If you are running your OS on a VM hypervisor (e.g.; Qemu, kvmtool), then you can just use virtio for disk and network I/O. No boring and complex device specification (e.g.; ATA) to implement.
Yeah, I'm avoiding modern interfaces due to my peculiar bootstrapping requirements (e.g. must run on old HW; no SW dependencies). virtio is probably a better choice for anyone trying to build something practical. On the other hand, it looks like using virtio would need a lot more code than accessing the BIOS and I couldn't easily find examples in assembly.