Why is it that when OS X freezes the mouse still moves?

[Esulatell]

I've always wondered this, when my Mac gets the little beach-ball and everything freezes (I have to hard-restart), why does the mouse still move?

 

[Tallest Skil]

I'll bet you that not everything is frozen. It's nigh-on impossible to freeze the entire system short of a kernel panic (and you'll know when you have a kernel panic...).

Try just Force Quitting the unruly application.

 

[Esulatell]

Ah but force quitting doesn't work because the menu bar has frozen too.
Plus the keyboard shortcut doesn't either.

But what happens during a kernel panic?

 

[Mal]

Technically, what you're seeing is not a freeze (though it's only a technical distinction). What's happening there is that some program is using up all of the CPU cycles, and so while the other programs are theoretically unaffected, they can't do any CPU time to do anything. It's like trying to talk to someone who won't shut up and so you can't get a word in edgewise. Oftentimes you can just wait and eventually one of them will slip through a command, but that could take a while, and you just hope it's what you needed to get some time in (like the Force Quit command, which you can trigger with Cmd-Opt-Esc, if you can't use the Menu bar). That's why the mouse still moves though, because the mouse cursor lives in the firmware and the graphics subsystem, so the CPU isn't involved in the movement of the cursor.

A kernel panic, btw, will result in one of two things: either a gray curtain will drop and a message will show up saying you need to restart your computer in 4 different languages, or white text on a black background will start filling up the screen and essentially give you the same message. The first is far more likely unless you're running an older system.

jW

 

[Tallest Skil]

Ah but force quitting doesn't work because the menu bar has frozen too.

Click into a different application via the Dock or by clicking on the Finder. You'll gain access to that app's Menu Bar.

 

[Esulatell]

A kernel panic, btw, will result in one of two things: either a gray curtain will drop and a message will show up saying you need to restart your computer in 4 different languages, or white text on a black background will start filling up the screen and essentially give you the same message. The first is far more likely unless you're running an older system.

jW

Essentially a BSoD?

 

[SnowLeopard2008]

Essentially a BSoD?

Yes and No. No because BSoD occurs before the OS has loaded. And yes because either way (BSoD or kernel panic) you need to reboot.

 

[John Jacob]

Yes and No. No because BSoD occurs before the OS has loaded. And yes because either way (BSoD or kernel panic) you need to reboot.

That's not true - you can have a BSoD at any time too. In fact, they a kernel panic in Mac OS X and a BSoD in Windows are quite equivalent.

 

[Omaha1]

Not sure where this will end up, but my iMac froze at startup. The cursor wouldn't move, but I could not select a profile to login.
Problem Solved!!! 2008 iMac OS X El Capitan. Version 10.11.6. Old I know... Anyway, I was using a wireless mouse and I could move the cursor, but I couldn't select anything at startup in order to get into my system. My last idea before I was going to take it in for repair/troubleshoot was to attach a wired mouse. Problem solved!!! I was able to select my profile and get into my system. So, there must be something that happens to a wireless mouse that prevents it from selecting, but remains able to move the cursor.
Here are a few things I did prior to changing my mouse.
1- Rebooted several times following various forum suggestions and then rebooted several times in safe mode. Nothing.
2- Shut down computer. Inserted Mac OS X Install Disc 1 and held 'D' key at start-up to run Hardware test. It came back that hardware was good.
3- Changed from wireless mouse to wired. Boom!!
4- Got in system and ran CleanMyMac 3 app.
Simple solution that may work for someone.