You can check out the xv6 64bit port by Brian Swetland. It contains code for handling the APIC https://github.com/swetland/xv6/blob/master/kernel/lapic.c / https://github.com/swetland/xv6/blob/master/kernel/ioapic.c and getting multiprocessing to work in 64bit. Pretty concise and nice code. He's now one of the contributors to Fuchsia.

I believe Redox has already implemented it in Rust here:

https://gitlab.redox-os.org/redox-os/kernel/blob/master/src/...

>What's next?

>As already mentioned, the 8259 APIC has been superseded by the APIC, a controller with more >capabilities and multicore support. In the next post we will explore this controller and learn how to >use its integrated timer and how to set interrupt priorities.