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Why are Additions required for using virtual machines through Remote Desktop?

Ben Armstrong, the Virtual PC Guy, recently answered the question "Why are Additions required for using virtual machines through Remote Desktop?" on his Blog site.

I was recently asked why it is required to have Virtual Machine Additions installed when trying to control virtual machines by connecting to the host operating system with Remote Desktop (as discussed here). 

To set the terminology correctly – we have two modes of mouse operation: integrated and relative mode.  Integrated mode is the mode that we use when Virtual Machine Additions are installed, it is also the mode used by Remote Desktop.  In this mode the absolute coordinates of the mouse (x,y) are translated to appropriate coordinates for the remote / virtual computer and are sent directly.  This is the most efficient and usable mode of mouse operation – however it requires software to be running in the remote / virtual environment that can understand this sort of data input.

Using integrated mode for a virtual machine through Remote Desktop works just fine.

Relative mode is the mode that we use when there are no Virtual Machine Additions installed.  In this mode the mouse is ‘captured’ – which we do by:

  1. Hiding the mouse cursor
  2. Repeatedly setting the mouse cursor position to the center of our window 

Every time that we reset the mouse cursor position (this is done ~60 times a second) we record the amount of space traveled by the mouse (the delta) and send this information to the emulated PS/2 device (this is the sort of information that PS/2 mice expect). 

Read the rest of the answer on his site, here.

Published Sunday, June 10, 2007 8:14 AM by David Marshall
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