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, SPU, M32C, and RL78 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.
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.
__memx AVR Named Address Spaces 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. AVR Built-in Functions``__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 AVR Variable Attributes``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 64KiB section boundary of the __flash or __flash``N`` address spaces shows undefined behavior. The only address space that supports reading across the 64KiB flash segment boundaries is __memx.
you must arrange your linker script to locate the .progmem``N.data`` sections according to your needs.
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.
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 http://sourceware.org/PR13503PR13503.
__far 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 64Ki bytes. If __far is used with the M32CM or M32C CPU variants, it has no effect.
__far 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 64KiB of the address space.
__ea SPU Named Address Spaces
On the SPU target variables may be declared as belonging to another address space by qualifying the type with the __ea address space identifier:
extern int __ea i;
The compiler generates special code to access the variable i. It may use runtime library support, or generate special machine instructions to access that address space.