info gcc

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 section __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 http://nongnu.org/avr-libc/user-manual/">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.

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