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What does a "Legacy USB Mouse" option in a BIOS mean?

Yes legacy usually means older revision, or obsolete. So legacy usb mouse means support for "old style usb mouses".

That's not really answering the question. As Mr. Tamm, my high school electricity teacher, liked to say, "Yes, and oranges taste orangey."

What does legacy USB mouse support mean?

  • What is a legacy USB mouse?
  • There's an "old style" USB mouse?
  • What changed between what version of usb and what version of usb that made mice incompatible?
  • Why was the change made?
  • When was the change made?
  • Who made the change?
  • What was the virtue of the "new USB mouses" over the "old USB mouses"?

Put it another way:

What is the BIOS doing when "Legacy USB Mouse" option is enabled?
What is the BIOS doing when "Legacy USB Mouse" option is disabled?

See also

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3 Answers 3

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As I understand it, it's not the Mouse or Keyboard that's legacy (PS/2 hasn't changed, USB is backwards compatible with other USB versions), it's the Operating System - legacy mouse mode emulates a PS/2 or AT device when a USB mouse is used with a OS that doesn't support it.

Not the best source, but this seems to confirm that, as does the last post here.

From Platform Compatibility for USB Boot Devices on the Windows Hardware Development Center on MSDN:

Architecture of Legacy Support with Both USB and PS/2 Support

Legacy support is shown in Figure 1 for both USB and PS/2-compatibile support. Important features include:

  • The BIOS traps events from the USB keyboard and mouse and presents them to the system as PS/2-compatible devices.

  • The legacy operating system recognizes the USB keyboard and USB mouse as PS/2-compatible devices, with limitations imposed by the USB boot protocol.

Figure 1. Architecture for Legacy Support for USB and PS/2
ditto

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4
  • i'm going to give it to you since you were first, and you explicitly mentioned the idea that legacy mode is where the BIOS will make the USB mouse look like a PS/2 mouse, so that operating systems that only support PS/2 mice (e.g. MS-DOS) will see a PS/2 mouse attached. Once you mentioned that emulation concept, everything clicked for me.
    – Ian Boyd
    Commented Jun 28, 2011 at 13:28
  • ooh. nice edit. I didn't know about this till you asked either - looked it up cause i just HAD to know ;)
    – Journeyman Geek ♦
    Commented Jun 28, 2011 at 13:29
  • i like to edit people's answers with referenced quotes and pretty screenshots; helps them get more free upvotes :P (although i'm more of a stackoverflow guy)
    – Ian Boyd
    Commented Jun 30, 2011 at 1:38
  • 1
    How does the BIOS figure out the OS needs USB mouse/kbd as PS/2 emulation? Or does it remap USB mouse/kbd to PS/2 signalling purely based on the BIOS setting "legacy USB", regardless of whether the OS understands USB input devices?
    – Lumi
    Commented Oct 12, 2015 at 21:59
15

I found a great explanation:

In a nutshell, legacy is a reroute of USB keyboard and/or mouse to allow an OS to use same. XP does not need legacy support of a USB keyboard or mouse.

An example of where legacy support is required of a USB keyboard is real mode msdos. And, a USB mouse would be addressed as a standard PS/2 mouse in msdos using its mouse driver with legacy support enabled.

http://help.wugnet.com/windows/USB-Legacy-BIOS-ftopict606120.html

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4

As an aside:

I disabled this setting on an Asus P5KPL-CM motherboard (following a suggestion it might be adding to boot time for this board) & am running XP Pro.

When I then attempted to flash the bios via usb flash drive (as this board is capable of doing), this drive was not detected at boot. It was, however, detected by the OS once booted to XP.

I re-enabled "legacy usb", rebooted, and the drive was then detected in bios, allowing me to successfully browse the usb drive & flash the bios.

Therefore, the legacy setting can affect usb detection in bios.

FYI. Cheers from AU!

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