One way to reduce an application's memory requirement is to use the same range of memory for multiple purposes. You can first use a range of memory for system initialization and startup. Once that phase is complete, the same memory can be repurposed as a collection of uninitialized data variables or a heap. To implement this scheme, use the following variation of the UNION statement to allow one section to be initialized and the remaining sections to be uninitialized.

Generally, an initialized section (one with raw data, such as .text) in a union must have its load allocation specified separately. However, one and only one initialized section in a union can be allocated at the union's run address. By listing it in the UNION statement with no load allocation at all, it will use the union's run address as its own load address.

For example:

UNION run = FAST_MEM
{ .cinit .bss }

In this example, the .cinit section is an initialized section. It will be loaded into FAST_MEM at the run address of the union. In contrast, .bss is an uninitialized section. Its run address will also be that of the union.