Memory alignment of arrays

Question

I am having trouble aligning memory for DMA transfer on the Cell processor. I need the last 4 bits of an address to be 0.

I have 4 arrays of unsigned int

Answer 1

If arr is an array of 32-bit elements, and the address of arr[0] is 0xXXXXXXX0, then the address of arr[1] will necessarily be 0xXXXXXXX4.

For your purpose, you need to use arrays of 16-byte elements:

typedef struct
{
    unsigned int x;
    unsigned char reserved[16-sizeof(unsigned int)];
}
element_t;

static element_t a[2] __attribute__ ((aligned (16)));
static element_t b[2] __attribute__ ((aligned (16)));
static element_t c[2] __attribute__ ((aligned (16)));
static element_t d[2] __attribute__ ((aligned (16)));

Alternatively, you can simply refrain from using arrays altogether.

Instead, use plain variables, and tell the compiler to align them to 16 bytes:

static unsigned int a0 __attribute__ ((aligned (16)));
static unsigned int a1 __attribute__ ((aligned (16)));
static unsigned int b0 __attribute__ ((aligned (16)));
static unsigned int b1 __attribute__ ((aligned (16)));
static unsigned int c0 __attribute__ ((aligned (16)));
static unsigned int c1 __attribute__ ((aligned (16)));
static unsigned int d0 __attribute__ ((aligned (16)));
static unsigned int d1 __attribute__ ((aligned (16)));