IIT 3C87-20

Definition

The IIT 3C87-20 is an X87-compatible math coprocessor designed as a plug-in alternative to the 80387DX for systems based on the 80386DX. The “-20” suffix indicates the nominal 20 MHZ version. Its purpose is to accelerate floating-point operations (and a subset of advanced math functions) versus software execution, while maintaining instruction-level compatibility with the X87 ecosystem expected on 386 platforms.

In practical use, it was selected as a market “second source” (i.e., an alternative supplier to INTEL) to improve availability, pricing, or power/performance, while remaining compatible with the expected socket and with software written for the platform.

Key point requested: lower power thanks to CMOS technology

Compared with several earlier solutions (or variants built on less efficient processes), the 3C87-20 was manufactured using CMOS technology, with the goal of reducing dissipation and improving energy efficiency. For a coprocessor in this class, the practical benefits of CMOS typically translate into:

• Lower power dissipation at the same frequency and numeric workload.

• Reduced thermal stress on the board, improving stability and reliability.

• Easier integration in systems with limited cooling constraints.

Compatibility and positioning (what “80387-compatible” means)

Saying the 3C87-20 is compatible with the 80387DX means, at system level:

• Electrical/mechanical compatibility: typically the same package and pinout expected by the coprocessor socket on 386DX motherboards.

• Software compatibility: support for the X87 instruction model required by applications that use a coprocessor (CAD, technical computing, scientific software, advanced spreadsheets).

• Transparent integration: software that “sees” an external FPU works without being rewritten, because floating-point instructions are executed by the dedicated unit.

Technical note: some third-party compatible implementations may include extensions or minor differences compared with the INTEL reference; however, the general goal remains operational compatibility for real-world use in DOS/UNIX environments and typical applications of the era.

Board-level integration (system architecture)

On an 80386DX motherboard, the coprocessor:

• Is installed into the dedicated socket (when present).

• “Works alongside” the CPU: floating-point instructions are handled by the FPU, while the CPU continues to manage overall program flow and integer operations.

• Accesses operands and stores results through system memory according to the X87 architecture model.

Sketch of the most important connections

                 ┌──────────────────────────────┐
                 │            CPU 80386DX        │
                 │  fetch/execute integer + ctrl │
                 │  bus + system signals         │
                 └──────────────┬───────────────┘
                                │ coprocessor interface (387DX socket)
                                ▼
                       ┌──────────────────┐
                       │    IIT 3C87-20   │
                       │      X87 FPU     │
                       │  floating-point  │
                       │  dedicated unit  │
                       └─────────┬────────┘
                                 │ operands/results in memory
                                 ▼
                 ┌────────────────────────────────┐
                 │ System RAM / ROM / I/O          │
                 │ (data, code, results)           │
                 └────────────────────────────────┘

Table 1 – Identification data and specifications (English)

Table 2 – Operational and usage aspects (English)