(I assume we're discussing an AVR processor.)
The processor itself does not alter SRAM in any way during "boot".
Memory is only initialized by Libc. Anything outside of .data (global / static with non-zero values) and .bss (global / static set to zero) is untouched. The heap and stack are outside of those regions.
The electricity generated by an LED (yes, I wrote that correctly) has been shown to preserve SRAM values.
In other words, simply connecting an LED to an AVR processor is enough to trigger what @cattledog described. Even through a power cycle.