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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];
               }

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)));