re PR target/49868 (Implement named address space to place/access data in flash memory)
PR target/49868 * doc/extend.texi (AVR Named Address Spaces): Move sample code up. Remove note on size/offset limitation. (AVR Variable Attributes): Add example how to read data located with progmem. Refer to named address spaces. * doc/invoke.texi (AVR Options): Fix typo. From-SVN: r183473
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Georg-Johann Lay
Georg-Johann Lay
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2012-01-24 Georg-Johann Lay <avr@gjlay.de>
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PR target/49868
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* doc/extend.texi (AVR Named Address Spaces): Move sample code up.
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Remove note on size/offset limitation.
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(AVR Variable Attributes): Add example how to read data located
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with progmem. Refer to named address spaces.
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* doc/invoke.texi (AVR Options): Fix typo.
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2012-01-24 Richard Guenther <rguenther@suse.de>
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Forward-port to trunk
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@ -1237,9 +1237,10 @@ in order to put read-only data into the flash memory and access that
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data by means of the special instructions @code{LPM} or @code{ELPM}
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needed to read from flash.
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Per default, any data including read-only data is located in RAM so
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that address spaces are needed to locate read-only data in flash memory
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@emph{and} to generate the right instructions to access the data
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Per default, any data including read-only data is located in RAM
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(the generic address space) so that non-generic address spaces are
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needed to locate read-only data in flash memory
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@emph{and} to generate the right instructions to access this data
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without using (inline) assembler code.
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@table @code
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@ -1265,9 +1266,9 @@ address space @code{__pgm@var{N}}.
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The compiler will set the @code{RAMPZ} segment register approptiately
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before reading data by means of the @code{ELPM} instruction.
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On devices with less 64kiB flash segments as indicated by the address
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On devices with less 64@tie{}kiB flash segments as indicated by the address
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space, the compiler will cut down the segment number to a number the
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device actually supports. Counting starts at @code{0}
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device actually supports. Counting starts at@tie{}@code{0}
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for space @code{__pgm}. For example, if you access address space
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@code{__pgm3} on an ATmega128 device with two 64@tie{}kiB flash segments,
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the compiler will generate a read from @code{__pgm1}, i.e.@: it
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@ -1284,6 +1285,31 @@ with @code{RAMPZ} set according to the high byte of the address.
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Objects in this address space will be located in @code{.progmem.data}.
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@end table
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@b{Example}
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@example
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char my_read (const __pgm char ** p)
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@{
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/* p is a pointer to RAM that points to a pointer to flash.
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The first indirection of p will read that flash pointer
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from RAM and the second indirection reads a char from this
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flash address. */
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return **p;
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@}
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/* Locate array[] in flash memory */
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const __pgm int array[] = @{ 3, 5, 7, 11, 13, 17, 19 @};
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int i = 1;
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int main (void)
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@{
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/* Return 17 by reading from flash memory */
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return array[array[i]];
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@}
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@end example
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For each named address space supported by avr-gcc there is an equally
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named but uppercase built-in macro defined.
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The purpose is to facilitate testing if respective address space
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@ -1311,8 +1337,9 @@ int read_i (void)
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Notice that attribute @ref{AVR Variable Attributes,@code{progmem}}
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locates data in flash but
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accesses to these data will be to generic address space, i.e.@: RAM,
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so that you need special access functions like @code{pgm_read_byte}
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accesses to these data will read from generic address space, i.e.@:
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from RAM,
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so that you need special accessors like @code{pgm_read_byte}
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from @w{@uref{http://nongnu.org/avr-libc/user-manual,avr-libc}}.
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@b{Limitations and caveats}
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@ -1331,10 +1358,10 @@ you will have to arrange your linker skript to locate the
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@code{.progmem@var{N}.data} sections according to your needs.
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@item
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Any data or pointers to the AVR address spaces spaces must
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also be qualified as @code{const}, i.e.@: as read-only data.
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Any data or pointers to the non-generic address spaces must
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be qualified as @code{const}, i.e.@: as read-only data.
