__attribute__((bitband)) type attribute

__attribute__((bitband)) is a type attribute that gives you efficient atomic access to single-bit values in SRAM and Peripheral regions of the memory architecture. It is possible to set or clear a single bit directly with a single memory access in certain memory regions, rather than having to use the traditional read, modify, write approach. It is also possible to read a single bit directly rather than having to use the traditional read then shift and mask operation. Example 10 illustrates the use of __attribute__((bitband)).

typedef struct {
  int i : 1;
  int j : 2;
  int k : 3;
} BB __attribute__((bitband));

BB bb __attribute__((at(0x20000004));

void foo(void)
{
  bb.i = 1;
}

For peripherals that are sensitive to the memory access width, byte, halfword, and word stores or loads to the alias space are generated for char, short, and int types of bitfields of bit-banded structs respectively.

In Example 11, bit-banded access is generated for bb.i.

typedef struct {
  char i : 1;
  int j : 2;
  int k : 3;
} BB __attribute__((bitband));

BB bb __attribute__((at(0x20000004)));

void foo()
{
  bb.i = 1;
}

If you do not use __attribute__((at())) to place the bit-banded variable in the bit-band region, you must relocate it using another method. You can do this by either using an appropriate scatter-loading description file or by using the --rw_base linker command-line option. See the Linker Reference for more information.