The WDC 65816 (also: 65C816), a 16-bit microprocessor CPU developed by the Western Design Center (WDC), is an expanded and compatible successor to the venerable MOS Technology 6502. The number 65 in the 65816's name comes from the 6502 compatibility, and 816 signifies that it can work in both 8 and 16 bit modes.
If you're like me and don't really have a clue about interrupts, Interrupts in 65xx processors is a starting point.