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/**
 * Ó²Å̲Ù×÷£º"¶Á"²Ù×÷(0x42)£¬"д"²Ù×÷(0x43),\n 
 * "cmd" must be Diskread(0x42) or Diskwrite(0x43).
 * @return >=0: ³É¹¦,¶ÁдµÄÊý¾Ý¿éÊý;
 * @return -1: ʧ°Ü.
 */
short _diskop (unsigned char drv, unsigned char cmd,
     unsigned char * buffer, unsigned long startlow, unsigned short copyblks)
{
 /* ´ÅÅÌ´æÈ¡Êý¾Ý°ü½á¹¹ */
 struct disk_address_packet {
  unsigned char size_of_packet;
  unsigned char reserved;
  unsigned short number_of_blocks_to_transfer;
  unsigned short transfer_buffer_offset;
  unsigned short transfer_buffer_segment;
  unsigned long starting_absolute_block_low;
  unsigned long starting_absolute_block_high;
 };
 struct disk_address_packet dap;
 unsigned char av;

 unsigned char * pt = (unsigned char *) &dap;

 dap.size_of_packet = sizeof (dap);
 dap.reserved = 0;
 dap.number_of_blocks_to_transfer = copyblks;
 dap.transfer_buffer_offset = FP_OFF(buffer);
 dap.transfer_buffer_segment = FP_SEG(buffer);
 dap.starting_absolute_block_low = startlow;
 dap.starting_absolute_block_high = 0; /* ºöÂÔ¸ßλ */

 asm {
  push ds
  push si
  mov ah, cmd
  mov dl, drv
  lds  si, DWORD PTR pt
  int  13h
  pop si
  pop ds
  mov av, ah
  jc  error
 }
 return (dap.number_of_blocks_to_transfer);

error:
 ErrorNum = av;
 return (-1);
}