The RapidCAD is not a coprocessor, strictly seen, although it was marketed as one. Rather, it is a full replacement for a 80386 CPU: basically, an Intel 486DX CPU chip without the internal cache and with a standard 386 pinout. RapidCAD is delivered as a set of two chips. RapidCAD-1 (S- Spec: SZ624) goes into the 386 socket and contains the CPU and FPU. RapidCAD-2 (S-Spec: SZ625) goes into the coprocessor (387) socket and contains a simple PAL(Programmable Array of Logic) whose only purpose is to generate the FERR signal normally generated by a coprocessor (This is needed by the motherboard circuitry to provide 287 compatible coprocessor exception handling in 386/387 systems.) The RapidCAD instruction set is compatible with the 386, so it doesn't have any newer, 486-specific instructions like BSWAP. However, since the RapidCAD CPU core is very similar to 80486 CPU core, most of the register- to-register instructions execute in the same number of clock cycles as on the 486.