AArch64 is the 64 bit version of the ARM architecture, as published by ARM Holdings. It has gained much headway in recent years, being used in many mobile phones. It also is gaining market share in the server world, due to performance advancements and its power management. Apple in 2020 started using it for most of its newest Macs, and this chip, the Apple M1 chip, has great performance comparable to Intel and AMD processors. ARM SystemReady is a fairly new standard that aims to standardize ARM hardware, to allow for OSes like Windows, Linux, and the BSDs to be ran out of box on compliant hardware. This means that implementing AArch64 support in your hobby OS may be a good thing, as its popularity and usage continue to expand.