To understand the Quad Core architecture, you first need to understand the core microarchitecture, based upon which your Dual & Multi-core processors work.
Just to give you a glimpse, let me discuss Core2Quad Q9300 which is based on Intel's 45nm "Yorkfield" architecture, that replaces their 65nm Kentsfield designs that dominated Intel's product lineup throughout 2007 and most of early 2008. The "Yorkfield" architecture is, more or less, two 45nm "Wolfdale" cores connected together into a single chip design. The Core 2 Quad Q9300 is not a native quad-core like AMD's Phenom processor, but rather is two dual-cores connected under a single heat spreader - a multi-chip module. But then Q9300 is a desktop processor and I have not discussed Quad-core Xeons here which are far more powerful.
I am posting a few important URLs that will assist you to better understand the core microarchitecture (multi-core processors) and hopefully when you are done, you will have a very fair understanding on how quad-core CPU works;
Intel Quad-Core Technology:
Intel Core (Microarchitecture):
Intel Core 2 (Core 2 Quad):
To Learn About Intel Xeon 5300 Clovertown Architecture:
If you are interested in architecture details of the Core 2 micro architecture (which powers the new Xeon), I suggest you read this IDF Spring 2006 article: http://www.tomshardware.com/2006/03/13/idf_spring_2006/index.html IDF Spring 2006: Will Intel's Core Architecture Close the Technology Gap?.
Hope this answers your question, however just in case you want any further help, please feel free to write.
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