The PC-9801 (often simply called "PC-98") was an IBM-compatible personal computer released by NEC in 1985.
The PC-9801 is a genuine Wintel PC, making use of MS-DOS and Windows 3.x (and eventually 95). As such, old MS-DOS manuals and PC hardware references are applicable to it, particularly regarding the functioning of the mouse, video, sound devices, and BIOS. It was the system which, more than any other, liberated the Japanese gaming industry. RPG Maker got its start here, as well as several other maker kits. It inherited the PC-8801's reputation for mature content, with more than 500 eroge projects being published during its production run. The PC-98 developed a reputation for independent RPGs, which flourished thanks to the availability of dial-up modem and the rise of pre-internet private networks called "bulletin board systems" (BBS). By the early 1990s, the PC-98 was ubiquitous... even Sailor Moon owned one.
Hacking the PC-98
The PC-98 is distinct from western PCs in that it features a full set of SHIFT-JIS character ROM and a special version of DOS capable to exploiting it to the full. This ROM features both Japanese (katakana, hiragana) and Chinese characters (Kanji). The explicit reliance on SHIFT-JIS makes syllable tables something of a rarity in PC-98 software... provided knowledge of the compression is available, a hacker can simply open files in a SHIFT-JIS editor and search for phrases to find the locations of text data. This data is usually serialized string data and can be edited quite easily. Compression on the PC-98 is pretty standard, consisting of the ARC, ZIP, and LZW formats typical of its era.