On Tue, 29 Mar 2005, Bob Ammerman wrote:

>> Afaik on Macs before OS X there was a strong division between system and 
>> user tasks. There was only one user task running at any one time, but 
>> several systems tasks could run concurrently. To have a server running one 
>> had to make it a system task (I'm probably using the wrong terminology for 
>> the Mac). Example: file sharing worked in parallel with whatever the user 
>> was doing with not visible side effects.
>> 
>> Peter
>
> AFAIK, and I have done a fair bit of Mac OS <X code, the only preemptive 
> multitasking on pre OS-X was provided by code that ran off the 'vertical 
> blanking interrupt'. Such code was strictly constrained in what it could do 
> (most system calls were off limits to it). So, unless file sharing were 
> driven off the VBI, it would have been constrained to cooperative 
> multitasking only.
>
> It is interesting to note that there is a slider control in one of the 
> preference panels for the OS to choose between good application and good 
> file-sharing performance.

I think that the filesharing (at least some part of it) was driven by 
the interrupt system in the OS, probably the ethernet card interrupt 
source. Together with the TCP/IP stack callbacks and a couple of other 
things that run in the background on any internet and multimedia capable 
machine. I did not *look* at code but the machines work in that way (at 
least iMacs above OS9.x). I don't know how they do it but they do it.

Peter
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