Author here; yes, the sole focus here is on the ISA-relevant parts. Obviously disk formats are out of scope. It's correct that IBM originally used custom IBM d

Author here; yes, the sole focus here is on the ISA-relevant parts. Obviously disk formats are out of scope. It's correct that IBM originally used custom IBM disk drives with 520-byte sectors. I believe 528 byte sectors were also seen where both tags and parity (for some RAID-like technology, I don't recall the details) were desired. On devices with a 4096 byte sector size, this leads to equivalent sector sizes of 4160 or 4224 bytes respectively. Nowadays you can also run IBM i on virtualised disks provided by IBM's Virtual I/O System (VIOS) under PowerVM. VIOS is AIUI basically a customised AIX install which runs as a VM under PowerVM and provides I/O functionality similar to that provided by a Xen dom0. In this case things are virtualised on top of normal hard drives with 512 byte sectors, so it's possible for these weird sector sizes to be emulated nowadays. I believe you can still pass a hard drive directly to an IBM i VM, though, and that IBM still offers hard drives with these special sector sizes for this purpose. Of course IBM doesn't make drives nowadays, so these are obviously an IBM rebrand of some other manufacturer's hard drives. What's really interesting though is that if you read the manuals for modern hard drives (yes, hard drives have manuals, who knew?), many of them advertise a selectable sector size, where you can reformat the drive to a new sector size. For example, see this manual, which says you can select a sector size of 512, 520, or 528 bytes (or 4096, 4160 or 4224 bytes): https://www.seagate.com/www-content/product-content/enterprise-hdd-fam/exos-7e10/_shared/docs/100870879e.pdf#page=10 https://www.seagate.com/www-content/product-content/enterpri... So theoretically most modern hard drives can be reformatted to support IBM i. I believe this functionality is also maintained to support modern hardware RAID systems, though.