A feature available in Cyclone IV devices that employs a phase-locked loop (PLL). The multipurpose PLL (MPLL) is a special-purpose PLL optimized for use by the circuitry inside the gigabit transceiver block (GXB). However, it can also be used as a general-purpose PLL (GPLL).
You can take advantage of the MPLL with the altgx or altpll megafunctions. The altgxb and altpll megafunctions instantiate the PLL instance, which is implemented as a GPLL or MPLL, depending on the results of the compilation.