Interaction With Unix's Memory Manager

The memory manager often begins with a single chunk of free memory. This chunk is not as large as the virtual address space of the process - only a small part of this space is initially mapped by the operating system.

When the memory manager runs out of memory in its free list, it invokes the brk() system call, to ask the operating system to map more of the process's available memory.

The system just enlarges the valid map, without actually allocating new physical memory pages - they are allocated when they are first accessed.