Interrupts and Forth

Mecrisp uses R7 as data stack pointer and R6 as TOS. If Forth rules the system R7 is always the data stack pointer and you can use the data stack pointer within a interrupt service routine. But in a mixed system, where C routines are used, there is no guarantee that the register R7 remains unchanged. When the interrupt occurs while a C routine is executed, the data stack pointer is invalid and Forth words can't be used in interrupt service routines.