As I understand it, if, when my system boots and I switch to using PSP (process stack pointer) and allow the CPU to handle exceptions using MSP? Have I got that the right way around?The reason is that for the sake of the fastest code, I need to use PSP as a pointer (allowing multiple reads & with free post increment) in my MP3 decoder. I could also get around the issue by disabling interrupts while the inner-loops that use SP (r13) are running.Obviously it would be nice if interrupt latency were minimised but I am of the 8-bit console era so I try to use every byte, every register and every cycle.I say without rancour that the Thumb instruction set has by far the steepest learning curve of any instruction-set I have ever encountered and I have professionally written commercial computer games in 14 different processors. It is very hard to learn in isolation, it needs people to bounce ideas off.