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.
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.
On such devices, there is no need for attribute progmem
or
__flash
qualifier at all.
Just use standard C / C++. The compiler will generate LD*
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
avoiding progmem
and __flash
. This applies to devices from
families avrtiny
and avrxmega3
, see AVR Options for
an overview.
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]; }
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 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)));
absdata
Variables in static storage and with the absdata
attribute can
be accessed by the LDS
and STS
instructions which take
absolute addresses.
0x40
…0xbf
accessible by
LDS
and STS
. One way to achieve this as an
appropriate linker description file.
LDS
and STS
, there is currently (Binutils 2.26) just an unspecific
warning like
module.cc:(.text+0x1c): warning: internal error: out of range error
See also the -mabsdata command-line option.