Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [121]
If you were designing a new computer system that included a new type of bus, you could choose whether to publish (or otherwise make available) the specifications of the bus or to keep them secret.
If the specifications of a particular bus are made public, other manufacturers—so-called third-party manufacturers—can design and sell expansion boards that work with that bus. The availability of these additional expansion boards makes the computer more useful and hence more desirable. More sales of the computer create more of a market for more expansion boards. This phenomenon is the incentive for designers of most small computer systems that adhere to the principle of open architecture, which allows other manufacturers to create peripherals for the computer. Eventually, a bus might be considered an industry-wide standard. Standards have been an important part of the personal computer industry.
The most famous open architecture personal computer was the original IBM PC introduced in the fall of 1981. IBM published a Technical Reference manual for the PC that contained complete circuit diagrams of the entire computer, including all the expansion boards that IBM manufactured for it. This manual was an essential tool that enabled many manufacturers to make their own expansion boards for the PC and, in fact, to create entire clones of the PC—computers that were nearly identical to IBM's and ran all the same software.
The descendants of that original IBM PC now account for about 90 percent of the market in the desktop computers. Although IBM itself has only a small share of this market, it could very well be that IBM's share is larger than if the original PC had a closed architecture with a proprietary design. The Apple Macintosh was originally designed with a closed architecture, and despite occasional flirtations with open architecture, that original decision possibly explains why the Macintosh currently accounts for less than 10 percent of the desktop market. (Keep in mind that whether a computer system is designed under the principle of open architecture or closed architecture doesn't affect the ability of other companies to write software that runs on the computer. Only the manufacturers of certain video games have restricted other companies from writing software for their systems.)
The original IBM PC used the Intel 8088 microprocessor, which can address 1 megabyte of memory. Although internally the 8088 is a 16-bit microprocessor, externally it addresses memory in 8-bit chunks. The bus that IBM designed for the original PC is now called the ISA (Industry Standard Architecture) bus. The expansion boards have 62 connectors. The signals include 20 address signals, 8 combined data input and output signals, 6 interrupt requests, and 3 direct memory access (DMA) requests. DMA allows storage devices (which I'll describe toward the end of this chapter) to perform more quickly than would otherwise be possible. Normally, the microprocessor handles all reading from and writing to memory. But using DMA, another device can bypass the microprocessor by taking over the bus and reading from or writing to memory directly.
In an S-100 system, all components are mounted on expansion boards. In the IBM PC, the microprocessor, some support chips, and some RAM are located on what IBM called the system board but which is also often called a motherboard or a main board.
In 1984, IBM introduced the Personal Computer AT, which used the 16-bit Intel 80286 microprocessor that can