How The IBM PC Went 8-Bit

If you were being all around when the IBM Computer rolled out, two issues probably caught you by shock. Just one is that the enterprise that made the Selectric put that ridiculous keyboard on it. The other was that it experienced an 8-little bit CPU onboard.  It was essentially even stranger than that. The Personal computer sported an 8088 which was a 16-bit 8086 stripped down to an 8 bit external bus. You have to speculate what brought on that, and [Steven Leibson] has a excellent post that explains what went down all individuals several years in the past.

In advance of the IBM Laptop, just about all private computers had been 8-bit and experienced 16-little bit tackle buses. Though 64K may have seemed plenty of for any individual, numerous understood that was heading to be a brick wall reasonably before long. So the respond to was larger deal with buses and addressing modes.

Intel knew this and was doing work on the flagship iAPX 432. This was heading to depict a radical departure from the 8080-sequence CPUs built from the commence for substantial-degree languages like Ada. Having said that, the radical style and design took lengthier than expected. The project commenced in 1976 but wouldn’t see the mild of working day until eventually 1981. It was crystal clear they needed some thing quicker, so the 8086 — a 16-bit processor evidently derived from the 8080 was born.

There had been other options, much too. The Motorola 68000 was a great design and style, but it was pricey and not extensively offered when IBM was deciding on a processor. TI had the TMS9900 in creation, but they had bet on CPU throughput becoming the essential to achievement and trapped with the similar previous 16-bit tackle bus. That processor, way too, experienced a novel way of storing registers in key memory which was good if your CPU was sluggish, but as CPU speeds outpaced memory speeds, that was a dropping design and style conclusion.

That nevertheless leaves the dilemma: why the 8088 in its place of the 8086? Value. IBM experienced a aim to fork out below $5 for a CPU and Intel couldn’t meet that value with the 8086. Apparently, it was not a complex problem but a contractual a single. However, folding the chip to an 8-bit external bus authorized for a smaller die, reduce value, and independence from contractual obligations that plagued the 8086. That last point was significant, as the producing value was not that various, but placing the cost was all about paperwork.

There is a good deal extra to read through. At the bottom of the publish, you will obtain hyperlinks to the resource oral historical past transcripts from the Computer Record Museum. Fascinating stuff. If you lived nevertheless it, you probably didn’t know all these specifics and if you didn’t, it presents a great flavor of how many possibilities there have been in people days and how numerous style and design trades you had to make to get a products into the marketplace.

We borrowed the title graphic from [Steven’s] write-up and he, in flip, borrowed it from the well-known [Ken Shirriff].

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