Unless there is an explicit description in the Reference Manual (RM) of any portion of RAM being cleared upon reset, data are preserved as long as power supply (VDD) is uninterrupted.
However, the C standard requires the translator (compiler and its suite, part of toolchain) to write to initialized variables, and to write zero to uninitialized global and static variables, upon program startup. This happens in the startup code, which is toolchain-dependent.
JW
