to create new structures are foiled and the process has to be abandoned. This can depend on what else is happening on the machine at the time. The Poplog garbage collector is unusually fast for an AI development environment, so most of the time users will not notice when garbage collection happens.
The above is an over simplification in that the Poplog system has to be able to cope not only with ordinary Poplog structures, but also data structures linked in via external programs, which might have been written in C or Fortran. Moreover some of the Poplog structures may be made accessible to external procedures. These complications require the heap to have a mixture of sub-regions with different characteristics. For example some regions are used by Poplog structures that cannot be relocated by the garbage collector because external procedures will then be confused. Also the heap may include `holes' corresponding to regions of memory that are used by external procedures to create structures that Poplog (and Pop-11) cannot access. Readers who wish to know more should consult the online files REF EXTERNAL and REF EXTERNAL_DATA