I want to use an older version of the compiler for compatibility reasons, If I buy a current XE 2019 license, can I use to install the XE 2015 version.
This is for Windows
Yes you can. Follow the instructions at https://software.intel.com/en-us/articles/older-version-product
However, most "compatibility reasons" are bogus. Please try to make your application work with the 2019 version first. Feel free to ask here if you need help.
Thanks Steve - yes I did try the 2019 version first and got 1000s of compiler errors. Then decided to try the 2015 version - got very few errors which I was able to fix (the code is over 15 years old from the early DEC version).
I plan to upgrade to 2016, 2017 one at a time to figure out which version breaks it and then I will try to address the problem from there. Right now just want to get the code working to meet customer needs.
Interesting - that suggests errors in the code. Can you tell me what some of the error messages were? If you're building in Visual Studio, see what happens when you set the project property Fortran > Diagnostics > Check Routine Interfaces to No. If this eliminates many or all of the errors, then, when you get the chance, re-enable the option and review the messages.
Over the years, compilers catch more invalid code. This does not mean the newer compiler is incompatible.