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| <h4 class="subsection">6.56.4 AVR Built-in Functions</h4> |
| |
| <p>For each built-in function for AVR, there is an equally named, |
| uppercase built-in macro defined. That way users can easily query if |
| or if not a specific built-in is implemented or not. For example, if |
| <code>__builtin_avr_nop</code> is available the macro |
| <code>__BUILTIN_AVR_NOP</code> is defined to <code>1</code> and undefined otherwise. |
| |
| <p>The following built-in functions map to the respective machine |
| instruction, i.e. <code>nop</code>, <code>sei</code>, <code>cli</code>, <code>sleep</code>, |
| <code>wdr</code>, <code>swap</code>, <code>fmul</code>, <code>fmuls</code> |
| resp. <code>fmulsu</code>. The three <code>fmul*</code> built-ins are implemented |
| as library call if no hardware multiplier is available. |
| |
| <pre class="smallexample"> void __builtin_avr_nop (void) |
| void __builtin_avr_sei (void) |
| void __builtin_avr_cli (void) |
| void __builtin_avr_sleep (void) |
| void __builtin_avr_wdr (void) |
| unsigned char __builtin_avr_swap (unsigned char) |
| unsigned int __builtin_avr_fmul (unsigned char, unsigned char) |
| int __builtin_avr_fmuls (char, char) |
| int __builtin_avr_fmulsu (char, unsigned char) |
| </pre> |
| <p>In order to delay execution for a specific number of cycles, GCC |
| implements |
| <pre class="smallexample"> void __builtin_avr_delay_cycles (unsigned long ticks) |
| </pre> |
| <p class="noindent"><code>ticks</code> is the number of ticks to delay execution. Note that this |
| built-in does not take into account the effect of interrupts that |
| might increase delay time. <code>ticks</code> must be a compile-time |
| integer constant; delays with a variable number of cycles are not supported. |
| |
| <pre class="smallexample"> char __builtin_avr_flash_segment (const __memx void*) |
| </pre> |
| <p class="noindent">This built-in takes a byte address to the 24-bit |
| <a href="AVR-Named-Address-Spaces.html#AVR-Named-Address-Spaces">address space</a> <code>__memx</code> and returns |
| the number of the flash segment (the 64 KiB chunk) where the address |
| points to. Counting starts at <code>0</code>. |
| If the address does not point to flash memory, return <code>-1</code>. |
| |
| <pre class="smallexample"> unsigned char __builtin_avr_insert_bits (unsigned long map, unsigned char bits, unsigned char val) |
| </pre> |
| <p class="noindent">Insert bits from <var>bits</var> into <var>val</var> and return the resulting |
| value. The nibbles of <var>map</var> determine how the insertion is |
| performed: Let <var>X</var> be the <var>n</var>-th nibble of <var>map</var> |
| <ol type=1 start=1> |
| <li>If <var>X</var> is <code>0xf</code>, |
| then the <var>n</var>-th bit of <var>val</var> is returned unaltered. |
| |
| <li>If X is in the range 0<small class="dots">...</small>7, |
| then the <var>n</var>-th result bit is set to the <var>X</var>-th bit of <var>bits</var> |
| |
| <li>If X is in the range 8<small class="dots">...</small><code>0xe</code>, |
| then the <var>n</var>-th result bit is undefined. |
| </ol> |
| |
| <p class="noindent">One typical use case for this built-in is adjusting input and |
| output values to non-contiguous port layouts. Some examples: |
| |
| <pre class="smallexample"> // same as val, bits is unused |
| __builtin_avr_insert_bits (0xffffffff, bits, val) |
| </pre> |
| <pre class="smallexample"> // same as bits, val is unused |
| __builtin_avr_insert_bits (0x76543210, bits, val) |
| </pre> |
| <pre class="smallexample"> // same as rotating bits by 4 |
| __builtin_avr_insert_bits (0x32107654, bits, 0) |
| </pre> |
| <pre class="smallexample"> // high nibble of result is the high nibble of val |
| // low nibble of result is the low nibble of bits |
| __builtin_avr_insert_bits (0xffff3210, bits, val) |
| </pre> |
| <pre class="smallexample"> // reverse the bit order of bits |
| __builtin_avr_insert_bits (0x01234567, bits, 0) |
| </pre> |
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