Editing MOS Technology VIC-II (section)

===Raster interrupts===
Utilization of raster interrupts is an essential part of C64 game programming. In the computer's power-on default state, the first [[MOS Technology CIA]] chip generates a [[Interrupts in 65xx processors|maskable interrupt]] (IRQ) 60 times per second (whether NTSC or PAL, this is unrelated to video refresh) which sends the CPU to the kernel IRQ handler at {{mono|$EA31}}. The handler acknowledges the CIA's IRQ, updates the clock, scans the keyboard, and blinks the cursor in BASIC.

Games normally disable the CIA's interrupt and instead set up the VIC-II to generate interrupts when a specific scanline is reached, which is necessary for split-screen scrolling and playing music. The game remaps the [[interrupt vector|IRQ vector]] at {{mono|$0314}}/{{mono|$0315}} to its raster handler which performs these functions and then optionally executes a JMP {{mono|$EA31}} instruction to return control to the kernel.

Some games use only one IRQ; however, chained IRQs are more common and improve program stability. In this setup, the IRQ is remapped to the second routine and so forth for each one until the last one restores it to the address of the first IRQ. When chained IRQs are used, only one JMP {{mono|$EA31}} instruction is needed in the chain and the others can be ended with JMP {{mono|$EA81}}, which simply goes to the end of the kernel handler. Also it is not uncommon for games to switch out the kernal and use their own IRQ handler instead. The NMI can be used for an additional interrupt thread, although undesirable side effects can result from accidentally pressing the Restore key as it triggers an NMI if pressed.

The VIC-II may also generate a raster interrupt from the collision registers, but this feature is rarely used as it provides insufficient information to the program in most cases.