A look at the die of the 8086 processor (2020)

The 8086's microcode has been disassembled (and is mostly documented) by the way [1]

1. https://www.reenigne.org/blog/8086-microcode-disassembled/

So where's the line drawn between instruction decoding, and microcoding? It sounds like microcoding is when a single instruction in the program resolves to a _sequence_ of operations that the processor executes?

The 8008, Intel's first 8-bit processor, direct ancestor to the 8080 and indirectly to the Z80 and 8086, also came in a far-too-small 18-pin package as well.

In hindsight it seems rather inscrutable. Why were they hobbling their product like that? Well it seems just the arbitrary preference of one executive. But the seeming absurdity of it is partially an artifact of later thinking, from the era of single-chip microcontrollers. The 4004 and 8008 both needed a large amount of dedicated support electronics. Not just bus multiplexers, but clock drivers, special memory interfaces, etc. You couldn't turn an 8008 on without at least half a dozen support chips which were as much an integral part of the "computer" to the designers as the 8008 itself was. And while multiplexing brutally hindered the chip's performance, oddly enough pure performance doesn't seem to have been a major design goal for either the 4004 or 8008. Basically anything would be fast enough -- even multiplexed at 0.5 MHz -- for the industrial control and calculator-like applications envisioned.

For details on how Intel was fixated on 16-pin chips, see the Oral History of Federico Faggin [1]. He describes how 16-pin packages were a completely silly requirement, but the "God-given 16 pins" was like a religion at Intel. He hated this requirement because it was throwing away performance. When Intel was forced to 18 pins by the 1103 memory chip, it "was like the sky had dropped from heaven" and he had "never seen so many long faces at Intel."

[1] pages 55-56 of http://archive.computerhistory.org/resources/text/Oral_Histo...

I think it would have been impossible otherwise. It's still annoying with the multiplexed address and data pins in the DIP-40: you have to demultiplex them with latches and transcievers. And don't forget the 8088 (used in the PC) couldn't do data in one clock due to the 8-bit data bus (386SX was similar in the next step up.) Pretty cool for us that the 8086/8 are breadboardable though.

ARM code is a fair bit bigger than 16 bit 8086 code, so there is that to consider. It makes total cost of ownership much higher when we're talking about systems with RAM measured in hundreds of kilobytes or single digit megabytes.

But basically what you noticed is exactly why RISC swept everything else away in the late 80s to early 90s. If you have a 50,000 or 100,000 transistor budget and RAM is relatively cheap and fast, then complex microcoded designs really are a bad idea. You can get so, so much more performance out of a design like MIPS or ARM rather than an 80186, etc.

Hindsight is 20/20. If you sent me back to 1976 - 77 in a time machine, I would propose not something like the 68000 or 8086, but something very much like MIPS, or Berkeley RISC (minus the register windows). It could have been done in the late 70s. It would have been easily 3 - 5x as fast per MHz, and could probably be clocked faster. Doing so just wasn't obvious until later. Everyone was trying to pack as much sophistication into the instruction set and architecture, to ease assembly language programming, as possible.

> 29000 transistors? But it's the same as an ARM2 which apparently was full 32bit and had an integer multiplier.

It's a very good point. I think it's worth to ask a few questions in return.

How many years separated the two designs?

Does ARM2 support an equivalent ISA, in terms of features (not encoding)?

For instance, the 8086 has support for BCD integers, specialized instructions for loops, the ability to use its registers as 16 bits or 8bits (doubling the register count in the later case).

Yes, for the most part you can reconstruct the circuits from the photos. (Which I'm doing.) A key thing that doesn't show up is which transistors are enhancement and which are depletion; this depends on the doping. It's straightforward to figure this out from the layout, though. Except in the Z80 processor: the designers famously used some depletion transistors in unexpected places as "traps", so anyone who tried to clone the chip visually would encounter subtle failures. I'm pretty sure that 8086 didn't do that, though.

The other aspect of your question is there's a lot of implicit manufacturing knowledge that you'd need if you tried to reconstruct the chip. Things like the resistance of the lines and the characteristics of the transistors that you'd need to get right for the chip to work reliably.

The Computer History Museum has some oral histories from folks who worked in this era. They don't have the 8086 design team, but they have different panels featuring people from the 8080, Z80, and 680x0 design teams. (Some from an 8086 marketing team too)

searching for "8086 design team" seems to get good results, including the Wikipedia article for the processor which lists names.