3.1.1 Load and Run Addresses

Consider an embedded device for which the program's load image is burned onto EPROM/ROM. Variable data in the program must be writable, and so must be located in writable memory, typically RAM. However, RAM is volatile, meaning it will lose its contents when the power goes out. If this data must have an initial value, that initial value must be stored somewhere else in the load image, or it would be lost when power is cycled. The initial value must be copied from the non-volatile ROM to its run-time location in RAM before it is used. See Section 8.8 for ways this is done.

The load address is the location of an object in the load image.

The run address is the location of the object as it exists during program execution.

An object is a chunk of memory. It represents a section, segment, function, or data.

The load and run addresses for an object may be the same. This is commonly the case for program code and read-only data, such as the .const section. In this case, the program can read the data directly from the load address. Sections that have no initial value, such as the .bss section, do not have load data and are considered to have load and run addresses that are the same. If you specify different load and run addresses for an uninitialized section, the linker provides a warning and ignores the load address.

The load and run addresses for an object may be different. This is commonly the case for writable data, such as the .data section. The .data section's starting contents are placed in ROM and copied to RAM. This often occurs during program startup, but depending on the needs of the object, it may be deferred to sometime later in the program as described in Section 3.5.

Symbols in assembly code and object files almost always refer to the run address. When you look at an address in the program, you are almost always looking at the run address. The load address is rarely used for anything but initialization.

The load and run addresses for a section are controlled by the linker command file and are recorded in the object file metadata.

The load address determines where a loader places the raw data for the section. Any references to the section (such as references to labels in it) refer to its run address. The application must copy the section from its load address to its run address before the first reference of the symbol is encountered at run time; this does not happen automatically simply because you specify a separate run address. For examples that specify load and run addresses, see Section 8.5.6.1.

For an example that illustrates how to move a block of code at run time, see Example 10. To create a symbol that lets you refer to the load-time address, rather than the run-time address, see the .label directive. To use copy tables to copy objects from load-space to run-space at boot time, see Section 8.8.

ELF format executable object files contain segments. See Section 2.3 for information about sections and segments.