386 motherboard using the OPTi 82C391 chipset © 199? Unknown
The 82C391 served as the System Controller (SYSC) for the 386WB chipset. Unlike entry-level chipsets of the time, the 82C391 integrated a write-back cache controller—a significant feature that differentiated "high-end" 386 systems from standard, "write-through" budget boards. It was almost always paired with the 82C392 (Data Buffer Controller) and the 82C206 (Integrated Peripheral Controller).
System Controller: 82C391 (SYSC).
Data Buffer: 82C392 (DBC).
Peripheral Controller: 82C206 (IPC).
CPU Support: Intel 386DX (supporting 25MHz, 33MHz, and up to 40MHz).
Cache: Advanced Copy-Back (Write-Back) Cache Controller with support for 32KB, 64KB, 128KB, and 256KB of SRAM.
Memory: Supports up to 64MB of high-speed page-mode DRAM.
Features: Included hardware support for burst-line-fills, shadow RAM, and "hidden refresh" (which allowed the system to perform memory refresh cycles without stopping the CPU, improving overall throughput).
The "Write-Back" Advantage: The most notable feature of the 82C391 was its internal "Dirty Bit" logic. In write-back mode, the cache only updates the main DRAM when a line is replaced, rather than on every write operation. This significantly reduced bus traffic on the 386's memory bus, allowing these systems to outperform basic 386 boards by a noticeable margin.
Chipset Confusion: Be aware that the 82C391 is often visually mistaken for the 82C491. While they share the "392/206" companion chips, the 82C391 is specifically for 386 designs, whereas the 82C491 was the dedicated version for the 486.