1.元素顺序是固定的，通常是由结构体来表述这些描述符。

2.描述符的下一个起始地址可以事先定义好，也可以在代码中生成而更新，需要指定。

3.如果你的描述符是1维的，需要X就可以了，无需Y。

发一个描述符描述的部分代码例子：

typedef struct
{
unsigned short NDPL;  // next descriptor pointer (lower 16-bits)
unsigned short NDPH;  // next descriptor pointer (upper 16-bits)
unsigned short SAL;  // start address (lower 16-bits)    
unsigned short  SAH;        // start address (upper 16-bits)    
unsigned short CFG;     // DMA config                      
unsigned short XCNT;       // X Count                          
unsigned short XMOD;       // X Mod                            
unsigned short YCNT;       // Y Count                          
unsigned short YMOD;       // Y Mod                            
} sDMA_Descriptor_Long;

extern sDMA_Descriptor_Long current_des[2];

//////////////////////////////////////////////////

while(1)
{
  ((audio_list_t *)sport_input_ptr)->des.SAL = (unsigned long)ptr;
  ((audio_list_t *)sport_input_ptr)->des.SAH = ((unsigned long)ptr)>>16;
  ((audio_list_t *)sport_input_ptr)->des.CFG = 0x7900 | WDSIZE_16|DMAEN|DI_EN;
  ((audio_list_t *)sport_input_ptr)->des.XCNT = 1024*16;
  ((audio_list_t *)sport_input_ptr)->des.XMOD = 2;
  ((audio_list_t *)sport_input_ptr)->flag = 1;
  sport_input_ptr = ((audio_list_t *)sport_input_ptr)->next;
  current_play_buffer ++;
  
  ptr += 1024*16*2;

}

/////////////////////////////////////////////////

EX_INTERRUPT_HANDLER(Sport0_TX_ISR)
{
*pDMA2_IRQ_STATUS = 0x0001;

if(current_play_buffer == 0)
{
  if(*pDMA2_NEXT_DESC_PTR == &des1)
  {
   des1.SAL = mute_des.SAL;
   des1.SAH = mute_des.SAH;
   des1.CFG = mute_des.CFG;
   des1.XCNT = mute_des.XCNT;
   des1.XMOD = mute_des.XMOD;
  }else
  {
   des2.SAL = mute_des.SAL;
   des2.SAH = mute_des.SAH;
   des2.CFG = mute_des.CFG;
   des2.XCNT = mute_des.XCNT;
   des2.XMOD = mute_des.XMOD;
  }  
}else
{
  flag = 1;
  if(*pDMA2_NEXT_DESC_PTR == &des1)
  {
   des1.SAL = ((audio_list_t *)sport_output_ptr)->des.SAL;
   des1.SAH = ((audio_list_t *)sport_output_ptr)->des.SAH;
   des1.CFG = ((audio_list_t *)sport_output_ptr)->des.CFG;
   des1.XCNT = ((audio_list_t *)sport_output_ptr)->des.XCNT;
   des1.XMOD = ((audio_list_t *)sport_output_ptr)->des.XMOD;
   ((audio_list_t *)sport_output_ptr)->flag = 0;
  }else
  {
   des2.SAL = ((audio_list_t *)sport_output_ptr)->des.SAL;
   des2.SAH = ((audio_list_t *)sport_output_ptr)->des.SAH;
   des2.CFG = ((audio_list_t *)sport_output_ptr)->des.CFG;
   des2.XCNT = ((audio_list_t *)sport_output_ptr)->des.XCNT;
   des2.XMOD = ((audio_list_t *)sport_output_ptr)->des.XMOD;
   ((audio_list_t *)sport_output_ptr)->flag = 0;
  }
  current_play_buffer --;
  sport_output_ptr = ((audio_list_t *)sport_output_ptr)->next;
}
}