80386
RapidCAD

(80386 die core)
 

Intel
A80386-16	A80386DX-16	A80386-20	A80386DX-20 IV	A80386-25	A80386DX-25
			SX217

A80386DX-25 IV	A80386-33 IV

NG80386SX-16	KU80386SLB1A-20	NG80386SX-20	NG80386SX-25	KU80386EX-25

		RapidCAD
		RAPIDCAD-1	RAPIDCAD-1
		Evaluation Sample USA Q612	USA SZ624
		RAPIDCAD-2	RAPIDCAD-2
		Evaluation Sample USA Q613	USA SZ625

The Intel RapidCAD is, in rough terms, an Intel 486DX with the 8 KB on-chip cache removed and a 386 pinout. To software, it appears to be a 386DX with a math coprocessor, as the 486-specific instructions have been removed from its instruction set. It is marketed by Intel as "the ultimate coprocessor" and, as the name implies, its main purpose is to replace the 386DX in existing systems and thereby boost the performance of floating-point intensive applications such as CAD software, spreadsheets, and math packages.

RapidCAD is delivered as a set of two chips. RapidCAD-1 is a 132-pin PGA (pin grid array) chip that goes into the CPU socket and replaces the 386DX. It contains the CPU and the FPU (floating-point unit). RapidCAD-2 is a 68-pin PGA chip that goes into the 387 coprocessor (or EMC) socket; it contains a simple PLA whose only purpose it to generate the FERR signal for the motherboard logic: this provides full PC-compatible handling of floating-point exceptions. Many CPU instructions execute in one clock cycle in the RapidCAD, just as on the Intel 486. The RapidCAD is constrained, however, by use of the standard 386DX bus interface, since every bus cycle takes two CPU clock cycles. This means that instructions may be executed by the RapidCAD faster than they can be fetched from memory. But as most FPU instructions take longer to execute than CPU instructions, they are not greatly slowed down by this slow bus interface, and most of them execute in the same time as in the Intel 486DX. Therefore, the Intel RapidCAD provides higher overall floating-point performance than any 386/387 combination. Results from the SPEC benchmarks, a common workstation UNIX benchmark suite, show that the RapidCAD provides 85% more floating-point and 15% more integer performance than an Intel 386DX/387DX combination at the same clock frequency.

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