(From a yahho goup I subscribe to)
By definiton, virtual clock is a clock which doesn’t have any design object associated with it.
Virtual Clocks are generally used to constraint the IOs (Inputs & Outputs) of a block.
Consider a big design with two sub-blocks, block ‘A’ and block ‘B’. Suppose there is a timing path going from a register x_reg (clocked by ‘clk1′) (in block A) to register y_reg (clocked by ‘clk2′) (in block B).
If you are analyzing this design at the top-level, the analysis tool can associate both the clocks with some design objects and hence there is no need for virtual clocks.
Now if you are doing analysis at block-level (say Block ‘B’), and you have to tell the tool that timing path ending at y_reg register is launched by a clock "clk1". The only way to do this is to create a virtual clock.
For creating clk2 you have some design object (port/pin) available, since it exist in block ‘B’ itself. But block ‘A’ is out of the scope of the current environment, and you have to define "clk1" as virtual clock.
RAIN: A SPECIAL FEELING
Rain is the best thing which helps us cherish every lovely moment of our lives…When it rains…The earth becomes a paradise… and watching it from my windowpane is a special feeling….The pure smell of dry earth giving forth its intoxicating aroma when moistened by rain is deeply enshrined in the heart. Yes, listening to the rain drops, fall on the tree leaves… it’s magnificent… a heavenly feeling fills every living soul. The feeling is as passionate as falling in love. When I miss my love with all my might… the rain brings me all lovely thoughts soothing my heart…
Rain….on my Windowpane
There I watch the rain…
From my window pane
The pitter-patter of those drops…
Dripping down the tree tops
How it clatters along the roofs…
Like the tramp of hoofs
The cool breeze blows along…
Adding rhythm to the holy song
The rain….
Showering down my windowpane…
My heart turns gray…
As I long for yesterday
When I looked into your eyes…
Revealing the refrained love…
Listening to the rain…. on my windowpane……….
I know that I need you…
In the time that I stray
But Now I stand alone….
And watch for the rainbow
Then those sweet words take hold
Open your eyes to the love around you…
You may feel you’re alone
But I’m still here with you
You can do what you dream
Just remember… to listen to the rain…
………on my windowpane
Just reinforce my belief (after I read it see here) that most of the innovative concepts in the EDA industry come from graduate research. And the big sharks buy them. Thats understandble in a way I guess. Calls for more industry acedemia relationship?
Point to myself—-gotta understand how the EDA industy acedemia relationship is currently structured.
and yeah….long live the start-up business model. 
Hmm….that a bit disheartening isn’t it? But then again this is a personal space for me rather than a business opportunity. Again this is nothing but just indicative of how many ppl are linking to your blog i think
I have been reading on a lot of things regarding ad-sense and I can understand that there is a lot of money to be made….if you are really serious about it….but I guess I have got bigger fish to fry 
Well check it out for your blog
http://www.business-opportunities.biz/projects/how-much-is-your-blog-worth/
