Nios II 中的缓存和内存数据的读写

nios 使用地址中31bit来表示访问是否bypass cache。
如果bit 31=0 表示不bypass cache，即使用cache里的数据；如果bit 31=1表示bypass cache，即直接使用mem中的数据。
如alt_remap_uncached函数

#ifdef NIOS2_MMU_PRESENT
/* Convert KERNEL region address to IO region address */
#define BYPASS_DCACHE_MASK   (0x1 << 29)
#else
/* Set bit 31 of address to bypass D-cache */
#define BYPASS_DCACHE_MASK   (0x1 << 31)
#endif

/*
 * Convert a pointer to a block of cached memory, into a block of
 * uncached memory.
 */

volatile void* alt_remap_uncached (void* ptr, alt_u32 len)
{
  alt_dcache_flush (ptr, len);
  return (volatile void*) (((alt_u32) ptr) | BYPASS_DCACHE_MASK);
}

 其中

#ifdef NIOS2_FLUSHDA_SUPPORTED
#define ALT_FLUSH_DATA(i) __asm__ volatile ("flushda (%0)" :: "r" (i));
#else
#define ALT_FLUSH_DATA(i) __asm__ volatile ("flushd (%0)" :: "r" (i));
#endif /* NIOS2_FLUSHDA_SUPPORTED */

/*
 * alt_dcache_flush() is called to flush the data cache for a memory
 * region of length "len" bytes, starting at address "start".
 *
 * Any dirty lines in the data cache are written back to memory.
 */

void alt_dcache_flush (void* start, alt_u32 len)
{
#if NIOS2_DCACHE_SIZE > 0

  char* i;
  char* end; 

  /*
   * This is the most we would ever need to flush.
   *
   * SPR 196942, 2006.01.13: The cache flush loop below will use the
   * 'flushda' instruction if its available; in that case each line
   * must be flushed individually, and thus 'len' cannot be trimmed.
   */
  #ifndef NIOS2_FLUSHDA_SUPPORTED
  if (len > NIOS2_DCACHE_SIZE)
  {
    len = NIOS2_DCACHE_SIZE;
  }
  #endif

  end = ((char*) start) + len; 

  for (i = start; i < end; i+= NIOS2_DCACHE_LINE_SIZE)
  { 
    ALT_FLUSH_DATA(i); 
  }

  /* 
   * For an unaligned flush request, we've got one more line left.
   * Note that this is dependent on NIOS2_DCACHE_LINE_SIZE to be a 
   * multiple of 2 (which it always is).
   */

  if (((alt_u32) start) & (NIOS2_DCACHE_LINE_SIZE - 1))
  {
    ALT_FLUSH_DATA(i);
  }

#endif /* NIOS2_DCACHE_SIZE > 0 */
}