[Daniel]

There are high and low priority interrupts. We are to only use low priority interrupts.

High priority interrupts can interupt low priority interrupts (call it nesting if you would like). BUT low priority interrupts can not interrupt other low priority interrupts. In you are servicing a low priority interrupt and do not clear the interrupt flag for another interrupt then as soon as you exit the interrupt service routine then another interrupt will occur - no nesting.

I discussed this with Dave at IFI while writing Interrupts for Dummies. The C Compiler Usrs guide can get you a bit confused.