How IBM PC 8-bit went

If you were around when IBM PC was launched, two things probably surprised you. One is that the company that made the electric has put the ridiculous keyboard in it. The other was that it had an 8-bit CPU onboard. It was actually more unfamiliar than that. The PC sported an 8088 which was a 16-bit 8086 and got off an 8-bit external bus. You have to wonder for that reason, and [Steven Leibson] There’s a great post that explains what went down all those years ago.

Before IBM PCs, almost all personal computers had 8-bit and 16-bit address buses. While 64K may seem like enough for some, many realized that it was going to be a brick wall soon. So the answer was big address bus and address mode.

Intel knew this and was working on the flagship iAPX 432 This represents a radical departure from the 8080-Series CPUs designed from scratch for high-end languages ​​like Ada. However, the radical design took longer than expected. The project started in 1976 but will not see the light of day until 1981. It was clear that they needed something sooner, so the 8086 – a 16-bit processor that clearly evolved from the 8080.

There were other choices, too. The Motorola 68000 was a great design, but when IBM was choosing a processor it was expensive and not widely available. The TI had TMS9900 in production, but they bet CPU throughput was the key to success and stuck with the same old 16-bit address bus. That processor, too, had a fancy way of storing registers in main memory which was great when your CPU was slow, but because CPU speeds out exceeded memory speeds, it was a lost design decision.

This still leaves the question: why 8088 instead of 8086? Price. IBM’s goal was to pay less than $ 5 for a CPU and could not meet that price with the Intel 8086. Obviously, this was not a technical issue but a contractual one. However, folding the chip on an 8-bit external bus allowed a small dye, low cost, and freedom from contractual obligations that plagued the 8086. That last point was important, because the cost of production was not different, but the set price was all about the paperwork.

There is much more to read. Below the post, you will find links to oral history transcripts from the Computer History Museum. Impressive thing. If you survived it you probably wouldn’t know all these details and if you don’t it gives a good taste of how many likes there were in those days and how many designs you had to trade to get a product. Market

We borrow from the title graphic [Steven’s] Post and he, in turn, borrowed it from the well-known [Ken Shirriff].

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