What is an "interruptNumber" [mostly moot for me now]

Arduino - SPIusingInterrupt says
SPI.usingInterrupt(interruptNumber)
Parameters
interruptNumber: the associated interrupt number

I was asking

  1. What is an interruptNumber?

The question is still a interesting to me, but it moot to me now given that the 255 I am using (thinking I was taking the lazy way out) is the way you do it with the timer compare A interrupt and all timer interrupts...

Seems odd that the timers would be all part of a generic case, but that is an easy thing to do once you find the magic 255.

This is from SPI.h:

// If SPI is used from within an interrupt, this function registers
  // that interrupt with the SPI library, so beginTransaction() can
  // prevent conflicts.  The input interruptNumber is the number used
  // with attachInterrupt.  If SPI is used from a different interrupt
  // (eg, a timer), interruptNumber should be 255.
  static void usingInterrupt(uint8_t interruptNumber);

You can use 'digitalPinToInterrupt(pin)' to get the interrupt number for a pin.

There's more info under 'attachInterrupt()' in the reference:-
https://www.arduino.cc/en/Reference/AttachInterrupt

Thanks!

This probably has more info than you need http://courses.cs.washington.edu/courses/csep567/10wi/lectures/Lecture7.pdf