Only a very small part of the CPU actually understands the "x86_64 language" (the "ISA", Instruction Set Architecture). Most of the CPU executes a completely different, much simpler language, where instructions are called "micro-operations" (or µops). There's a hardware component called the front-end which is responsible for parsing the x86_64 instructions and emitting these µops. One x86_64 instruction often produces multiple µops.

You can change the mapping from x86_64 instruction to sequence of micro-operations during boot on modern CPUs. That's what we mean by updating the microcode.

At least that's my understanding, as someone who has implemented a few toy CPUs in digital logic simulation tools and has consumed a bunch of material on the topic as hobbyist but have no actual knowledge of the particulars of how AMD and Intel does stuff.

that makes a lot more sense now, thanks!

So the decoder is like an emulator. If so it would theoretically be possible to provide a different ISA and have it execute by the cops as well. Not saying it would be fast, or practically possible due to how locked down it is.

Transmeta tried that approach, but it wasn't enough of a competitive edge for it to prosper.

Transmeta couldn't bring their product fast enough to the market because Intel was suing them. It had nothing to do with the quality of the product itself.

Then NVidia re-used their design on ARM as Project Denver.

Is this kinda what thumb mode on (some) ARM chips is?

Micro-ops aren’t simpler than the AMD64 instructions, the complexity is about the same. For instance, the following instruction

    vfmadd231ps     ymm3, ymm1, YMMWORD PTR [rax-256]

does quite a few things (memory load, and 8-wide fused multiply+accumulate), yet it decodes into a single micro-op.

Most AMD64 instructions decode into a single micro-op. Moreover, there’s a thing called “macro-ops fusion”, when two AMD64 instructions are fused into a single micro-op. For example, scalar comparison + conditional jump instructions are typically fused when decoding.

That's an important detail, not all macro-ops are more complex than micro-ops, and most of our everyday x86 instructions are simpler than the more complex micro-ops.

But we can agree that the complexity ceiling is much higher on macro-ops than micro-ops, right? The µop you mentioned does one (vector) FMA operation on two (vector) registers and stores the result to RAM. While in x86, we have things like the rep instruction which repeats an action until ECX is zero, or the ENTER and LEAVE instructions to set up and tear down a stack frame. Those are undoubtedly implemented in terms of lots of micro-ops.

> complexity ceiling is much higher on macro-ops than micro-ops, right?

Other examples are crc32, sha1rnds4, aesdec, aeskeygenassist - the math they do is rather complicated, yet on modern CPUs they are single micro-op each.

> one (vector) FMA operation on two (vector) registers and stores the result to RAM.

It loads from there.

> Those are undoubtedly implemented in terms of lots of micro-ops.

Indeed, but I don't think it's complexity. I think they use microcode for 2 things: instructions which load or store more than 1 value (a value is up to 32 bytes on AVX, 64 bytes on AVX512 processors), or rarely used instructions.

The CPU only pretends to be a CPU. In reality, it is a small datacenter comprised of several small special purpose computers doing all the work. I gave up understanding CPUs in depth by the time I read an introducion to Intel's then-new i860 CPU in the April issue of a magazine and it turned out to be a real device.

Best explanation in my opinion.

Almost like a CPU has an JIT emulator for x86.

ucode cracking is relatively straightforward, I wouldn't call it a JIT. https://intelxed.github.io/ref-manual/

It's more like an intepreter for x86, seeing as it is actually executed on a dataflow architecture.

Macro op fusion is more what I had in mind when I called it JIT and not interpretation.

Not sure what the distinction is but that's where it is to my mind

It's true that the distinction is a bit vague, the term JIT is overloaded to enough that it stopped being a useful technical term.

Compared to 'JVM JIT' or 'LuaJIT': there is no instrumentation to detect what is hot or not. The CPU frontend will crack x86 instructions into micro-ops, while looking for some patterns to merge certain instructions or uops into larger micro-ops. The micro-coded instructions (like many of the legacy instructions) are likely just lookups.

Most of this is my speculation, mind. Modern CPU frontends are still kind of a black magic box to me, but I think they are limited by relatively simple transformations by virtue of being on the critical execution path.

There is a Micro-op cache in play in many cases like Zen [0], but I'm not entirely sure what it does.

[0] - https://chipsandcheese.com/2022/11/05/amds-zen-4-part-1-fron...

Micro-op cache is literally L1 cache after the instructions are broken into micro-ops.

It's not a magic arc profiling optimization either.

Heck, at this point I think phone chargers run a "real" OS to manage USB-C charging.

Slightly off-topic, but I highly recommend Inside The Machine by Jon Stokes if you'd like to understand a bit more about how CPUs work... it's an extremely accessible book (I also knew next to nothing about the hardware world)

It's nothing new. Most of us might think one instruction will only do one thing but it's actully much more complicated than that. Instructions can actually be broken down to multiple steps and some of those are in common. And thus modern CPUs have a concept of μOp which refers to aforementioned step. What and especially how an instruction does things can be updated by uploading a new firmware to the CPU.

