Dwedit wrote:
Acknowledge means that it TURNS OFF the IRQ pin. Otherwise it would keep triggering interrupts over and over again.
To clarify, the IRQ signal is
level-sensitive (compared to NMI, which is edge-triggered) - once the timer expires, the IRQ signal is activated, and the CPU will jump to the IRQ handler at
every possible opportunity (i.e. whenever the "I" flag is clear). To prevent the CPU from getting stuck in a loop rerunning the IRQ handler over and over, it needs to tell the hardware that generated the interrupt that it's done handling it and that it can deactivate the signal, and for the MMC3 this involves writing any value to $E000.