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 - ))
   {
     ALT_FLUSH_DATA(i);
   }

 #endif /* NIOS2_DCACHE_SIZE > 0 */
 }