PIC24 Interrupt auto_psv vs no_auto_psv
PSV window size is 64K which could be smaller than whole ROM size on certain parts (like the one you're using). On those, if you have more than 64K of constants you'd have to move the window to access them all. If you have an ISR which accesses constants 'auto_psv' will make sure you can do this. On the other hand, if your ISR doesn't use constants set this attribute to 'no' to reduce code size.