The 6502 is designed to be used in systems with devices that may need to cause an interrupt. The way this is accommodated is by having each device use an open-collector/open-drain output to drive the /IRQ pin, which is passively pulled up with a resistor. If any device wants the /IRQ pin low, it will be low. Only if no device wants it low will it be high.

When an IRQ occurs, the handler will initially have no idea which device requested it. The usual practice is for each device to provide a means of asking whether it requested an interrupt, and for the device to keep on requesting an interrupt until the CPU does whatever the device needed it to do. Suppose there are two devices and the second one needs service. It will drive IRQ, whereupon the interrupt service routine will check whether the first device needs service (it won't), so the interrupt service routine will check the second device, see that it needs service, and start servicing it.

If the first device decided it needed service at some point while the second device was being serviced, the first device would start driving IRQ (which the second device may or may not still be driving). After code finishes servicing the second device, it will execute an RTI which will re-enable interrupts. As soon as that happens, the asserted /IRQ line will cause the CPU to re-invoke the interrupt handler, which will then ask whether the first device needs service (it will), and thus service the first device.

For some kinds of devices such as UARTS ("serial ports") and interrupts (e.g. "incoming character received") it may not be necessary to explicitly acknowledge interrupts. If an interrupt indicates that a character has been received but code hasn't yet read it, the act of reading all characters that have been received would clear the condition until more characters arrive. For something like the raster interrupt on the VIC-II, however, the only way the VIC-II can know that the processor has noticed that it has requested an interrupt is for the processor to indicate that by writing to $D019. Using ASL is a handy way of writing a suitable value.

The 6502 is designed to be used in systems with devices that may need to cause an interrupt. The way this is accommodated is by having each device use an open-collector/open-drain output to drive the /IRQ pin, which is passively pulled up with a resistor. If any device wants the /IRQ pin low, it will be low. Only if no device wants it low will it be high.

When an IRQ occurs, the handler will initially have no idea which device requested it. The usual practice is for each device to provide a means of asking whether it requested an interrupt, and for the device to keep on requesting an interrupt until the CPU does whatever the device needed it to do. Suppose there are two devices and the second one needs service. It will drive IRQ, whereupon the interrupt service routine will check whether the first device needs service (it won't), so the interrupt service routine will check the second device, see that it needs service, and start servicing it.

If the first device decided it needed service at some point while the second device was being serviced, the first device would start driving IRQ (which the second device may or may not still be driving). After code finishes servicing the second device, it will execute an RTI which will re-enable interrupts. As soon as that happens, the asserted /IRQ line will cause the CPU to re-invoke the interrupt handler, which will then ask whether the first device needs service (it will), and thus service the first device.

For some kinds of devices such as UARTs ("serial ports") and interrupts (e.g. "incoming character received") it may not be necessary to explicitly acknowledge interrupts. If an interrupt indicates that a character has been received but code hasn't yet read it, the act of reading all characters that have been received would clear the condition until more characters arrive. For something like the raster interrupt on the VIC-II, however, the only way the VIC-II can know that the processor has noticed that it has requested an interrupt is for the processor to indicate that by writing to $D019. Using ASL is a handy way of writing a suitable value.