AVR Variable Attributes (Using the GNU Compiler Collection (GCC))

6.34.3 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.

AVR cores with flash memory visible in the RAM address range:

On such devices, there is no need for attribute progmem or __flash qualifier at all. Just use standard C / C++. The compiler will generate LD* instructions. As flash memory is visible in the RAM address range, and the default linker script does not locate .rodata in RAM, no special features are needed in order not to waste RAM for read-only data or to read from flash. You might even get slightly better performance by avoiding progmem and __flash. This applies to devices from families avrtiny and avrxmega3, see AVR Options for an overview.

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.

io

io (addr)

Variables with the io attribute are used to address memory-mapped peripherals in the I/O address range. No memory is allocated. 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 (__AVR_SFR_OFFSET__ + 0x2)));

Otherwise, the variable is not assigned an address, but the compiler will still use in and out instructions where applicable, assuming some other module assigns an address in the I/O 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 (addr)

Variables with the address attribute can be used to address memory-mapped peripherals that may lie outside the I/O address range. Just like with the io and io_low attributes, no memory is allocated.

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

This attribute can also be used to define symbols in C/C++ code which otherwise would require assembly, a linker description file or command line options like -Wl,--defsym,a_symbol=value. For example,

int a_symbol __attribute__((weak, address (1234)));

will be compiled to

.weak a_symbol
a_symbol = 1234

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 0x400xbf 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.cc:(.text+0x1c): warning: internal error: out of range error