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Tracing the roots of the 8086 instruction set to the Datapoint 2200 minicomputer

The Intel 8086 processor started the x86 architecture that is still extensively used today. The 8086 has some quirky characteristics: it is little-endian, has a parity flag, and uses explicit I/O instructions instead of just memory-mapped I/O. It has four 16-bit registers that can be split into 8-bit registers, but only one that can be used for memory indexing. Surprisingly, the reason for these characteristics and more is compatibility with a computer dating back before the creation of the microprocessor: the Datapoint 2200, a minicomputer with a processor built out of TTL chips. In this blog post, I’ll look in detail at how the Datapoint 2200 led to the architecture of Intel’s modern processors, step by step through the 8008, 8080, and 8086 processors.

The Datapoint 2200
In the late 1960s, 80-column IBM punch cards were the primary way of entering data into computers, although CRT terminals were growing in popularity. The Datapoint 2200 was designed as a low-cost terminal that could replace a keypunch, with a squat CRT display the size of a punch card. By putting some processing power into the Datapoint 2200, it could perform data validation and other tasks, making data entry more efficient. Even though the Datapoint 2200 was typically used as an intelligent terminal, it was really a desktop minicomputer with a “unique combination of powerful computer, display, and dual cassette drives.” Although now mostly forgotten, the Datapoint 2200 was the origin of the 8-bit microprocessor, as I’ll explain below.

The memory storage of the Datapoint 2200 had a large impact on its architecture and thus the architecture of today’s computers. In the 1960s and early 1970s, magnetic core memory was the dominant form of computer storage. It consisted of tiny ferrite rings, threaded into grids, with each ring storing one bit. Magnetic core storage was bulky and relatively expensive, though. Semiconductor RAM was new and very expensive; Intel’s first product in 1969 was a RAM chip called the 3101, which held just 64 bits and cost $99.50. To minimize storage costs, the Datapoint 2200 used an alternative: MOS shift-register memory. The Intel 1405 shift-register memory chip provided much more storage than RAM chips at a much lower cost (512 bits for $13.30).”

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