What are NMI's used for on Amiga?

[roondar]

When coding, I often disable the OS using the normal method of disabling all interrupts (plus saving/restoring views, vectors, etc ). But, disabling interrupts only affects maskable interrupts, not non-maskable interrupts. However, just disabling maskable interrupts seems to work very well and most OS started demo's/games that hit the hardware seem to use this method.

The question I have is what are NMI's used for on the Amiga?
I'm assuming the basic machines (without expansions) don't use them, otherwise disabling only maskable interrupts will still potentially cause issues when NMI's hit.

Are they used by common hardware? If so, what kinds of hardware?
Or are NMI's not used at all (I don't think this is true)?

[Thomas Richter]

Originally Posted by roondar

When coding, I often disable the OS using the normal method of disabling all interrupts (plus saving/restoring views, vectors, etc ). But, disabling interrupts only affects maskable interrupts, not non-maskable interrupts.

This may make your machine unusable or unstable as certain hardware may require or generate periodic interrupts. Also note that not all interrupts are necessarily routed through PAULA.

Originally Posted by roondar

The question I have is what are NMI's used for on the Amiga?

Not at all. Some A2000 boards (from the 4.x area) have a slightly broken IRQ logic that generates sporadic NMIs from time to time.

[roondar]

Not at all. Some A2000 boards (from the 4.x area) have a slightly broken IRQ logic that generates sporadic NMIs from time to time.

Interesting!

That makes me wonder: are these A2000 boards incompatible with the many games/demos (and WHDLoad for that matter) that throw out or disable the OS, or do they merely generate the odd spurious interrupt here and there without much issues?

[StingRay]

Originally Posted by roondar

Are they used by common hardware? If so, what kinds of hardware?

Beermon and other monitors/freezers use the NMI for activating the monitor. ASM-One (and its derivates) use it to be return to the system if the code hangs (endless loops for example).

This may make your machine unusable or unstable

Are there any real world examples for this claim?

[roondar]

Originally Posted by StingRay

Beermon and other monitors/freezers use the NMI for activating the monitor. ASM-One (and its derivates) use it to be return to the system if the code hangs (endless loops for example).

Right, that's pretty clever. So at least monitors/freezers use NMI's.
Still wondering if expansions also use it (outside of AR, which is a freezer so it probably does as well), but this is already helpful information thanks!

[mr.spiv]

Originally Posted by roondar

Interesting!

That makes me wonder: are these A2000 boards incompatible with the many games/demos (and WHDLoad for that matter) that throw out or disable the OS, or do they merely generate the odd spurious interrupt here and there without much issues?

Odd crashes granted unless the level 7 IRQ handler points to a valid IRQ handler (that is also operable in the current state of the machine). There are also more recent expansion hw e.g. for A500 that has the same issue.. There are threads in EAB about these.

[Toni Wilen]

Reason for spurious interrupts (at least the most common one): Paula IPL pins don't change at the same time but it seems Paula does it in two steps, clears pins first, then sets pins, delay is about 1/2 to 1/4 CCKs. If accelerator board does not sync IPL sampling to CCK and samples IPL state during this transition: spurious interrupt. Because IPL is active low, some IPL transition combinations can cause spurious NMI (all lines low for 1/4 CCKs or so).