中断服务程序不会跳回到ARM Cortex M0上的中断处理程序

Currently I'm trying to establish a communication between my PC and an ARM Cortex M0 over UART. The system is simple: The custom-made UART module has a one Byte buffer. If the buffer is full the interrupt flag is set. The interrupt handler gets called and calls the interrupt service routine (ISR) in return. The ISR reads the buffer (therefore the interrupt flag of the modules gets reset) and writes the obtained Byte into a global array.Currently I'm trying to establish a communicati