[rwmj]

Hi, virt engineer here. Partly because it a very hard problem (in fact, theoretically impossible if you include timing attacks), but mainly because you don't need to emulate the hardware very accurately in order to get common operating systems to run. Getting them to run is all that we're paid to do, and that's a difficult enough job already.

One strange aspect of this is that only a narrow range of current OSes run under virtualization. Qemu is great for running, say, current versions of Linux or Windows, but absolutely terrible if you try to run Linux 1.0 or Windows 95 or Solaris/x86 or any uncommon OS. (I tried a few of these several years ago out of curiosity, and none of them would even boot.) The reason is that we don't emulate enough of the corner cases in CPUs and devices to run those operating systems. eg. The SATA device only emulates the commands issued by drivers of modern operating systems, not every single command and dark corner of the real hardware.

To be fair there are emulators that try much harder to be cycle accurate, especially the ones designed to run old games. The MisTER is the current king here, but that uses an expensive FPGA and can just about emulate a 486 PC.

[swayvil]

So step 1 is to emulate the world within which the emulated machine exists?

[anthk]

That's bullshit because Qemu it's an emulator too, so it will run Solaris and W95 perfectly.

I am not a virt engineer but I could run W95, OS2 and heck, even Mac OS 9 under Qemu, recently.

Seriously, if you are a virt engineer, drop your title down :).

Qemu has an ISA pc module, and you need to disable kvm just to be sure. Set the CPU to Pentium and everything will be fine.

[rwmj]

You might want to experiment yourself before making bold assertions, because you are wrong. I've just tried these (with qemu-system-x86-6.0.0-7.fc35.x86_64):

• Microsoft_Windows_NT_Server_Version_4.0_227-075-385_CD-KEY_419-1343253_1996.iso (1996, own copy)

Installer starts, locks up with screen corruption about 5 seconds in.

• https://archive.org/details/windows-95_fixcpu_iso_windows_is... (1994-ish)

Cannot read the emulated CD-ROM.

• https://archive.org/details/redhat-9.0_release (2003)

Installer boots, but fails at partitioning stage, the first time it accesses the disk.

• https://archive.org/details/IBMOS2Warp4Collection (1996)

Cannot read the emulated CD-ROM.

• Plan 9, 4th ed. (2003, own copy)

Gets quite far, up to the login, although with a lot of errors, but later hangs hard. (Out of all of them this looks closest to being possible to make work.)

I can also tell you that we're moving away from emulating i440fx entirely (to q35), and nothing prior to 2005 will work once that change has been made. In addition, changes to how virtio works means that guests before about 2010 that use virtio will have problems unless you take special steps.

[anthk]

Ah, sure?

1) Don't use Qemu from the kvm binary.

2) Don't use VirtIO

3) Don't set the CPU higher than a Pentium for w95/w98/NT4, Pentium2 may be fine for w98SE.

   >qemu-system-i386  -M help 
   isapc                ISA-only PC

   >qemu-system-i386  --version
   QEMU emulator version 6.1.0
   Copyright (c) 2003-2021 Fabrice Bellard and the QEMU Project developers

This should work for NT4

   qemu-system-i386 -cdrom $CDROM -m 32 -vga cirrus -net nic,model=pcnet -net user -cpu pentium -hda $DISK

Also, if qemu enables kvm by default, set the machine acceleration method to TCG.

Bye, "virt engineer".

[fomine3]

What's your parameter for Qemu?

Could you provide link to discussion that i440fx going to be removed?

[anthk]

Nah, he is a "virt engineer", ofc he/she doesn't have a clue on Qemu without KVM ;).

JK. Old stuff it's difficult to emulate if you only know qemu from KVM and you didn't use Qemu since the Bellard days, or Bochs.

Meanwhile, I emulated w95, w98, Linux 2.2/2.4 based distros (my first Linux), OS/2 and so on just fine. Even BeOS.

[lokedhs]

I installed NT 3.51 in Qemu not long ago. I can look up the Qemu settings I used if it's of interest.

[xattt]

How does software-based x86 emulation (ie OG Connectix Virtual PC) compare to current hardware-assisted virtualization? Were older methods more cycle accurate than what’s in use now?

[emteycz]

You reminded me of my father showing up home one time (around 2005, I was 7-8) proudly showing a random CD. Then after few hours he called to show off a virtual Windows 98 PC running in a window on our Windows XP computer. I was fascinated, total awe for a few minutes. Virtual PC became the basis for my experimentation with Windows Server 2003 and newer + Windows clients (even multiple networked PCs ran nicely!), later Linux servers inside Virtualbox, and led to my career in software engineering.

Anyways to answer your question, Virtual PC and VirtualBox can fully run old as well as new software, and the performance hit is not that bad (I ran multiple virtualized Windows Servers when a PC had 1GB of RAM). However more modern virtualization methods can offer bare metal-like performance, which Virtual PC/Virtualbox will never be able to make.

[xattt]

Thank you for your answer but also thank you for making me feel like I’m an old man.

I was in my last year of high school around the time you mentioned when I was experimenting with running Windows NT with a copy of Virtual PC.