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#### 6.32.2 AVR Variable Attributes

`progmem`

  The `progmem` attribute is used on the AVR to place read-only data in the non-volatile program memory (flash). The `progmem` attribute accomplishes this by putting respective variables into a section whose name starts with `.progmem`.

  This attribute works similar to the `section` attribute but adds additional checking.

  • Ordinary AVR cores with 32 general purpose registers:

    `progmem` affects the location of the data but not how this data is accessed. In order to read data located with the `progmem` attribute (inline) assembler must be used.

        /* Use custom macros from AVR-LibC */
        #include <avr/pgmspace.h>

        /* Locate var in flash memory */
        const int var[2] PROGMEM = { 1, 2 };

        int read_var (int i)
        {
            /* Access var[] by accessor macro from avr/pgmspace.h */
            return (int) pgm_read_word (& var[i]);
        }

    AVR is a Harvard architecture processor and data and read-only data normally resides in the data memory (RAM).

    See also the AVR Named Address Spaces section for an alternate way to locate and access data in flash memory.

  • Reduced AVR Tiny cores like ATtiny40:

    The compiler adds `0x4000` to the addresses of objects and declarations in `progmem` and locates the objects in flash memory, namely in section `.progmem.data`. The offset is needed because the flash memory is visible in the RAM address space starting at address `0x4000`.

    Data in `progmem` can be accessed by means of ordinary C code, no special functions or macros are needed.

        /* var is located in flash memory */
        extern const int var[2] __attribute__((progmem));

        int read_var (int i)
        {
            return var[i];
        }

    Please notice that on these devices, there is no need for `progmem` at all. Just use an appropriate linker description file like outlined below.

          .text :
          { ...
          } > text
          /* Leave .rodata in flash and add an offset of 0x4000 to all
             addresses so that respective objects can be accessed by
             LD instructions and open coded C/C++.  This means there
             is no need for progmem in the source and no overhead by
             read-only data in RAM.  */
          .rodata ADDR(.text) + SIZEOF (.text) + 0x4000 :
          {
            *(.rodata)
            *(.rodata*)
            *(.gnu.linkonce.r*)
          } AT> text
          /* No more need to put .rodata into .data:
             Removed all .rodata entries from .data.  */
          .data :
          { ...

`io`
`io (addr)`

  Variables with the `io` attribute are used to address memory-mapped peripherals in the io address range. If an address is specified, the variable is assigned that address, and the value is interpreted as an address in the data address space. Example:

      volatile int porta __attribute__((io (0x22)));

  The address specified in the address in the data address range.

  Otherwise, the variable it is not assigned an address, but the compiler will still use in/out instructions where applicable, assuming some other module assigns an address in the io address range. Example:

      extern volatile int porta __attribute__((io));

`io_low`
`io_low (addr)`

  This is like the `io` attribute, but additionally it informs the compiler that the object lies in the lower half of the I/O area, allowing the use of `cbi`, `sbi`, `sbic` and `sbis` instructions.

`address`
`address (addr)`

  Variables with the `address` attribute are used to address memory-mapped peripherals that may lie outside the io address range.

      volatile int porta __attribute__((address (0x600)));

`absdata`

  Variables in static storage and with the `absdata` attribute can be accessed by the `LDS` and `STS` instructions which take absolute addresses.

  - This attribute is only supported for the reduced AVR Tiny core like ATtiny40.
  - You must make sure that respective data is located in the address range `0x40`…`0xbf` accessible by `LDS` and `STS`. One way to achieve this as an appropriate linker description file.
  - If the location does not fit the address range of `LDS` and `STS`, there is currently (Binutils 2.26) just an unspecific warning like

    > `module.c:(.text+0x1c): warning: internal error: out of range error`

  See also the `-mabsdata` command-line option.

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