This library utilizes the XMOS XS3 architecture's vector processing unit (VPU). All loads and stores to and from the XS3 VPU have the requirement that the loaded/stored addresses must be aligned to a 4-byte boundary (word-aligned).

In the current version of the API, this leads to the requirement that most API functions require vectors (or the data backing a BFP vector) to begin at word-aligned addresses. Vectors are not required, however, to have a size (in bytes) that is a multiple of 4.

Writing Alignment-safe Code

The alignment requirement is ultimately always on the data that backs a vector. For the low-level API, that is the pointers passed to the functions themselves. For the high-level API, that is the memory to which the data field (or the real and imag fields in the case of bfp_complex_s16_t) points, specified when the BFP vector is initialized.

Arrays of type int32_t and complex_s32_t will normally be guaranteed to be word-aligned by the compiler. However, if the user manually specifies the beginning of an int32_t array, as in the following..

uint8_t byte_buffer[100];

int32_t* integer_array = (int32_t*) &byte_buffer[1];

.. it is the responsibility of the user to ensure proper alignment of data.

For int16_t arrays, the compiler does not by default guarantee that the array starts on a word-aligned address. To force word-alignment on arrays of this type, use __attribute__((align 4)) in the variable definition, as in the following.

int16_t __attribute__((align 4)) data[100];

Occasionally, 8-byte (double word) alignment is required. In this case, neither int32_t nor int16_t is necessarily guaranteed to align as required. Similar to the above, this can be hinted to the compiler as in the following.

int32_t __attribute__((align 8)) data[100];