3.6.1.4 Const-data-in-config-mapped-progmem Option

The -mconst-data-in-config-mapped-progmem option stores objects qualified with const to an area of program memory that the compiler will ensure is mapped into the data memory space for some devices.

Certain devices, such as the AVR DA and AVR DB family, have an SFR (e.g. FLMAP) that specifies which 32 KB section of program memory will be mapped into the data memory space. The -mconst-data-in-config-mapped-progmem option can be used to have the linker place all const-qualified data within one of these 32 KB sections and automatically initialize the relevant SFR register to ensure that these objects are mapped into data memory, where they will be accessed more efficiently in terms of program size. The option must be used in conjunction with the -mconst-data-in-progmem option; however, this option is enabled by default.

The -mconst-data-in-config-mapped-progmem option is automatically enabled for devices that can support this mapping feature. In that case, the feature can be disabled by using the -mno-const-data-in-config-mapped-progmem option, forcing const-qualified objects to be accessed from program memory.

As the -mconst-data-in-config-mapped-progmem option affects both how code is generated and the choice of library to link against, it (or its negated no- form) must be used with both compile and link steps of the build process.

When this option is selected, the runtime startup code will set the FLMAPLOCK bit. Doing so prevents any modification of the FLMAP register. This is necessary since the code generated by the compiler assumes the page mapped by the compiler at compile time will still be mapped at runtime. If a program could change the mapping at runtime, any code that reads const data might fail.

Although the mapped page cannot be changed at runtime, you have some control over which page will be mapped. If you use the -mconst-data-in-config-mapped-progmem option as well as the __at() construct to place const data in a page of your choice, the linker will attempt to allocate all const data in the same page and ensure that page is mapped. For example:

const char __at(0xc000) arr[3] = {3, 1, 2};  // Force arr into page 1
const char arr2[3] = {3, 1, 2};              // arr2 will be placed in the same page

If __at() is used more than once to force objects into different pages, the compiler will generate an error and suggest disabling the -mconst-data-in-config-mapped-progmem option. If you place some const data using __at() but you also need other const data in an alternate page that is not mapped (will be accessed using LPM instructions), then use the __memx specifier with the other data. For example:

const char __at(0xc000) arr[3] = {8, 1, 2};        // Force arr into mapped page 1
const char __at(0x300) __memx arr2[3] = {7, 1, 2}; // Force arr2 into page 0

If more than 32 KB of const-qualified data is defined or the section holding this data cannot otherwise be located in the mapped memory section, an error from the linker will be issued.

The macro __AVR_CONST_DATA_IN_CONFIG_MAPPED_PROGMEM__ is defined if this option is enabled and const-qualified objects are located in a mapped section of memory.