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239 lines
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239 lines
11 KiB
HTML
4 years ago
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<html lang="en">
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<title>Named Address Spaces - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Fixed_002dPoint.html#Fixed_002dPoint" title="Fixed-Point">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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<body>
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<div class="node">
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<a name="Named-Address-Spaces"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Zero-Length.html#Zero-Length">Zero Length</a>,
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Previous: <a rel="previous" accesskey="p" href="Fixed_002dPoint.html#Fixed_002dPoint">Fixed-Point</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.16 Named Address Spaces</h3>
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<p><a name="index-Named-Address-Spaces-2972"></a>
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As an extension, GNU C supports named address spaces as
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defined in the N1275 draft of ISO/IEC DTR 18037. Support for named
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address spaces in GCC will evolve as the draft technical report
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changes. Calling conventions for any target might also change. At
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present, only the AVR, SPU, M32C, and RL78 targets support address
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spaces other than the generic address space.
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<p>Address space identifiers may be used exactly like any other C type
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qualifier (e.g., <code>const</code> or <code>volatile</code>). See the N1275
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document for more details.
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<p><a name="AVR-Named-Address-Spaces"></a>
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<h4 class="subsection">6.16.1 AVR Named Address Spaces</h4>
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<p>On the AVR target, there are several address spaces that can be used
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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</code> or <code>ELPM</code>
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needed to read from flash.
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<p>Devices belonging to <code>avrtiny</code> and <code>avrxmega3</code> can access
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flash memory by means of <code>LD*</code> instructions because the flash
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memory is mapped into the RAM address space. There is <em>no need</em>
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for language extensions like <code>__flash</code> or attribute
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<a href="AVR-Variable-Attributes.html#AVR-Variable-Attributes"><code>progmem</code></a>.
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The default linker description files for these devices cater for that
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feature and <code>.rodata</code> stays in flash: The compiler just generates
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<code>LD*</code> instructions, and the linker script adds core specific
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offsets to all <code>.rodata</code> symbols: <code>0x4000</code> in the case of
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<code>avrtiny</code> and <code>0x8000</code> in the case of <code>avrxmega3</code>.
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See <a href="AVR-Options.html#AVR-Options">AVR Options</a> for a list of respective devices.
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<p>For devices not in <code>avrtiny</code> or <code>avrxmega3</code>,
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any data including read-only data is located in RAM (the generic
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address space) because flash memory is not visible in the RAM address
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space. In order to locate read-only data in flash memory <em>and</em>
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to generate the right instructions to access this data without
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using (inline) assembler code, special address spaces are needed.
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<dl>
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<dt><code>__flash</code><dd><a name="index-g_t_0040code_007b_005f_005fflash_007d-AVR-Named-Address-Spaces-2973"></a>The <code>__flash</code> qualifier locates data in the
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<code>.progmem.data</code> section. Data is read using the <code>LPM</code>
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instruction. Pointers to this address space are 16 bits wide.
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<br><dt><code>__flash1</code><dt><code>__flash2</code><dt><code>__flash3</code><dt><code>__flash4</code><dt><code>__flash5</code><dd><a name="index-g_t_0040code_007b_005f_005fflash1_007d-AVR-Named-Address-Spaces-2974"></a><a name="index-g_t_0040code_007b_005f_005fflash2_007d-AVR-Named-Address-Spaces-2975"></a><a name="index-g_t_0040code_007b_005f_005fflash3_007d-AVR-Named-Address-Spaces-2976"></a><a name="index-g_t_0040code_007b_005f_005fflash4_007d-AVR-Named-Address-Spaces-2977"></a><a name="index-g_t_0040code_007b_005f_005fflash5_007d-AVR-Named-Address-Spaces-2978"></a>These are 16-bit address spaces locating data in section
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<code>.progmem</code><var>N</var><code>.data</code> where <var>N</var> refers to
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address space <code>__flash</code><var>N</var>.
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The compiler sets the <code>RAMPZ</code> segment register appropriately
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before reading data by means of the <code>ELPM</code> instruction.
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<br><dt><code>__memx</code><dd><a name="index-g_t_0040code_007b_005f_005fmemx_007d-AVR-Named-Address-Spaces-2979"></a>This is a 24-bit address space that linearizes flash and RAM:
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If the high bit of the address is set, data is read from
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RAM using the lower two bytes as RAM address.
