My guess is the software. It was probably written in C++. Computer code that crucial should be written in processor machine code. But maybe nobody knows that language anymore.
and how would using x86 or whatever processor being used make any difference? What you'd end up with is a much harder job of writing code. As it is C/C++, PL/1, Ada etc have code-checkers, well defined, certified compilers, assertion libraries which make writing in these high(er) level langauges MUCH safer than writing at the level of bare metal.