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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 __builtin_avr_nop is available the macro __BUILTIN_AVR_NOP is defined to 1 and undefined otherwise.
These built-in functions map to the respective machine instruction, i.e. nop, sei, cli, sleep, wdr, swap, fmul, fmuls resp. fmulsu. The three fmul* built-ins are implemented as library call if no hardware multiplier is available.
Delay execution for ticks cycles. Note that this built-in does not take into account the effect of interrupts that might increase delay time. ticks must be a compile-time integer constant; delays with a variable number of cycles are not supported.
Insert bits from bits into val and return the resulting value. The nibbles of map determine how the insertion is performed: Let X be the n-th nibble of map
0xf, then the n-th bit of val is returned unaltered.0xe, then the n-th result bit is undefined.One typical use case for this built-in is adjusting input and output values to non-contiguous port layouts. Some examples:
// same as val, bits is unused __builtin_avr_insert_bits (0xffffffff, bits, val);
// same as bits, val is unused __builtin_avr_insert_bits (0x76543210, bits, val);
// same as rotating bits by 4 __builtin_avr_insert_bits (0x32107654, bits, 0);
// 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);
// reverse the bit order of bits __builtin_avr_insert_bits (0x01234567, bits, 0);
Rotate the 8-bit constant value mask by an offset of offs, where mask is in { 0x01, 0xfe, 0x7f, 0x80 }. This built-in can be used as an alternative to 8-bit expressions like 1 << offs when their computation consumes too much time, and offs is known to be in the range 07.
__builtin_avr_mask1 (1, offs) // same like 1 << offs __builtin_avr_mask1 (~1, offs) // same like ~(1 << offs) __builtin_avr_mask1 (0x80, offs) // same like 0x80 >> offs __builtin_avr_mask1 (~0x80, offs) // same like ~(0x80 >> offs)
The open coded C versions take at least 5 + 4 * offs cycles (and 5 instructions), whereas the built-in takes 7 cycles and instructions (8 cycles and instructions in the case of mask = 0x7f).
Insert count NOP instructions. The number of instructions must be a compile-time integer constant.
All of the following built-in functions are only available for GNU-C
This built-in takes a byte address to the 24-bit named address space __memx and returns the number of the flash segment (the 64 KiB chunk) where the address points to. Counting starts at 0. If the address does not point to flash memory, return -1.
These built-ins return the length of a string located in named address-space __flash, __flashx or __memx, respectively. They are used to support functions like strlen_F from AVR-LibCs header avr/flash.h.
There are many more AVR-specific built-in functions that are used to implement the ISO/IEC TR 18037 Embedded C fixed-point functions of section 7.18a.6. You dont need to use these built-ins directly. Instead, use the declarations as supplied by the stdfix.h header with GNU-C99:
#include <stdfix.h>
// Re-interpret the bit representation of unsigned 16-bit
// integer uval as Q-format 0.16 value.
unsigned fract get_bits (uint_ur_t uval)
{
return urbits (uval);
}
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