Is it possible to use a system with a modern, multi-core CPU with older operating systems that only support single core operation? Is this something that needs to be set in the BIOS? Does it depend on the CPU, the BIOS, the OS, or something else?

  • I think this has been closed as off-topic because the core question is about how to use modern CPUs with any retrocomputing as ancillary information.
    – wizzwizz4
    Nov 21, 2018 at 21:01
  • I agree this is off topic. And I apologize, I'm still trying to aclimate to the whole stack exchange model.
    – user11220
    Nov 21, 2018 at 21:08
  • Oh, no; it's fine! I was just posting a comment because the system complained about a close-without-comment. We need off-topic questions almost as much as on-topic questions, since that way we can figure out the site scope (although that's less important now, since we're pretty sure what to allow and what not to). It's all part of the learning process.
    – wizzwizz4
    Nov 21, 2018 at 21:13

2 Answers 2


The previous answer talks about various issues that may arise when attempting to run an older OS on a modern system, but doesn’t really focus on the questions asked.

When a multicore CPU powers on, a single core is activated. The BIOS typically has a configuration option to control how many additional cores should be activated. If this BIOS setting is set to a single core, then any OS that boots will see only that one core.

When the BIOS starts up the boot loader, followed by the OS itself, again that is done with only a single core active. The BIOS reports the number of cores available to the OS. If the OS supports multicore operation, the OS activates the additional cores. If not, the OS will proceed on the original single core. Operating systems such as Linux have a boot option to specify the number of cores to activate, so the OS can be limited to one core. This is useful for debugging.

On a processor with hyperthreading, every place I wrote “core” above can be replaced with “hardware thread”.


Preface: I assume the question to be deleted or moved soon, as this is not in any way RC related. After all, the issue in question is not the classic OS, but workings modern hard- and software.

Since every multi core CPU starts up in single core mode, it is basically possible to use any old, single core OS. As long as the CPU is compatible - which no longer always is true - but that's less due having multiple cores (or threads) but changes in basic CPU design.

In compatibility mode even a modern BIOS should be able to to initiate classic upstart procedures. With modern (UEFI) BIOS, you may have to activate CSM mode (Compatibility Support Module).

Past that hurdle it depends much on the OS to be used. Emulation on BIOS level for like Kbd/Mouse and basic Video will work fine. Even basic Disk may work (Size and filesystem may of course be a bigger issue).

As soon as direct hardware drivers are installed, the chances will reduce. While chipsets do emulate several classic ports one way or another, they are not really foolproof. In fact, they are usually only tested to boot some minimal DOS for BIOS updates, not so much for OS support in all possible configurations. So DOS 1.25 may work better than 5.0 with its GUI. Similar systems that rely on special, no longer supported processor modes - like 286 protected mode and alike got a high chance of failure. So I wouldn't really try for OS/2 - not even 1.2 without any GUI.

  • To add to that, compatibility with basic peripherals may be an issue. While the South Bridge (or equivalent) should present the older interfaces (PS2 keyboard and mouse, etc.) as fully-emulated devices sitting on an ISA bus one may still encounter problems. USB keyboards and mice, even in "backwards-compatible" mode might not function. Video presents its own set of problems; while pretty much any card will support basic VGA graphics, support for the physical interface (especially anything besides parallel PCI or ISA) may be lacking in older OSes. Nov 21, 2018 at 3:17
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    @AlexHajnal Well, yes. Then again it depends much on the OS in use. DOS got a good chance to succeed. Kbd/Mouse will work fine. Similar for basic Video. Even Disk may work great. Ofc, only as long as everything works via the ROM-BIOS. As soon as direct hardware drivers are installed, the chances will reduce. So DOS 1.25 may work better than 5.0 :)) Similar systems that rely on special, no longer supported processor modes - like 286 protected mode and alike.
    – Raffzahn
    Nov 21, 2018 at 3:32
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    @AlexHajnal Actually, most of these things are still 100% backwards compatible (in fact, most of the time it's hacked to work even on Windows - Doom still runs just fine on 32-bit Windows 10, for example). You can still start Turbo Pascal on Windows 10 and write old-school VGA DMA graphics code, because both VGA and Windows have a great track record for compatibility. Sound is another matter - it wasn't standardized at all until DirectX.
    – Luaan
    Nov 21, 2018 at 8:20
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    @AlexHajnal I agree with Luaan. Current graphics support on systems with a CSM is still register-compatible with VGA. DOS itself hardly has any hardware-specific code, so running DOS 1 (without folders!) or DOS 5 or 6 won’t make any difference. As Luaan says, sound is another matter; even PCI-hosted sound hardware is usually not Adlib or SB-register-compatible without a TSR. Nov 21, 2018 at 8:34
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    Probably worth pointing out that modern video cards often do an awful job of emulating CGA and EGA as well. vogons.org/viewtopic.php?f=63&t=61223
    – mnem
    Nov 21, 2018 at 15:47