Using the GNU Compiler Collection (GCC): Named Address Spaces

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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

- Reading across the 64 KiB section boundary of the `__flash` or `__flashN` address spaces shows undefined behavior. The only address space that supports reading across the 64 KiB flash segment boundaries is `__memx`.
- If you use one of the `__flashN` address spaces you must arrange your linker script to locate the `.progmemN.data` sections according to your needs.
- Any data or pointers to the non-generic address spaces must be qualified as `const`, i.e. as read-only data. This still applies if the data in one of these address spaces like software version number or calibration lookup table are intended to be changed after load time by, say, a boot loader. In this case the right qualification is `const` `volatile` so that the compiler must not optimize away known values or insert them as immediates into operands of instructions.
- The following code initializes a variable `pfoo` located in static storage with a 24-bit address:

      extern const __memx char foo;
      const __memx void *pfoo = &foo;

  Such code requires at least binutils 2.23, see PR13503.

- On the reduced Tiny devices like ATtiny40, no address spaces are supported. Data can be put into and read from flash memory by means of attribute `progmem`, see AVR Variable Attributes.

#### 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.

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