about the IRQ

You can set interrupt entry to a onchip memory, and save the interrupt stack there.

That is, handle intrrupt service yourself instead of by HAL.

You can look at the source of HAL, and modify them to improve response time.

Alternately, you could start with a full_featured design, which locates the HAL's exception handling code in a tightly coupled memory (an onchip RAM that should be just as quick as cache). If you're using >=5.0 Nios II/Quartus II, you should also be able to place the exception stack and, with some effort, your own ISR, there, as well!

Best of luck!

Cheers,

- slacker