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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, M32C, RL78, and x86 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.
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.
Devices belonging to avrtiny and avrxmega3 can access
flash memory by means of LD* instructions because the flash
memory is mapped into the RAM address space. There is no need
for language extensions like __flash or attribute
progmem.
The default linker description files for these devices cater for that
feature and .rodata stays in flash: The compiler just generates
LD* instructions, and the linker script adds core specific
offsets to all .rodata symbols: 0x4000 in the case of
avrtiny and 0x8000 in the case of avrxmega3.
See AVR Options for a list of respective devices.
For devices not in avrtiny or avrxmega3,
any data including read-only data is located in RAM (the generic
address space) because flash memory is not visible in the RAM address
space. In order to locate read-only data in flash memory and
to generate the right instructions to access this data without
using (inline) assembler code, special address spaces are needed.
__flashThe __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__flash5These 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.
__memxThis 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 __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 AVR-LibC
together with attribute progmem.
Limitations and caveats
__flash or __flashN address spaces
shows undefined behavior. The only address space that
supports reading across the 64 KiB flash segment boundaries is
__memx.
__flashN address spaces
you must arrange your linker script to locate the
.progmemN.data sections according to your needs.
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.
pfoo
located in static storage with a 24-bit address:
extern const __memx char foo; const __memx void *pfoo = &foo;
progmem is supported but works differently,
see AVR Variable Attributes.
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 64 Ki bytes. If
__far is used with the M32CM or M32C CPU variants, it has no
effect.
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
64 KiB of the address space.
On the x86 target, variables may be declared as being relative
to the %fs or %gs segments.
__seg_fs__seg_gsThe object is accessed with the respective segment override prefix.
The respective segment base must be set via some method specific to
the operating system. Rather than require an expensive system call
to retrieve the segment base, these address spaces are not considered
to be subspaces of the generic (flat) address space. This means that
explicit casts are required to convert pointers between these address
spaces and the generic address space. In practice the application
should cast to uintptr_t and apply the segment base offset
that it installed previously.
The preprocessor symbols __SEG_FS and __SEG_GS are
defined when these address spaces are supported.
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