Overwriting the stack-pointer

While gaining an additional register might sound promising, this has a few pitfalls. Bits [0,1] are “Read As Zero/Write Ignored” thus, add sp, #5 will add 4 to the stack pointer instead of 5. It is also essential to have some way to recover the stack pointer, this could be a hard-coded address or a FPU register. Finally, there is the issue that any interrupts that rely on a valid stack pointer will crash. An example of this is the interrupt handler used in PQM4 that keeps track of overflowing cycle counts. Working on a system that seemingly crashes randomly when an interrupt is triggered in the background would cause headaches for any programmer.