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174 lines
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<title>Using the GNU Compiler Collection (GCC): AVR Variable Attributes</title>
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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="Common-Variable-Attributes.html#Common-Variable-Attributes" accesskey="p" rel="prev">Common 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">6.32.2 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. Notice that just like the
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<code>section</code> attribute, <code>progmem</code> affects the location
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of the data but not how this data is accessed.
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</p>
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<p>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><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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</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="Common-Variable-Attributes.html#Common-Variable-Attributes" accesskey="p" rel="prev">Common 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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