x86

I am not entirely sure, this is an educated guess.

I'm guessing that the 86 means that there are 86 instructions the processor could use.

For instance, add, sub, mult, etc etc (assembly level instructions).

In order to maintain compatibility, a processor must support these 86 instructions. Newer processors have added instructions (MMX, SSE, etc) but they all support these original 86 instructions.

If a processor did not support any of the original 86 instructions, then it would not be backwards compatibile.

This is all a GUESS, please someone chime in if I'm wrong.
 
I'm guessing that the 86 means that there are 86 instructions the processor could use.
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It's not even that technical.

The "x86" refers to a family of processors starting with the 8086, a 16-bit version of the 8088.

The first two digits of a processors name generally tie it to a particular manufacturer or peripheral chips. For example, MOS Technologies' 6502 is a modified version of the 6500 (by the same manufacturer), which is a plug-in replacement for Motorola's 6800. The 6502 can use peripheral chips such as the 6522 VIA or the 6551 ACIA.

In the case of the 8086, the big peripheral chip was the 8087, a 16-bit floating point math coprocessor.

As it was part of the same family, the next chip in the line was the 80186, which just added a few features to the 8086. The "80" at the beginning designates which family of processors this chip belongs in, and what peripheral chips it can interface with. But, the rest of the name went up by 100. This can be compared to the 65C02, which is a 6502 with a different manufacturing process and some extra assembly language instructions.

Next, the number was incremented again to make the 80286, with its 80287 coprocessor. Then, the 80386 and 80387. The '386 is an improvement on the original '86 chip, and the family has come to be known as x86 to symbolize this.

Companies don't necessarily have to follow a numbering scheme like this, but there's usually some reason behind a particular number being assigned to an enhanced version of a particular chip, even if it's just the marketing department being cute. For example, why was the first chip in this family called 8086 in the first place? It's a 16-bit version of the 8088! The 8088 was almost identical to the 8080, except it had an extra 8 bits in its address bus. The 8080 was an enhancement of the 8008, which was little more than an 8-bit version of the 4004.
 
The "8" and the "6" came about because of the 4004. No, seriously! I know "4004" has neither an "8" nor a "6" in it, but trust me on this one.

At the time the 4004 came about, Intel primarily made memory chips, and they had a series of memory and general purpose chips consisting of the 4001, 4002, and 4003. Intel grabbed the next number in the series when they got a contract with a firm called Busicom for a general-purpose calculator chip. What Intel delivered came to be known as the Intel 4004 Single Chip 4-bit P-Channel Microprocessor. This chip became quite popular, and companies started finding uses for it besides making calculators.

Later on, a company called "Computer Terminal Corporation" wanted an 8-bit chip for making computer terminals. Well, since Intel's flagship product was a 4-bit microprocessor called the "4004", why not call their new 8-bit microprocessor the "8008".

The 8008 didn't do so well, though. So Intel made an improved version of the 8008 with some features that seem so elementary these days, like replacing the hardware stack with a stack pointer to a location in memory. A more powerful 8008? Let's call it the 8080! Isn't that clever?

The 8080 was followed up by a few variations...mostly different sizes for the address & data busses. This series included the unpopular 8085, the 8086 (with a 16-bit data bus), and the 8088 (a low-cost 8-bit chip). Actually, they were all picked as "catchy" names: 8085 is a 5V 8080, 8086 is a 16-bit 8080, 8088 is an enhanced 8-bit 8080.

The 4.77MHz 8086 was used in the first IBM PC, and the rest is history (see my previous post).
 
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