Fedora opens up to bundling

Posted Oct 26, 2015 20:53 UTC (Mon) by javispedro (guest, #83660)
In reply to: Fedora opens up to bundling by Cyberax
Parent article: Fedora opens up to bundling

> But it's hard to blame MS about this one - there's no 16-bit support in 64-bit mode.

Nitpick: actually that's wrong. 64-bit processors certainly support 16-bit mode, otherwise they wouldn't be proper supersets of 32-bit processors. MS not supporting 16-bit is just a cost-benefit trade-off. In fact, for similar reasons Linux does not support vm86 on amd64, but there's a patch floating around for it, and it works quite well.

Fedora opens up to bundling

Posted Oct 26, 2015 21:00 UTC (Mon) by mjg59 (subscriber, #23239) [Link] (4 responses)

> actually that's wrong

No it's not. 64-bit mode has no support for vm86 mode, and so you can't run 16-bit code. You can certainly switch back to 32-bit mode and use vm86, which is presumably what the patch you're talking about does - but that's a pretty awful hack.

Fedora opens up to bundling

Posted Oct 27, 2015 2:27 UTC (Tue) by deater (subscriber, #11746) [Link] (2 responses)

You can certainly run 16-bit x86 *code* in 64-bit mode although the stealing of opcodes for REX prefixes as well as various Linux security features certainly make it difficult. See https://github.com/deater/ll_asm/blob/master/linux16/16.s for one example.

Whether you can run unmodified code written for x86 real mode operating systems is a different story.

I know this is a bit of a nitpick, but there's a lot of confusion out there about this.

Fedora opens up to bundling

Posted Oct 27, 2015 2:33 UTC (Tue) by mjg59 (subscriber, #23239) [Link]

That… is horrifying.

Fedora opens up to bundling

Posted Oct 27, 2015 3:08 UTC (Tue) by mathstuf (subscriber, #69389) [Link]

Oh man, I can't wait for there to be one for Mill :D .

Fedora opens up to bundling

Posted Oct 27, 2015 6:41 UTC (Tue) by javispedro (guest, #83660) [Link]

> You can certainly switch back to 32-bit mode and use vm86

So why you keep saying you can't if it turns out you can? It was even documented on the original AMD64 manual albeit they did certainly mention it was tricky.

I've been always curious about this, since I've been doing exactly that for at least a decade, it's a ~4k patch in 3.x, performance is still better than qemu or vt-x, and I guess no one wants to talk about the security/races of vm86 in general.