8.5.7.2 Overlaying Sections With the UNION Statement

For some applications, you may want to allocate more than one section that occupies the same address during run time. For example, you may have several routines you want in fast external memory at different stages of execution. Or you may want several data objects that are not active at the same time to share a block of memory. The UNION statement within the SECTIONS directive provides a way to allocate several sections at the same run-time address.

In Example 13, the .bss sections from file1.c.obj and file2.c.obj are allocated at the same address in FAST_MEM. In the memory map, the union occupies as much space as its largest component. The components of a union remain independent sections; they are simply allocated together as a unit.