STM32F1 system timer

Hi, I'm using STM32F103. I encountered a problem when using the system timer to generate a delay. The processor runs at 8MHz from reset, well I think, and I'm using that clock to generate a 1Ms delay. So, I've to load the SysTick LOAD register with 8000-1.

But it does't work as I expected. I assumed it would generate 1Ms but actually it generates a 0.1Ms. I literally don't know WHY. My code and the simulated output are below in the picture. Thanks in advance!