Install DOS 6.22 for PC-98x1 on an HDD
(This is a WIP guide, if any JP speaker wants to fill in the blanks be my guest. It'll eventually go in the main OS Install thread)
Pre-requisites as per MAME 0.169:
1. Enable "Load IDE BIOS" in Machine Configuration;
2. Create a blank HDD via chdman, anything with a sense like previous Amiga 600 example works just fine for PC-98 too (i.e. CHS 332,16,63).
1. Launch PC-98x1 with your shiny new Hard Disk and disk 1 of DOS in drive A.
2. Wait until it prompts you to this, press ENTER on first option (?):
3. It then prompts you to partition select (?). Select first option and press ENTER again:
4. "Ready to install, ENTER to proceed, ESC to double check configuration" (?)
5. After some time, it eventually ask you to format HDD (?). Press ENTER twice to confirm:
6. "SYSTEM SHUTDOWN", which is a not-so-clever way to tell you to restart the machine, F3 should be enough.
(At this point, your HDD has bare minimum for DOS, i.e. COMMAND.COM and nothing else)
7. Once restarted, DOS asks you to tell the directory which it should be installed, also notice that the Yen symbol is an alias for backslash for PC-98xx. Next screen is a Yes/No confirmation:
8. At this point it'll start to feed DOS files into your HDD. When it prompts you to following screen just swap first drive disk into the designated disk number (so disk no. 2 here) and press ENTER:
9. If everything went correctly, DOS tells you detected HW (here: screen for default PC-9821), press ENTER here, then I guess it tells you to remove disk  here, press ENTER again, "SYSTEM SHUTDOWN" -> F3
Congrats, you just installed an OS without understanding a single word of the UI :p
1. PC-98xx drive letters are inverted compared to PC/AT when in normal config, i.e. HDD is A: / B: if present, floppy drives are C: / D:
2. Each time that you start up a PC-98 machine with a valid HDD, a screen with a list of bootable OS-es will appear (see previous post), so you could potentially install Windows / *nix / DOS / Miscellanea at the same time on the same HDD and just switch them at boot time.