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<h4 class="subsection">6.58.9 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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