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PC-9801
The PC-9801 (often simply called "PC-98") was an x86-based 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 (however, the BIOS calls, ports, and memory mapping are different from IBM-compatible PCs; see below). The PC-98, more than any other machine 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, much like the Amiga and Atari-ST in the west, 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, emerging as it did out of the open BBS community, is pretty standard, consisting of the ARC, ZIP, and LZW formats typical of its era. The lack of technical obstacles makes PC-98 product easily translatable even by non-programmers, or at least with very little outside expert assistance.
While games since 1992 generally used MS-DOS FAT-12 as their file system, games made in previous years tended to use varying approaches to encoding files. Some companies, like Elf, made their own DOS variants ("Elf DOS"), while others relied on more straightforward clones like MEG-DOS. Fortunately, operating systems like DOS are little more than wrappers for the BIOS, making their dissection straightforward. The earliest games employed custom file management systems, or read data off the disk a sector at a time without the use of a FAT.
Reference
The PC Game Programmers Encyclopaedia offers an overview of IBM-compatible architecture as regards games. The IBM and NEC standards diverge in terms of memory addressing and port access, however. Consult The Undocumented 9801 for specifics on how to interface with PC-98 memory and hardware.