So, FE4 was originally going to be called Holy Sword Elm Kaiser according to Masahiro Higuchi, who was a graphic designer at Intelligent Systems at the time. You can read about that here.
Although that name didn’t stick around and the project eventually became another Fire Emblem game, references to the old name still exist within both FE4 and FE5. FE4’s sound engine is labeled ELM SND1960327W internally. FE5’s is labeled FE5 SND19907074, though. We’re getting off-track, oops.
The common reference to the old title between the two games occurs during their boot sequences. When the games start up successfully* they write a short string right before the stack (oh, uh, the stack is a part of memory used by the CPU for temporarily storing return addresses during subroutine calls and register contents. A register is…). That string reads ELM0.
Also during startup, the games check to see if ELM0 is already written to RAM. If it is, the game has been reset by software and we skip checking for bad things*.
I like to call FE4 and FE5’s engine the ELM0 Engine because of tthis.
*FE5 (and FE4) can fail to start up. We’ll cover that in another tidbit.