>>12018351
You need to get familiar with x86 assembly first. Then my main tools for reverse-engineering are IDA 5, NekoProject 2 with debugger, and an hex editor.
For PC-98 hardware (BIOS, I/O ports, graphics...) I use Japanese websites like https://radioc.web.fc2.com or http://software.aufheben.info/contents.html?contents_key=oohpc_menu