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<title>AVR Variable Attributes (Using the GNU Compiler Collection (GCC))</title>
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</head>
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<a name="AVR-Variable-Attributes"></a>
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<div class="header">
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<p>
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Next: <a href="Blackfin-Variable-Attributes.html#Blackfin-Variable-Attributes" accesskey="n" rel="next">Blackfin Variable Attributes</a>, Previous: <a href="ARC-Variable-Attributes.html#ARC-Variable-Attributes" accesskey="p" rel="prev">ARC Variable Attributes</a>, Up: <a href="Variable-Attributes.html#Variable-Attributes" accesskey="u" rel="up">Variable Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="AVR-Variable-Attributes-1"></a>
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<h4 class="subsection">6.32.3 AVR Variable Attributes</h4>
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<dl compact="compact">
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<dt><code>progmem</code></dt>
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<dd><a name="index-progmem-variable-attribute_002c-AVR"></a>
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<p>The <code>progmem</code> attribute is used on the AVR to place read-only
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data in the non-volatile program memory (flash). The <code>progmem</code>
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attribute accomplishes this by putting respective variables into a
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section whose name starts with <code>.progmem</code>.
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</p>
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<p>This attribute works similar to the <code>section</code> attribute
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but adds additional checking.
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</p>
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<dl compact="compact">
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<dt>• Ordinary AVR cores with 32 general purpose registers:</dt>
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<dd><p><code>progmem</code> 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</code> attribute
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(inline) assembler must be used.
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</p><div class="smallexample">
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<pre class="smallexample">/* Use custom macros from <a href="http://nongnu.org/avr-libc/user-manual/">AVR-LibC</a><!-- /@w --> */
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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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</pre></div>
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<p>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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</p>
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<p>See also the <a href="Named-Address-Spaces.html#AVR-Named-Address-Spaces">AVR Named Address Spaces</a> section for
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an alternate way to locate and access data in flash memory.
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</p>
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</dd>
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<dt>• AVR cores with flash memory visible in the RAM address range:</dt>
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<dd><p>On such devices, there is no need for attribute <code>progmem</code> or
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<a href="Named-Address-Spaces.html#AVR-Named-Address-Spaces"><code>__flash</code></a> qualifier at all.
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Just use standard C / C++. The compiler will generate <code>LD*</code>
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instructions. As flash memory is visible in the RAM address range,
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and the default linker script does <em>not</em> locate <code>.rodata</code> in
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RAM, no special features are needed in order not to waste RAM for
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read-only data or to read from flash. You might even get slightly better
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performance by
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avoiding <code>progmem</code> and <code>__flash</code>. This applies to devices from
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families <code>avrtiny</code> and <code>avrxmega3</code>, see <a href="AVR-Options.html#AVR-Options">AVR Options</a> for
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an overview.
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</p>
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</dd>
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<dt>• Reduced AVR Tiny cores like ATtiny40:</dt>
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<dd><p>The compiler adds <code>0x4000</code>
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to the addresses of objects and declarations in <code>progmem</code> and locates
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the objects in flash memory, namely in section <code>.progmem.data</code>.
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The offset is needed because the flash memory is visible in the RAM
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address space starting at address <code>0x4000</code>.
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</p>
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<p>Data in <code>progmem</code> can be accessed by means of ordinary C code,
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no special functions or macros are needed.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/* var is located in flash memory */
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extern const int var[2] __attribute__((progmem));
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int read_var (int i)
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{
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return var[i];
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}
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</pre></div>
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<p>Please notice that on these devices, there is no need for <code>progmem</code>
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at all.
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</p>
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</dd>
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</dl>
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</dd>
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<dt><code>io</code></dt>
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<dt><code>io (<var>addr</var>)</code></dt>
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<dd><a name="index-io-variable-attribute_002c-AVR"></a>
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<p>Variables with the <code>io</code> attribute are used to address
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memory-mapped peripherals in the io address range.
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If an address is specified, the variable
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is assigned that address, and the value is interpreted as an
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address in the data address space.
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Example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">volatile int porta __attribute__((io (0x22)));
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</pre></div>
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<p>The address specified in the address in the data address range.
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</p>
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<p>Otherwise, the variable it is not assigned an address, but the
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compiler will still use in/out instructions where applicable,
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assuming some other module assigns an address in the io address range.
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Example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">extern volatile int porta __attribute__((io));
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</pre></div>
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</dd>
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<dt><code>io_low</code></dt>
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<dt><code>io_low (<var>addr</var>)</code></dt>
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<dd><a name="index-io_005flow-variable-attribute_002c-AVR"></a>
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<p>This is like the <code>io</code> attribute, but additionally it informs the
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compiler that the object lies in the lower half of the I/O area,
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allowing the use of <code>cbi</code>, <code>sbi</code>, <code>sbic</code> and <code>sbis</code>
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instructions.
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</p>
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</dd>
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<dt><code>address</code></dt>
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<dt><code>address (<var>addr</var>)</code></dt>
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<dd><a name="index-address-variable-attribute_002c-AVR"></a>
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<p>Variables with the <code>address</code> attribute are used to address
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memory-mapped peripherals that may lie outside the io address range.
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</p>
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<div class="smallexample">
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<pre class="smallexample">volatile int porta __attribute__((address (0x600)));
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</pre></div>
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</dd>
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<dt><code>absdata</code></dt>
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<dd><a name="index-absdata-variable-attribute_002c-AVR"></a>
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<p>Variables in static storage and with the <code>absdata</code> attribute can
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be accessed by the <code>LDS</code> and <code>STS</code> instructions which take
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absolute addresses.
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</p>
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<ul>
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<li> This attribute is only supported for the reduced AVR Tiny core
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like ATtiny40.
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</li><li> You must make sure that respective data is located in the
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address range <code>0x40</code>…<code>0xbf</code> accessible by
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<code>LDS</code> and <code>STS</code>. One way to achieve this as an
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appropriate linker description file.
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</li><li> If the location does not fit the address range of <code>LDS</code>
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and <code>STS</code>, there is currently (Binutils 2.26) just an unspecific
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warning like
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<blockquote>
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<p><code>module.c:(.text+0x1c): warning: internal error: out of range error</code>
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</p></blockquote>
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</li></ul>
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<p>See also the <samp>-mabsdata</samp> <a href="AVR-Options.html#AVR-Options">command-line option</a>.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Blackfin-Variable-Attributes.html#Blackfin-Variable-Attributes" accesskey="n" rel="next">Blackfin Variable Attributes</a>, Previous: <a href="ARC-Variable-Attributes.html#ARC-Variable-Attributes" accesskey="p" rel="prev">ARC Variable Attributes</a>, Up: <a href="Variable-Attributes.html#Variable-Attributes" accesskey="u" rel="up">Variable Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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