Early demoscene problems with Sound Blaster/GUS IRQ reception about dosbox-x

[joncampbell123]

I think I figured it out.

DOSBox's PIC IRQ dispatching has a minor but fatal flaw. It raises a "PIC_IRQCheck" flag when a IRQ signal is raised, and resets the flag if an IRQ is lowered. It also resets the flag after PIC_runIRQs() dispatches a single interrupt.

But that's the flaw: Sooner or later, one device will raise an IRQ, and within DOSBox's 1ms "interval", another device will lower the IRQ. That resets the flag, so the one device with it's IRQ still raised is effectively ignored by PIC_runIRQs() and it's never taken care of. In some DOS demos like Crystal Dream II, this flaw eventually causes the demo music to stop, and the demo to forever loop at that one point, or in other demos using Sound Blaster, the demo eventually misses the SB IRQ and music abruptly stops.

[joncampbell123]

Bug fix confirm: Extreme "Lunatic" demo (1993) SB Pro music no longer cuts out at a random point under load. It just slows down audibly if you set a low cycles count and the demo is doing intense effects, but it doesn't cut out anymore.