Gcc/10.2.0/gcc/AVR-Variable-Attributes
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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). Theprogmem
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 theprogmem
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 generateLD*
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 avoidingprogmem
and__flash
. This applies to devices from familiesavrtiny
andavrxmega3
, see AVR Options for an overview.- • Reduced AVR Tiny cores like ATtiny40:
The compiler adds
0x4000
to the addresses of objects and declarations inprogmem
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 address0x4000
.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 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 ofcbi
,sbi
,sbic
andsbis
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)));
absdata
Variables in static storage and with the
absdata
attribute can be accessed by theLDS
andSTS
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
0x40
…0xbf
accessible byLDS
andSTS
. One way to achieve this as an appropriate linker description file. If the location does not fit the address range of
LDS
andSTS
, there is currently (Binutils 2.26) just an unspecific warning likemodule.c:(.text+0x1c): warning: internal error: out of range error
See also the
-mabsdata
command-line option.
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