Yamaha YM2164
The Yamaha YM2164 a.k.a.OPP (FM Operator Type P), is an FM synthesis sound chip developed by Yamaha, an enhanced version of their YM2151 a.k.a. OPM. The OPP was used in various MIDI-based synthesizers by Yamaha - DX21, DX27, DX100, SFG-05, FB-01 (= a standalone SFG-05) - plus several licensed products: the IBM Music Feature Card (effectively an FB-01 on an ISA card) and Korg's DS-8 and Korg 707. The OPP has the following features: Compared to the OPM, the OPP has the same pinout and functional features but some minor changes to control registers. Differences are the  test register address (9), the timer B resolution (period doubled: 2048 clock cycles on OPP vs. 1024 on OPM), and 8 undocumented registers (0-7). Due to how the FB-01 and IBM MFC hardware use those registers, swapping an OPM into these hosts does not operate correctly. While the OPP was available for use by IBM and Korg, the chips were unavailable for purchase outside of Yamaha, compared to the almost identical and widely available OPM, which found its way into countless arcade game PCBs.