CDVD Driver
Using the CDVD drive, the PS2 has the ability to read CDROMs, single-layer DVDs, and dual-layer DVDs. It also sports backwards compatibility with the PS1's CDROM drive in PS1 mode.
CDVD commands are either asynchronous (N commands) or synchronous (S commands). Seeks and reads fall into the former category, and miscellaneous commands, such as RTC access, fall into the latter.
CDVD Reference