"In 1951, Maurice Wilkes came up with the idea of microcode: instead of building the control circuitry from complex logic gates, the control logic could be replaced with another layer of code (i. e. microcode) stored in a special memory called a control store. To execute a machine instruction, the computer internally executes several simpler micro-instructions, specified by the microcode. In other words, microcode forms another layer between the machine instructions and the hardware."
Note the "1951".
What microcode really does is make it possible to share hardware resources inside the execution of the same instruction. Hardware is really (really) expensive, and a naive implementation of a CPU needs a lot of it that you can't really afford.
Take adders. You need adders everywhere in a CPU. You need to compute the result of an ADD instruction, sure, but you also need to compute the destination of a relative branch, or for that matter just compute the address of the next instruction to fetch. You might have segment registers like the 8086 that require an addition be performed implicitly. You might have a stack engine for interrupts that needs to pop stuff off during return. That's a lot of adders!
But wait, how about if you put just one adder on the chip, and share it serially by having each instruction run a little state machine program in "micro" code! Now you can have a cheap CPU, at the cost of an extra ROM. ROMs are, technically, a lot of transistors; but they're dense and cheap (both in dollars for discrete logic designs and in chip area for VLSI CPUs).
That is, microcode started as a size optimization for severely constrained devices. It was only much later that it was used to implement features that couldn't be pipelines. Only the last bit was a mistake.
(As an aside, the 8086 has a separate adder for address computations, independent of the ALU. You can see this in the upper left of the die photo.)
Only with some other kind of state machine, though. I was maybe being a little loose with my definition for "microcode" vs. other state paradigms.
Maybe the converse point makes more sense: "RISC", as a design philosophy, only makes sense once there's enough hardware on the chip to execute and retire every piece of an entire instruction in one cycle (not all in the same cycle, of course, and there were always instructions that broke the rules and required a stall). Lots and lots of successful CPUs (including the 8086!) were shipped without this property.
My point was just that that having 50k+ transistor budgets was a comparatively late innovation and that given the constraints of the time, microcode made a ton of sense. It's not a mistake even if it seems like it in hindsight.
I believe every other pdp-11 was microcoded.
ALSO: these days, assy lang makes it easy for compiler writers to express programs written in a high-level language!
It's software, all the way down..... almost.
Some small minicomputers like the PDP-8 in the 60s/70s were hardwired logic; they were so simple that microcode wasn't necessary to cheaply implement them. Along the same lines, some early microprocessors like the 6502 also weren't microcoded (though they still tended to use large ROM lookup tables driven by a custom state machine so sort of half-microcoded).
Some very large early supercomputers were also hardwired; the CDC 6000 series, Cray-1, or IBM's largest System/370 models, for example. Microcode was avoided to minimize the logic delay during a single cycle and get things running as fast as possible. The same philosophy would resurface in the 1980s with RISC. But then RISC chips started adding microcode back in to handle complex multi-step instructions, etc.
Nearly everything else has been microcoded: everything from 8-bit microcontrollers to the Motorola 68K to the VAX, later RISC designs like POWER and ARM, every iteration of x86, etc.
The same thing happened with microprocessors. Early microprocessors didn't have enough space for microcode so they all had hard-wired control logic. It wasn't until the 8086 generation that you could put enough transistors on a chip to make microcode practical in a microprocessor. The 8087 co-processor is an interesting example. In order to fit the microcode onto the 8087 chip, Intel had to use weird semi-analog multi-sized transistors. This let them store 2 bits per transistor in the microcode ROJM.
Even today, I would be shocked if ARM designs weren't partially microcoded as well. Something like the ERET instruction is begging to be.
In short the ARFM has decode logic that makes a bunch of clocks worth of ALU controls directly, while the x86 has decode logic that makes an index into a ROM from where that information is read/generated
There’s two types of microcode: vertical (the kind people think of when hearing “microcode”) and horizontal (a large PLA). So, the 6502 is not “sort of” microcoded, it is microcoded, just horizontal microcode.
A better example of horizontal microcode (and what I learned on) are the PDP-11s. A clear micro instruction address counter and the ability to jump around the micro instruction ROM, but very wide control words where it's clear 'these three signals go into this mux, this signal is an enable over here, etc'.
[*] For standard definitions of microcode.
You could use a PLA to produce exactly the same outputs from the same inputs, but it wouldn't be microcode. The PLA is "just" an optimization. But with a PLA, you can't change a micro-instruction without ripping out some of the logic and replacing it. You have a hardware implementation rather than a software implementation, which is the hallmark of microcode. In other words, changing a control signal value in the 8086's microcode is trivial, but changing a control signal in the 6502 is considerably harder.
As a concrete example, the design of the Apollo Guidance Computer is clearly microcoded, with specific microinstructions. However, the implementation is gate logic that completely obscures the microcode design. (Not a ROM implemented with NOR gates, but highly-optimized logic.) So I consider the implementation of the AGC to not be microcode.
(This is kind of splitting hairs, and I'm not really into subtle linguistic arguments.)