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<a name="Named-Address-Spaces"></a>
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<hr>
<a name="Named-Address-Spaces-1"></a>
<h3 class="section">6.16 Named Address Spaces</h3>
<a name="index-Named-Address-Spaces"></a>

<p>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, RL78, and x86 targets support
address spaces other than the generic address space.
</p>
<p>Address space identifiers may be used exactly like any other C type
qualifier (e.g., <code>const</code> or <code>volatile</code>).  See the N1275
document for more details.
</p>
<a name="AVR-Named-Address-Spaces"></a><a name="AVR-Named-Address-Spaces-1"></a>
<h4 class="subsection">6.16.1 AVR Named Address Spaces</h4>

<p>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 <code>LPM</code> or <code>ELPM</code>
needed to read from flash.
</p>
<p>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
<em>and</em> to generate the right instructions to access this data
without using (inline) assembler code.
</p>
<dl compact="compact">
<dt><code>__flash</code></dt>
<dd><a name="index-_005f_005fflash-AVR-Named-Address-Spaces"></a>
<p>The <code>__flash</code> qualifier locates data in the
<code>.progmem.data</code> section. Data is read using the <code>LPM</code>
instruction. Pointers to this address space are 16 bits wide.
</p>
</dd>
<dt><code>__flash1</code></dt>
<dt><code>__flash2</code></dt>
<dt><code>__flash3</code></dt>
<dt><code>__flash4</code></dt>
<dt><code>__flash5</code></dt>
<dd><a name="index-_005f_005fflash1-AVR-Named-Address-Spaces"></a>
<a name="index-_005f_005fflash2-AVR-Named-Address-Spaces"></a>
<a name="index-_005f_005fflash3-AVR-Named-Address-Spaces"></a>
<a name="index-_005f_005fflash4-AVR-Named-Address-Spaces"></a>
<a name="index-_005f_005fflash5-AVR-Named-Address-Spaces"></a>
<p>These are 16-bit address spaces locating data in section
<code>.progmem<var>N</var>.data</code> where <var>N</var> refers to
address space <code>__flash<var>N</var></code>.
The compiler sets the <code>RAMPZ</code> segment register appropriately 
before reading data by means of the <code>ELPM</code> instruction.
</p>
</dd>
<dt><code>__memx</code></dt>
<dd><a name="index-_005f_005fmemx-AVR-Named-Address-Spaces"></a>
<p>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 <code>RAMPZ</code> set according to the high byte of the address.
See <a href="AVR-Built_002din-Functions.html#AVR-Built_002din-Functions"><code>__builtin_avr_flash_segment</code></a>.
</p>
<p>Objects in this address space are located in <code>.progmemx.data</code>.
</p></dd>
</dl>

<p><b>Example</b>
</p>
<div class="smallexample">
<pre class="smallexample">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]];
}
</pre></div>

<p>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:
</p>
<div class="smallexample">
<pre class="smallexample">#ifdef __FLASH
const __flash int var = 1;

int read_var (void)
{
    return var;
}
#else
#include &lt;avr/pgmspace.h&gt; /* From AVR-LibC */

const int var PROGMEM = 1;

int read_var (void)
{
    return (int) pgm_read_word (&amp;var);
}
#endif /* __FLASH */
</pre></div>

<p>Notice that attribute <a href="AVR-Variable-Attributes.html#AVR-Variable-Attributes"><code>progmem</code></a>
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 <code>pgm_read_byte</code>
from <a href="http://nongnu.org/avr-libc/user-manual/"><span class="nolinebreak">AVR-LibC</span></a><!-- /@w -->
together with attribute <code>progmem</code>.
</p>
<p><b>Limitations and caveats</b>
</p>
<ul>
<li> Reading across the 64&nbsp;KiB section boundary of
the <code>__flash</code> or <code>__flash<var>N</var></code> address spaces
shows undefined behavior. The only address space that
supports reading across the 64&nbsp;KiB flash segment boundaries is
<code>__memx</code>.

</li><li> If you use one of the <code>__flash<var>N</var></code> address spaces
you must arrange your linker script to locate the
<code>.progmem<var>N</var>.data</code> sections according to your needs.

</li><li> Any data or pointers to the non-generic address spaces must
be qualified as <code>const</code>, 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 <code>const</code> <code>volatile</code> so that the compiler
must not optimize away known values or insert them
as immediates into operands of instructions.

</li><li> The following code initializes a variable <code>pfoo</code>
located in static storage with a 24-bit address:
<div class="smallexample">
<pre class="smallexample">extern const __memx char foo;
const __memx void *pfoo = &amp;foo;
</pre></div>

<p>Such code requires at least binutils 2.23, see
<a href="https://sourceware.org/PR13503">PR13503</a><!-- /@w -->.
</p>
</li><li> On the reduced Tiny devices like ATtiny40, no address spaces are supported.
Data can be put into and read from flash memory by means of
attribute <code>progmem</code>, see <a href="AVR-Variable-Attributes.html#AVR-Variable-Attributes">AVR Variable Attributes</a>.

</li></ul>

<a name="M32C-Named-Address-Spaces"></a>
<h4 class="subsection">6.16.2 M32C Named Address Spaces</h4>
<a name="index-_005f_005ffar-M32C-Named-Address-Spaces"></a>

<p>On the M32C target, with the R8C and M16C CPU variants, variables
qualified with <code>__far</code> are accessed using 32-bit addresses in
order to access memory beyond the first 64&nbsp;Ki bytes.  If
<code>__far</code> is used with the M32CM or M32C CPU variants, it has no
effect.
</p>
<a name="RL78-Named-Address-Spaces"></a>
<h4 class="subsection">6.16.3 RL78 Named Address Spaces</h4>
<a name="index-_005f_005ffar-RL78-Named-Address-Spaces"></a>

<p>On the RL78 target, variables qualified with <code>__far</code> 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&nbsp;KiB of the address space.
</p>
<a name="SPU-Named-Address-Spaces"></a>
<h4 class="subsection">6.16.4 SPU Named Address Spaces</h4>
<a name="index-_005f_005fea-SPU-Named-Address-Spaces"></a>

<p>On the SPU target variables may be declared as
belonging to another address space by qualifying the type with the
<code>__ea</code> address space identifier:
</p>
<div class="smallexample">
<pre class="smallexample">extern int __ea i;
</pre></div>

<p>The compiler generates special code to access the variable <code>i</code>.
It may use runtime library
support, or generate special machine instructions to access that address
space.
</p>
<a name="x86-Named-Address-Spaces"></a>
<h4 class="subsection">6.16.5 x86 Named Address Spaces</h4>
<a name="index-x86-named-address-spaces"></a>

<p>On the x86 target, variables may be declared as being relative
to the <code>%fs</code> or <code>%gs</code> segments.
</p>
<dl compact="compact">
<dt><code>__seg_fs</code></dt>
<dt><code>__seg_gs</code></dt>
<dd><a name="index-_005f_005fseg_005ffs-x86-named-address-space"></a>
<a name="index-_005f_005fseg_005fgs-x86-named-address-space"></a>
<p>The object is accessed with the respective segment override prefix.
</p>
<p>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 <code>uintptr_t</code> and apply the segment base offset
that it installed previously.
</p>
<p>The preprocessor symbols <code>__SEG_FS</code> and <code>__SEG_GS</code> are
defined when these address spaces are supported.
</p></dd>
</dl>

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