NMI: are VBL and DLI really not maskable?

[SoLo2]

Hello!

Why are the VBI and DLI

non-maskable interrupts?

Can the vertical blank and

display list interrupts

not be disabled?

Is there a way to disable them?

For example, NMIEN $D40E has

2 flags no enable this interrupts.

Why NMI?

Greetings,

SoLo2

P.S.: why isn't there an Atari XL

group, instead of the VCS5200?

Thanks for this great forum.

[Rybags]

They can be disabled, like you've said - the bitsettings in NMIEN.

 

The System Reset key on 400/800 is the only NMI source that can't be disabled - XL/XE doesn't use it, the Reset key there does an actual reset on the 6502 and system, which is why they can recover from a lockup condition.

 

That's your typical mode of operation on a 6502 system - NMI by design can't be disabled at the CPU level, it's up to the hardware that supplies that interrupt source to provide means to disable it (or not).

 

 

Also, there's the "NMI pulse too short" bug that's designed into Antic, which means that an IRQ can in some circumstances cause an NMI to be ignored. We had a thread or two about it in the last year or so.

[Bryan]

Yep.. the Antic NMI pulse is 2 cycles which early 6502 datasheets say will work. Unfortunately, the early datasheets are wrong if an IRQ happens at the same time. Most other 6502 machines don't have this problem because they force you to shut off the NMI manually.