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owl [soylentnews.org] writes:

The Intel 8086 microprocessor (1978) started the x86 architecture that continues to this day. In this blog post, I'm focusing on a small part of the chip: the address and data pins that connect the chip to external memory and I/O devices. In many processors, this circuitry is straightforward, but it is complicated in the 8086 for two reasons. First, Intel decided to package the 8086 as a 40-pin DIP, which didn't provide enough pins for all the functionality. Instead, the 8086 multiplexes address, data, and status. In other words, a pin can have multiple roles, providing an address bit at one time and a data bit at another time.

The second complication is that the 8086 has a 20-bit address space (due to its infamous segment registers), while the data bus is 16 bits wide. As will be seen, the "extra" four address bits have more impact than you might expect. To summarize, 16 pins, called AD0-AD15, provide 16 bits of address and data. The four remaining address pins (A16-A19) are multiplexed for use as status pins, providing information about what the processor is doing for use by other parts of the system. You might expect that the 8086 would thus have two types of pin circuits, but it turns out that there are four distinct circuits, which I will discuss below.

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