Quartus -multiple versions on one PC

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Are versions of Quartus installed as independent applications? I have 17.1 installed and now I need 15.x . Thanks.

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Basically, yes. They would be installed in entirely different directory structures. E.G., 17.1 under C:\Altera\Quartus\17.1, and 15.0 under C:\Altera\Quartus\15.0

I then use a command script to set the appropriate environment variables to point to one or other of the directory trees. In my case I use a shell command script:

#!/bin/bash# 
export QUARTUS_BASE=/cygdrive/C/Altera/Quartus/15.0
export QUARTUS_ROOTDIR=${QUARTUS_BASE}/quartus
if  ; then
  export QUARTUS_BIN=${QUARTUS_BASE}/quartus/bin64
else
  export QUARTUS_BIN=${QUARTUS_BASE}/quartus/bin
fi
export QSYS_ROOTDIR=${QUARTUS_BASE}/quartus/sopc_builder/bin
export SOPC_KIT_NIOS2=${QUARTUS_BASE}/nios2eds

under CYGWIN on Windows 7 64b, but there are lots of different ways to do this using windows batch command files or other programs.