GLCD_SPI_LPC1700.c: GLCD_ScrollVertical()

Hi All, Can anyone who are quite familiar with GLCD_SPI_LPC1700.c help me out? There is a vertically scroll screen function provided in that file. But that will scroll the whole screen. What i want is to scroll a certain size of window based on my design. But the code is really hard to understand. All the ops is directly to the reg without any comment. Here is just the code of the function. I don't know how to upload the attachment. The whole GLCD_SPI_LPC1700 contains 929 lines in all.

void GLCD_ScrollVertical (unsigned int dy) {
#if (LANDSCAPE == 0)
  static unsigned int y = 0;

  y = y + dy;
  while (y >= HEIGHT)//HEIGHT=320
    y -= HEIGHT;

  if (Himax) {
    wr_reg(0x01, 0x08);
    wr_reg(0x14, y>>8);                 /* VSP MSB                            */
    wr_reg(0x15, y&0xFF);               /* VSP LSB                            */
  }
  else {
    wr_reg(0x6A, y);
    wr_reg(0x61, 3);
  }
#endif
}

Anyone any advice TT? Thanks all!