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Atollic TrueSTUDIO 9.0.0 - PRO version is free
STMicroelectronics acquired Atollic in December 2017. After that, a lot of people speculated that Pro version could be free for everyone.
Atollic released a new version of IDE - 9.0.0 and the speculation is confirmed. The latest version and following are without license for STM32.
The new name of IDE is TrueSTUDIO for STM32. If you are using CubeMX, the process how to generate the code is the same, choose option SW4STM32 in the setting of project.

The IDE TrueSTUDIO for STM32 is available for Linux and Windows.

Download link: https://atollic.com/resources/download/