Static code. Also, very fine details of the machine because less important.

When emulating a NES (for example), you really have to emulate every register, how registers change, and also weird effects like instructions that take longer to read or write values, as games rely on that stuff. Once you have modern systems where much of the code was originally C, it becomes less important to ensure every register has exactly the right values when a subroutine finishes in most cases, you can rely that (most) of the code follows standard calling conventions.