8.5.8 Overlaying Pages

Some devices use a memory configuration in which all or part of the memory space is overlaid by shadow memory. This allows the system to map different banks of physical memory into and out of a single address range in response to hardware selection signals. In other words, multiple banks of physical memory overlay each other at one address range. You may want the linker to load various output sections into each of these banks or into banks that are not mapped at load time.

The linker supports this feature by providing overlay pages. Each page represents an address range that must be configured separately with the MEMORY directive. You then use the SECTIONS directive to specify the sections to be mapped into various pages.

NOTE

Overlay Section and Overlay Page Are Not the Same

The UNION capability and the overlay page capability (see Section 8.5.7.2) sound similar because they both deal with overlays. They are, in fact, quite different. UNION allows multiple sections to be overlaid within the same memory space. Overlay pages, on the other hand, define multiple memory spaces. It is possible to use the page facility to approximate the function of UNION, but it is cumbersome.