为什么要有这个while循环

我和玲佑有个约

#include <REGX52.H>
void Delay1ms(unsigned int xms)//@12.000MHz
{
unsigned char i, j;
while(xms)
{

i = 2;
j = 239;
do
{
while (--j);
} while (--i);
xms--;
}
}

void main()
{
while(1)
{P2=0xFE;
Delay1ms(100);
P2=0xFD;
Delay1ms(100);
P2=0xFB;
Delay1ms(100);
P2=0xF7;
Delay1ms(100);
P2=0xEF;
Delay1ms(100);
P2=0xDF;
Delay1ms(100);
P2=0xBF;
Delay1ms(100);
P2=0x7F;
Delay1ms(100);


}



}
为什么延时函数里要有while(xms)循环