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This still applies if the data in one of these address
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spaces like software version number or lookup tables are intended to
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spaces like software version number or calibration lookup table are intended to
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be changed after load time by, say, a boot loader. In this case
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the right qualification is @code{const} @code{volatile} so that the compiler
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must not optimize away known values or insert them
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@ -1349,52 +1376,12 @@ extern const __pgmx char foo;
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const __pgmx void *pfoo = &foo;
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@end example
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The code will throw an assembler warning and the high byte of
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@code{pfoo} will be initialized with @code{0}, i.e.@: the
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@code{pfoo} will be initialized with@tie{}@code{0}, i.e.@: the
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initialization will be as if @code{foo} was located in the first
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64@tie{}KiB chunk of flash.
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@item
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Address arithmetic for the @code{__pgmx} address space is carried out
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as 16-bit signed integer arithmetic. This means that in the following
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code array positions with offsets @code{idx}@tie{}>@tie{}8191 are
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inaccessible.
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@example
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extern const __pgmx long lookup[];
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long read_lookup (unsigned idx)
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@{
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return lookup[idx];
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@}
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@end example
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@end itemize
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@b{Example}
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@example
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char my_read (const __pgm ** p)
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@{
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/* p is a pointer to RAM that points to a pointer to flash.
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The first indirection of p will read that flash pointer
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from RAM and the second indirection reads a char from this
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flash address. */
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return **p;
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@}
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/* Locate array[] in flash memory */
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const __pgm int array[] = @{ 3, 5, 7, 11, 13, 17, 19 @};
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int i = 1;
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int main (void)
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@{
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/* Return 17 by reading from flash memory */
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return array[array[i]];
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@}
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@end example
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@subsection M32C Named Address Spaces
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@cindex @code{__far} M32C Named Address Spaces
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@ -3235,7 +3222,7 @@ is entered like for, e@.g@. task functions in a multi-threading operating
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system. In that case, changing the stack pointer register will be
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guarded by save/clear/restore of the global interrupt enable flag.
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The differences to the @code{naked} function attrubute are:
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The differences to the @code{naked} function attribute are:
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@itemize @bullet
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@item @code{naked} functions do not have a return instruction whereas
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@code{OS_main} and @code{OS_task} functions will have a @code{RET} or
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@ -4752,13 +4739,32 @@ data in the non-volatile program memory (flash). The @code{progmem}
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attribute accomplishes this by putting respective variables into a
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section whose name starts with @code{.progmem}.
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This attrubute wirks similar to the @code{section} attribute
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This attribute works similar to the @code{section} attribute
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but adds additional checking. Notice that just like the
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@code{section} attribute, @code{progmem} affects the location
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of the data but not how this data is accessed.
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In order to read data located with the @code{progmem} attribute
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(inline) assembler must be used.
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@example
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/* Use custom macros from @w{@uref{http://nongnu.org/avr-libc/user-manual,avr-libc}} */
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#include <avr/pgmspace.h>
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/* Locate var in flash memory */
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const int var[2] PROGMEM = @{ 1, 2 @};
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int read_var (int i)
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@{
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/* Access var[] by accessor macro from avr/pgmspace.h */
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return (int) pgm_read_word (& var[i]);
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@}
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@end example
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AVR is a Harvard architecture processor and data and read-only data
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normally resides in the data memory (RAM).
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See also the @ref{AVR Named Address Spaces} section for
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an alternate way to locate and access data in flash memory.
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@end table
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@subsection Blackfin Variable Attributes
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@ -10973,11 +10973,11 @@ before calling a function and popped afterwards.
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Popping the arguments after the function call can be expensive on
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AVR so that accumulating the stack space might lead to smaller
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executables because areguments need not to be removed from the
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executables because arguments need not to be removed from the
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stack after such a function call.
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This option can lead to reduced code size for functions that get
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their arguments on the stack like functions that perform several
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This option can lead to reduced code size for functions that perform
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several calls to functions which get their arguments on the stack like
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calls to printf-like functions.
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@item -mbranch-cost=@var{cost}
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