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

### 6.3 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, M32C, PRU, RL78, x86 and Xtensa 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.

#### 6.3.1 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.

Devices belonging to `avrtiny` and `avrxmega3` can access flash memory by means of `LD*` instructions because the flash memory is mapped into the RAM address space. There is no need for language extensions like `__flash` or attribute `progmem`. The default linker description files for these devices cater for that feature and `.rodata` stays in flash: The compiler just generates `LD*` instructions, and the linker script adds core specific offsets to all `.rodata` symbols: `0x4000` in the case of `avrtiny` and `0x8000` in the case of `avrxmega3`. See AVR Options for a list of respective devices.

For devices not in `avrtiny` or `avrxmega3`, any data including read-only data is located in RAM (the generic address space) because flash memory is not visible in the RAM address space. In order to locate read-only data in flash memory and to generate the right instructions to access this data without using (inline) assembler code, special address spaces are needed.

`__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.

`__flashx` ¶

  This is a 24-bit flash address space locating data in section `.progmemx.data`. 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 is not supported. The only address spaces that support reading across the 64 KiB flash segment boundaries are `__memx` and `__flashx`.
- 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. For an example, see the avr-gcc wiki
- 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;

- On the reduced Tiny devices like ATtiny40, no address spaces are supported. Just use vanilla C / C++ code without overhead as outlined above. Attribute `progmem` is supported but works differently.

#### 6.3.2 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.

#### 6.3.3 PRU Named Address Spaces ¶

On the PRU target, variables qualified with `__regio_symbol` are aliases used to access the special I/O CPU registers. They must be declared as `extern` because such variables will not be allocated in any data memory. They must also be marked as `volatile`, and can only be 32-bit integer types. The only names those variables can have are `__R30` and `__R31`, representing respectively the `R30` and `R31` special I/O CPU registers. Hence the following example is the only valid usage of `__regio_symbol`:

    extern volatile __regio_symbol uint32_t __R30;
    extern volatile __regio_symbol uint32_t __R31;

#### 6.3.4 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.

#### 6.3.5 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.

#### 6.3.6 Xtensa Named Address Spaces ¶

On the Xtensa target, when a variable qualified with `__force_l32` is loaded from memory, it is always read aligned to a 4-byte width regardless of whether its width is 1 or 2 bytes, and a bit-extraction instruction is applied to the read to obtain the desired result; writing with a width of 1 or 2 bytes is not supported (see also `force_l32` attribute described in Xtensa Attributes, and command-line option `-mforce-l32` described in Xtensa Options).

    char *strcpy_irom (char *dst, __force_l32 const char *src)
    {
      char *p = dst;
      /* "*src" is always read as an aligned 4-byte width, and then
         the desired one byte is extracted using bitwise operations.  */
      while (*p = *src)
        ++p, ++src;
      return dst;
    }

Qualifying a variable with `__force_l32` affects how that variable is read as mentioned above, but it does not affect the memory section in which the variable is placed (this can be specified separately using the `section` attribute).

    /* Instruction ROM reading requires aligned 4-byte width access.  */
    __force_l32 const char IROM_message[] __attribute__((section(".irom.text")))
        = "placed within the instruction ROM area.";

A pointer qualified with `__force_l32` can read memory regions in the generic address space (though not very efficiently), but not vice versa. Therefore, the conversion from a pointer for the generic address space to a pointer qualified with `__force_l32` is implicit, but not the other way around.

    extern char *strcpy_irom (char *, __force_l32 const char *);
    char buf[80], alt_buf[80];

    strcpy_irom (buf, IROM_message);
    strcpy_irom (alt_buf, "placed within read-only RAM area.");