I found these Ben Eater YouTube videos very useful to understanding what microcode is and how it works:

8-bit CPU control logic https://youtu.be/dXdoim96v5A

8-bit CPU reprogramming microcode https://youtu.be/JUVt_KYAp-I

In short, the microcode instructions are a bunch of flags that enable different parts of the processor during that clock cycle (eg is data being loaded off the bus into a register? Is the adder active? Etc), so to implement an instruction that says add value from memory a to value from memory b and store in memory c, the microcode might be: copy memory a onto bus, store bus to register, copy memory b to bus, store b to register, add both register and put result online bus, store value on bus to memory c. (In a hypothetical simple cpu like the one Ben built, a real one is obviously much more sophisticated). So in Ben’s toy CPU, the instructions are just indices nto an EEPROM that stored the control logic but pattern “microcode” for each instruction, and IIRC each instruction takes however many cycles the longest instruction requires (in real life that would be optimised of course).

This is also how some processors like the 6502 have “undocumented” instructions: they’re just bit patterns enabling parts of the processor that weren’t planned or intended.

So you can see that it may be possible to fix a bug in instructions by changing the control logic in this way, even though the actual units being controlled are hard wired. I guess it very much depends on what the bug is. Of course I only know how Ben’s microcode works and not how an advanced processor like the one in question does it, but I imagine the general theme is similar.

> I find it astounding that it's possible to fix the behaviour a CPU instruction by changing code.

Sometimes, CPU vendors run out of space for such bug fixes. They have to re-introduce another bug to free up space to fix a more serious one. That one kinda blew my mind.

Do you have any source? I want to read more about this.

The chip has quasi-compiler that compiles stream of assembler instructions into uops and dispatches it onto various parts to run, often in parallel.

That part is driven via microcode (kind of like firmware) and fixes in that can fix some CPU bugs.

Which also means that one core can essentially run multiple assembler instructions in parallel (say fetch memory at same time floating point operation is running, at the same time some other integer operation is running etc.) and it just makes it look like it was done serially.

This particular instruction - XSAVES - isn't the sort of simple building block that most user code is full of like ADD or MOV. It does quite a bit of work (saving a chunk of the CPU state) and is implemented more like calling a subroutine within the CPU than the way the normal number-crunching instructions are executed. These updates basically just change that subroutine code within the CPU.

The instructions that you the user see are in themselves little sequences of code. Think about it this way - you like code reuse, right? DRY? If you want a bit of hardware that can add two numbers in registers, why would you want to have another copy of the same thing that can add a value to the program counter? It's just a register, even if it's a bit special.

The thing is, the microcode is often using instructions that are a very different "shape" from sensible machine-code instructions, because quite often they have to drive gates within the chip directly and not all combinations might make sense. So you might have an instruction that breaks down as "load register A into the ALU A port, load register X in the the ALU B port, carry out an ADD and be ready to latch the result into X but don't actually latch it for another clock cycle in case we're waiting for carry to stabilise", much of which you simply don't want to care about. The instructions might be many many bits long, with a lot of those bits "irrelevant" for a particular task.

The 6502 CPU was a directly-wired CPU where everything was decoded from the current opcode. It doesn't really have "microcode" but it does have a state machine that'll carry out instructions in phases across a few clocks. It does actually have a lot of "undefined" instructions, which are where the opcode decodes into something nonsensical like "load X and Y at the same time into the ALU" which returns something unpredictable.

CPUs internally are made up of various components connected to various busses.

Take a simple example: the registers are made up of latches that hold onto values and have a set of transistors that switch their latches to connect to the BUS lines or disconnect from them, along with a line that makes them emit their latched value or take a new value to latch. This forms a simple read/write primitive.

If the microcode wants to move the result of an ADD out of the ALU into register R1 then it will assert the relevant control lines:

  1. The ALU's hidden SUM register WRITE high which connects the output of its latches to the lines of the bus. For a 64-bit chip there would be 64 lines, one per bit. Each bit line will then go high or low to match the contents of SUM.

  2. It will also set R1's READ line high, meaning the transistors that connect R1's bit latch inputs to the bus lines will switch ON, allowing the voltages on each bus line to force R1's latch input lines high or low (for 1 or 0). 

In a real modern CPU things are vastly more complex than this but it is just nicer abstractions built on top of these kinds of simple ideas. Microcode doesn't actually control the cache with control lines, it issues higher level instructions to the cache unit that takes responsibility. The cache unit itself may have a microcode engine that itself delegates operations to even simpler functional units until you eventually get to something that is managing control lines to connect/disconnect/trigger things. Much like software higher level components offer their "API" and internally break operations down into multiple simpler steps until you get to the lowest layer doing the actual work.