Intel 80387 is a co-processor for 80386 microprocessor family. The 80387 runs at the same speed as the 80386 and supports all execution modes of the main processor - real, protected and virtual. Being fully object-code compatible with 80287 and 8087 co-processors, the 80387 has much greater performance due to a few factors:
Higher co-processor clock frequency.
Execution of optimized FPU instructions requires fewer clock cycles.
Data bus is 32-bit.
The 80387 comes in two versions - 80387DX works with 80386DX processors, and 80387SX works with 80386SX CPUs (Since the SX and DX have different bus sizes)