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6.16 Named Address Spaces

As an extension, GNU C supports named address spaces as defined in the N1275 draft of ISO/IEC DTR 18037. Support for named address spaces in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. At present, only the AVR, SPU, M32C, RL78, and x86 targets support address spaces other than the generic address space.

Address space identifiers may be used exactly like any other C type qualifier (e.g., const or volatile). See the N1275 document for more details.

6.16.1 AVR Named Address Spaces

On the AVR target, there are several address spaces that can be used in order to put read-only data into the flash memory and access that data by means of the special instructions LPM or ELPM needed to read from flash.

Per default, any data including read-only data is located in RAM (the generic address space) so that non-generic address spaces are needed to locate read-only data in flash memory and to generate the right instructions to access this data without using (inline) assembler code.

__flash
The __flash qualifier locates data in the .progmem.data section. Data is read using the LPM instruction. Pointers to this address space are 16 bits wide.
__flash1
__flash2
__flash3
__flash4
__flash5
These are 16-bit address spaces locating data in section .progmemN.data where N refers to address space __flashN. The compiler sets the RAMPZ segment register appropriately before reading data by means of the ELPM instruction.
__memx
This is a 24-bit address space that linearizes flash and RAM: If the high bit of the address is set, data is read from RAM using the lower two bytes as RAM address. If the high bit of the address is clear, data is read from flash with RAMPZ set according to the high byte of the address. See __builtin_avr_flash_segment.

Objects in this address space are located in .progmemx.data.

Example

     char my_read (const __flash char ** p)
     {
         /* p is a pointer to RAM that points to a pointer to flash.
            The first indirection of p reads that flash pointer
            from RAM and the second indirection reads a char from this
            flash address.  */
     
         return **p;
     }
     
     /* Locate array[] in flash memory */
     const __flash int array[] = { 3, 5, 7, 11, 13, 17, 19 };
     
     int i = 1;
     
     int main (void)
     {
        /* Return 17 by reading from flash memory */
        return array[array[i]];
     }

For each named address space supported by avr-gcc there is an equally named but uppercase built-in macro defined. The purpose is to facilitate testing if respective address space support is available or not:

     #ifdef __FLASH
     const __flash int var = 1;
     
     int read_var (void)
     {
         return var;
     }
     #else
     #include <avr/pgmspace.h> /* From AVR-LibC */
     
     const int var PROGMEM = 1;
     
     int read_var (void)
     {
         return (int) pgm_read_word (&var);
     }
     #endif /* __FLASH */

Notice that attribute progmem locates data in flash but accesses to these data read from generic address space, i.e. from RAM, so that you need special accessors like pgm_read_byte from AVR-LibC together with attribute progmem.

Limitations and caveats

6.16.2 M32C Named Address Spaces

On the M32C target, with the R8C and M16C CPU variants, variables qualified with __far are accessed using 32-bit addresses in order to access memory beyond the first 64 Ki bytes. If __far is used with the M32CM or M32C CPU variants, it has no effect.

6.16.3 RL78 Named Address Spaces

On the RL78 target, variables qualified with __far are accessed with 32-bit pointers (20-bit addresses) rather than the default 16-bit addresses. Non-far variables are assumed to appear in the topmost 64 KiB of the address space.

6.16.4 SPU Named Address Spaces

On the SPU target variables may be declared as belonging to another address space by qualifying the type with the __ea address space identifier:

     extern int __ea i;

The compiler generates special code to access the variable i. It may use runtime library support, or generate special machine instructions to access that address space.

6.16.5 x86 Named Address Spaces

On the x86 target, variables may be declared as being relative to the %fs or %gs segments.

__seg_fs
__seg_gs
The object is accessed with the respective segment override prefix.

The respective segment base must be set via some method specific to the operating system. Rather than require an expensive system call to retrieve the segment base, these address spaces are not considered to be subspaces of the generic (flat) address space. This means that explicit casts are required to convert pointers between these address spaces and the generic address space. In practice the application should cast to uintptr_t and apply the segment base offset that it installed previously.

The preprocessor symbols __SEG_FS and __SEG_GS are defined when these address spaces are supported.