Computer Bus Architectures and Evolution

Key Insight

A computer comprises a processor, random access memory (RAM), input/output devices, and long-term storage. These integrated circuits are mounted on circuit boards, which communicate via a bus—a collection of digital signals provided to every board. These signals categorize into address signals (from microprocessor to RAM/devices), data output signals (from microprocessor to RAM/devices), data input signals (from RAM/devices to microprocessor), and control signals (miscellaneous signals from/to microprocessor, e.g., for writing data). The bus also supplies power. An early popular bus for home computers was the S-100, introduced in 1975 with the MITS Altair, featuring 100 connectors on 5.3 x 10 inch circuit boards, and originally designed for the 8080 microprocessor.

An 'open architecture' design, where bus specifications are public, allows third-party manufacturers to create compatible expansion boards, enhancing the computer's utility and market desirability. The original IBM PC, launched in 1981, exemplified this by publishing its Technical Reference manual, which enabled the creation of many expansion boards and 'clones,' leading to its descendants accounting for about 90 percent of the desktop market today. In contrast, the Apple Macintosh's initial closed architecture design may explain its less than 10 percent market share. The IBM PC used the ISA (Industry Standard Architecture) bus, which featured 62 connectors, 20 address signals, 8 combined data input/output signals, 6 interrupt requests, and 3 direct memory access (DMA) requests. DMA allows storage devices to bypass the microprocessor and directly read/write to memory, enhancing speed.

Busses require upgrades or replacement when microprocessors exceed their capabilities in data width (e.g., from 8 to 16 to 32 bits), address signal count, or speed. Early busses designed for megahertz speeds often caused radio frequency interference (RFI) at higher processor clock speeds. IBM's 1984 Personal Computer AT, with its 16-bit 80286 microprocessor, added a 36-connector socket to the existing bus for more signals. The proprietary Micro Channel Architecture (MCA) bus introduced by IBM in 1987, due to patenting and licensing fees, did not become an industry standard. Instead, a consortium introduced the 32-bit EISA (Extended Industry Standard Architecture) bus in 1988, and more recently, the Intel-designed Peripheral Component Interconnect (PCI) bus has become common in PC-compatibles.