CPU Upgrade for the 80386 System ( IEEE-754 FPU)
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.
RapidCAD’s use of the standard 386 bus interface causes instructions that access memory to execute at about the same speed as on the 386. The integer performance on the RapidCAD is definitely limited by the low memory bandwidth provided by this interface (2 clock cycles per bus cycle) and the lack of an internal cache. CPU instructions often execute faster than they can be fetched from memory, even with a big and fast external cache. Therefore, the integer performance of the RapidCAD exceeds that of a 386 by *at most* 35%. While CPU and integer instructions often execute in one clock cycle on the RapidCAD, floating-point operations always take more than seven clock cycles. They are therefore rarely slowed down by the low-bandwidth 386 bus interface; Tests show a 70%-100% performance increase for floating-point intensive benchmarks over a 386-based system using the Intel 387DX math coprocessor. This is consistent with the SPECfp rating reported by Intel. The 386/387 at 33 MHz is rated at 3.3 SPECfp, while the RapidCAD is rated at 6.1 SPECfp at the same frequency, an 85% increase. This means that a system that uses the RapidCAD is faster than *any* 386/387 combination, regardless of the type of 387 used, whether an Intel 387DX or a faster 387 clone.