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Comment by pcwalton | Hacker News Reader

This may be an unpopular opinion, but making ancient x86 features unreliable strikes me as a good thing. Virtual 8086 mode is an awful virtualization solution, as you can't enter protected mode from within it. This makes it only good for running a subset of DOS apps: either real-mode-only apps or those written to an unspecified subset of DPMI [1]. Ryzen CPUs are easily fast enough to emulate them. And if that's not enough, x86 has had real virtualization for years now, with a functioning protected mode (and long mode!) in addition to real mode.

We're long overdue for a simplification of the PC platform. Having CPUs out there with broken legacy features accelerates the migration away from those features. Hopefully this leads to dropping them outright at some point.

[1]: https://en.wikipedia.org/wiki/DOS_Protected_Mode_Interface#H...

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VME is an optional feature indicated by a CPUID flag. AMD could have simple not supported the feature flag.

Perhaps they'll submit a microcode update which does that. (assuming they can do that with microcode, but can't fix the problem with it)

I think this is just a bug though. But yes, I have been thinking of creating a new version of x86 that drops things like segmentation, real mode, virtual 8086 mode, and 16-bit protected mode.

Right, I know it's not intentional. My point is that this bug has unintended benefits. :)

Keep in mind that if you drop the old stuff, you'll have to provide a bootloader and even then it'll be weird; modern systems still boot in 16 bits, bootstrap in 32 bit mode, then configure 64 bit. Unless UEFI fixed that madness?

Yes it did.

UEFI starts you off in protected mode or long mode with identity paging. It even lets you package up your loader as a PE file.

If by "modern" you mean "EFI" you are incorrect.

A traditional BIOS is hardly suitable for describing a "modern" firmware; for chrissake it doesn't even use 32 bits!

The processor itself still boots up in 16-bit mode. UEFI does switch into 64-bit mode before booting an OS loader, but that doesn't mean the processor itself starts in 64-bit mode.

UEFI can drop you directly in 64bit mode.

The problem is it uses a different memory map than 32bit mode so a lot of write your own os manuals are just flat wrong.

BIOS itself can be implemented as 32-bit code, which contains software emulator of 16-bit code. This emulator runs until CPU is switched to 32-bit protected mode by OS bootloader.

Coincidentally I've also thought about creating a "better x86", but in the opposite direction: on the basis that segmentation is extremely powerful and useful, I think it would've been better for all the various virtualisation extensions to be based on the same model that V86 and protected mode started: segment descriptors of different types. That would allow V86, 16, 32, and 64-bit (using "double-wide" descriptors, to allow a 64-bit base and limit to fit) protected mode "tasks" or VMs to coexist completely, and the OS to switch between them easily. There are enough reserved spaces in the existing specification to allow these to fit nicely (in fact, what I came up with fits so neatly that I wonder if Intel had originally envisioned them for this purpose, if AMD hadn't beaten them with AMD64.)

Instead, now we have partially-backwards-compatible (and even slightly different) AMD64/Intel64 and at least two non-interoperable virtualisation extensions (AMD-V, VT-x).

If you're going to create a wildly incompatible "better x86", just start with a RISC design and be done with it. Compatibility with modern x86 OS's is the reason for x86's existence (note that v8086 mode is not required for this). Drop that and you might as well just fix all the other problems with the ISA while you're at it.

I'd be interested to know how much ARM have an advantage here in terms of their ISA? Presumably if more resources were dedicated to it, ARM cpus could be quite a lot faster than x86?

I doubt it. For one, 32-bit ARM is pretty complex too (though 64-bit ARM is a very nice simplification). More importantly, though, you can overcome the x86 complexity by simply throwing enormous manpower at the problem, and that is what Intel has done over the years.

(That doesn't mean the complexity is justified, just that it can be overcome. I think that people often assume that x86's dominance means that there's something inherently amazing about x86 from a microarchitectural point of view. That isn't true; rather it's simply that x86/Windows has historically earned Intel so much revenue that they've been able to fund the manpower needed to keep it alive and on top.)

It's complicated. There's a fixed amount of design overhead in supporting all the various x86 modes but compared to designing a top-end processor those aren't too big. And being able to decode 4 x86 instruction at once is hard compared to doing that with ARM but compared to the size of a 192 entry reorder buffer for deep out of order execution the cost isn't huge. Both x86 and ARM want to convert their ISA operations to a different format for use internally. ARM's is closer to their ISA instructions but it's not clear that that makes a big difference. The biggest issue in practice at the high end might memory ordering constraints with x86 being very tight and ARM being very loose. But I can't actually say for sure.

But this is probably why Intel's efforts to extend x86 down to lower power processors haven't been huge successes.

This a thousand times. Also, its usually Intel's policy to have a (very) strong backwards compatiblity. See the A20 line for a ridiculous example. But AMD has always been less strict on this issues, and I'm glad they aren't. This is good news.

And despite all that, as you say, a new architecture is badly needed.

One of the big selling points of x86 is backwards compatibility. If you have some OS from 1990 you can still run it (without emulation or virtualization, so long as it doesn't depend on clock speed), which is pretty crazy.

Slight aside, there are a lot of reasons that Itanium failed, but certainly one of them was lack of backwards compatibility.

> Slight aside, there are a lot of reasons that Itanium failed, but certainly one of them was lack of backwards compatibility.

Itanium did not aim at the x86 market. The x86 translation layer was retrospectively seen as a mistake as well, because it wasn't relevant, but required transistors that limited the design's performance overall, which was relevant.

> Itanium did not aim at the x86 market.

Maybe not. But x86 certainly took over the market Itanium was aiming for.

x86 did not. x64 did.

All 16 and 32 bit x86 code is valid on x64. It's an extension, not a new ISA.

You got it all backwards. AMD64 is a complete revamp which happens to support x86 in its legacy mode.

Server people care for 64-bit address spaces, and that's a feature introduced by AMD64 which is not available in x86.

Itanium was extraordinarily backwards incompatible. There's an enormous gulf between "runs software from the 80s" (which is something the PC platform only pretends to do anyway, because peripherals now are incompatible) and "can't run Windows at all". Breaking v8086 mode wouldn't prevent modern Windows from working (which is in fact why this bug wasn't noticed). You can't even enter it from long mode to begin with!

actually my windows 10 installation didn't used uefi. (I had a really old machine). Basically I upgrade/upgraded to ryzen. I don't think it will be easy to migrate to UEFI straight.

This is also one of the main reasons why Mainframes still exist.

Backwards compatibility straight back to 1964 is a big deal, there's lots of 50+ year old code still in production at banks, insurers, and the like.

Pedantic point, but you couldn't really run an OS from 1990. It wouldn't support any modern peripheral buses required for normal operation.

But you certainly can natively run user-mode 16-bit DOS programs on a modern CPU.

I find it somewhat ironic (apropos?) that this article is on OS/2 Museum.

Is there any decent estimate as to how much silicon you could expect to save by dropping stuff that was long ago shunted to microcode?

It's not so much silicon as engineering and maintenance time keeping all that stuff working.

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