LCD的疑问

最近在研究3E板子上的LCD的程序，有个疑问，请教一下各位。先谢谢了。

void lcdWrite(unsigned char datum, unsigned int isData, unsigned long delay) {    // local variables     unsigned long  upperNibble = ((datum >> 4) & 0x0F) << registerShiftOffset;    unsigned long  lowerNibble = (datum & 0x0F) << registerShiftOffset;    unsigned long *lcdDevice;

    lcdDevice = LCD_BASEADDR;

registerShiftOffset这个值为25，请问这个值的作用是什么，datum 这个是不是命令中的后面8位呢？

void LCDOn() { lcdWrite(0x0E,INSTRUCTION,40); }void LCDOff(){ lcdWrite(0x08,INSTRUCTION,40); }void LCDClear() {   lcdWrite(0x01,INSTRUCTION,1535); }