Prime95 "shouldn't" use 100% RAM, and if it did it would crash.
"By selecting Custom, the user can gain further control of the configuration. For example, by selecting 8-8KB as the FFT size, the program stresses primarily the CPU. By selecting 2048-4096KB and unchecking the "Run FFTs in-place" checkbox, providing the maximum amount of RAM free in the system, the program tests the memory and the chipset. If the amount of memory to use option is set too high then system will start using the paging file and the test will not stress the memory."