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If the high bit of the address is clear, data is read from flash
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with <code>RAMPZ</code> set according to the high byte of the address.
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See <a href="AVR-Built_002din-Functions.html#AVR-Built_002din-Functions"><code>__builtin_avr_flash_segment</code></a>.
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<p>Objects in this address space are located in <code>.progmemx.data</code>.
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</dl>
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<p><b>Example</b>
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<pre class="smallexample"> char my_read (const __flash 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 reads 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 __flash 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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</pre>
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<p class="noindent">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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support is available or not:
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<pre class="smallexample"> #ifdef __FLASH
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const __flash int var = 1;
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int read_var (void)
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{
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return var;
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}
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#else
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#include <avr/pgmspace.h> /* From AVR-LibC */
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const int var PROGMEM = 1;
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int read_var (void)
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{
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return (int) pgm_read_word (&var);
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}
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#endif /* __FLASH */
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</pre>
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<p class="noindent">Notice that attribute <a href="AVR-Variable-Attributes.html#AVR-Variable-Attributes"><code>progmem</code></a>
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locates data in flash but
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accesses to these data 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</code>
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from <a href="http://nongnu.org/avr-libc/user-manual/">AVR-LibC</a><!-- /@w -->
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together with attribute <code>progmem</code>.
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<p class="noindent"><b>Limitations and caveats</b>
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<ul>
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<li>Reading across the 64 KiB section boundary of
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the <code>__flash</code> or <code>__flash</code><var>N</var> address spaces
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shows undefined behavior. The only address space that
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supports reading across the 64 KiB flash segment boundaries is
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<code>__memx</code>.
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<li>If you use one of the <code>__flash</code><var>N</var> address spaces
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you must arrange your linker script to locate the
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<code>.progmem</code><var>N</var><code>.data</code> sections according to your needs.
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<li>Any data or pointers to the non-generic address spaces must
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be qualified as <code>const</code>, 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 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> <code>volatile</code> so that the compiler
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must not optimize away known values or insert them
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as immediates into operands of instructions.
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<li>The following code initializes a variable <code>pfoo</code>
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located in static storage with a 24-bit address:
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<pre class="smallexample"> extern const __memx char foo;
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const __memx void *pfoo = &foo;
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</pre>
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<li>On the reduced Tiny devices like ATtiny40, no address spaces are supported.
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Just use vanilla C / C++ code without overhead as outlined above.
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Attribute <code>progmem</code> is supported but works differently,
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see <a href="AVR-Variable-Attributes.html#AVR-Variable-Attributes">AVR Variable Attributes</a>.
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</ul>
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<h4 class="subsection">6.16.2 M32C Named Address Spaces</h4>
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<p><a name="index-g_t_0040code_007b_005f_005ffar_007d-M32C-Named-Address-Spaces-2980"></a>
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On the M32C target, with the R8C and M16C CPU variants, variables
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qualified with <code>__far</code> are accessed using 32-bit addresses in
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order to access memory beyond the first 64 Ki bytes. If
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<code>__far</code> is used with the M32CM or M32C CPU variants, it has no
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effect.
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<h4 class="subsection">6.16.3 RL78 Named Address Spaces</h4>
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<p><a name="index-g_t_0040code_007b_005f_005ffar_007d-RL78-Named-Address-Spaces-2981"></a>
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On the RL78 target, variables qualified with <code>__far</code> are accessed
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with 32-bit pointers (20-bit addresses) rather than the default 16-bit
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addresses. Non-far variables are assumed to appear in the topmost
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64 KiB of the address space.
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<h4 class="subsection">6.16.4 SPU Named Address Spaces</h4>
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<p><a name="index-g_t_0040code_007b_005f_005fea_007d-SPU-Named-Address-Spaces-2982"></a>
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On the SPU target variables may be declared as
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belonging to another address space by qualifying the type with the
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<code>__ea</code> address space identifier:
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<pre class="smallexample"> extern int __ea i;
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</pre>
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<p class="noindent">The compiler generates special code to access the variable <code>i</code>.
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It may use runtime library
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support, or generate special machine instructions to access that address
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space.
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</body></html>
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