If you decide to multi-task then the processor can split into two cpu's. One side concentrating on one task and the other side concentrating with the other task. The main thing is the built in dual cache, usually 2 X 2Mb.
This cache holds this information in store ready for the cpu to number crunch.
The bottom line is that when you open another application, when working on another, you should see less of the waiting hour glass and possibly instant loading of the program.
A pentium D 950 has 2 3.4 processors mounted on one chip........you effectively have 2 separate P4 3.4GHz processors to use.
Now, most games are single threaded....that means all the calculations needed for the game have to be done on one cpu.
With a dual core processor that means one of the two core's will run the game.....the other core will deal with any other processes,ie Windows and your antivirus etc.
In this case you have the power of a single P4 3.4GHz available for your game.
If you are lucky your game will support multi-threading........which means that both cores will be able to contribute to running the game.Call of Duty 2 and Quake IV are two of the games that can do that.
In this case you have the power of both cores available(2x3.4 GHz P4) and can have the equivalent of a much more powerful single processor available for your game.
This wont matter in the slightest with your games......as a single 3.4GHz cpu can handle any current game with ease.
Where having two 3.4GHz processors available really comes into it's own is when you are busy doing two things at once,like playing a game and burning a DVD.
Now you can do it without having the game performance ruined and your DVD take forever to finish.
It's like having an extra computer available to do the second task.
Everything is explained in the link from De Marcus™
And in spite of what Arthur Scrimshaw thinks (even though correct) the D950 is a very powerful real world cpu.
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