neviem ci ti to bude na nieco dobre...
Logic 5.5.1: according to some posts I have seen you need to set the affinity of the process to one CPU core - so no, Logic is not using both cores.
The old I/O plugin thing (any track with the i/o plugin inserted runs on second CPU) - that worked with hyperthreading but I haven't heard that it works with the dual core CPUs.
http://www.kvraudio.com/forum/viewtopic ... re#1564682
An update for anyone who wishes to run Logic PC 5.5.1 on X2 WinXP systems.
I just got around to installing Logic on my X2 4400 box and did some testing. I set up a tune with 4 VSTi tracks and some insert and bus FX - just to load up the CPU.
The Task Manager shows that the main load falls on the second CPU (CPU 1) BUT it seems that the currently selected AudioInstrument track is a different thread (lower latency than the rest ) and it (ie the VSTi intelf and all the insert plugins on that track) definitely runs on CPU 0 so the load is split to some degree if you have one of those AudioInstrument tracks selected. If you move the selection away - eg to an audio track - then the load is almost all on the second CPU.
I then tried the " I/O plugin trick" (worked on both hyperthreaded Intel CPUs and Logic 5.x and DP Macs with earlier versions of Logic Mac) on these VST tracks and it does indeed distribute the cpu from I/O plugin tracks to the first CPU. It works BUT only if you actually assign some outputs and inputs.
I tried then the I/O thing on an audio track - (imported a couple of wavs from the audio drive) and sent the signal in and out of some ADAT ports on my HDSP Multiface then bypassed the plugin. With the outputs unassigned nothing happened CPU wise - load balance stays the same.
I then experimented with the four VSTi tracks and as I installed and assigned the i/o plugin on each in succession it shifted the load to the other CPU so the load went from (say) 5% +50% all the way across to 50% + 5%.
With enough CPU heavy plugins running Logic's performance meter shows all the way up to the red while the Task Manager CPU meter shows 44% but there are no clicks in the audio. The interface is very responsive too. The Logic performance meter seems to show a smaller load when the load is balanced more equally across the cores with the I/O trick - might need to test that some more to see exactly how much it changes.
In conclusion - both the currently selected Audio Instrument track thread and threads associated with audio tracks or A.I. tracks with the I/O plugin instantiated go onto the otherwise underused CPU core - definitely some use of the second CPU - not the full deal which is automatic load balancing. This is pretty much what I expected given that other users had reported that the I/o plugin trick worked in WinXP on Intel CPUs with hyperthreading enabled.
This simple test song Is all I have played with so far so I don't know what the substance of those reports of needing to assign the affinity to just one CPU in the Task Manager to get around problems. No problems yet for me with Logic PC and the X2.
ten steinbergovsky piano roll editor je nieco neskutocne zle... 
DD
