Addresses are defined, that is, made available for potential use, in one of four ways:
| Fork | When a process is created using fork(), the new process is given a duplicate copy of the parent process's page table, so that any addresses that were defined in the parent's address space are defined in the address space of the new process. |
| Stack | The call stack is created and extended automatically. When a function is entered and more stack space is needed, IRIX makes the stack segment larger, defining new addresses if required. |
| Mapping | A process can ask IRIX to map (associate byte for byte) a segment of address space to one of a number of special objects, for example, the contents of a file. This is covered further under "Mapping Segments of Memory". |
| Allocation | The brk() function extends the heap, the segment devoted to data, to a specific virtual address. The malloc() function allocates memory for use, calling brk() as required. (See the brk(2), malloc(3), and malloc(3x) reference pages). |
An address is defined by entry in the page tables. A defined address is always related to a backing store, a source from which its contents can be retrieved. A page in the data or stack segment is related to a page in a swap partition on disk.
The total size of the defined pages in an address space is its virtual size, displayed by the ps command under the heading SZ (see the ps(1) reference page).
Once addresses have been defined in the address space, there is no way to undefine them except to terminate the process. To free allocated memory makes the freed memory available for reuse within the process, but the pages are still defined in the page tables and the swap space is still allocated.