The Intel 387DX is the second generation Intel 387 and was introduced in 1989. This version is done in a more advanced CMOS process than the 80387 that enables the coprocessor to run at a maximum frequency of 33 MHz, while the 80387 had a maximum frequency of 25 MHz.
The 387DX is about 20% faster than the 80387 at the same clock frequency. The Intel 387DX (and its predecessor 80387) are the only 387 coprocessors that support asynchronous operation of CPU and FPU. The 387 consists of a bus interface unit and a numerical execution unit. The bus interface unit always runs at the speed of the CPU clock.