应该发这里，KEIL C数值计算问题

使用KEIL c；stm32f103编写如下代码：
#define MAX_LAMPNUM_IN_CTRLDEV 4
unsigned short CharPos,ChangePos,MeterNo;
unsigned char LampNum;

MeterNo = 1;
LampNum = 0;
CharPos = ((MeterNo - 1) * MAX_LAMPNUM_IN_CTRLDEV + LampNum - 1) / 8;
ChangePos = ((MeterNo - 1) * MAX_LAMPNUM_IN_CTRLDEV + LampNum - 1) % 8;

为什么最后计算记过CharPos = 0x0000，ChangePos = 0xffff

(unsigned short)(-1/8)
(unsigned short)(-1%8)