I haven't read the spec. or followed the topic in general, but from a quick glance at it: the size of the registers isn't given, and the registers aren't memory-mapped.

Why not use an integer instead of Cell? 42 is memory address 42, -3 is register R, including for SP, PC, etc. Or addresses >= 0x10000 are registers, then you can just have

    def ife(self, a, b):
        m = self.mem
        self.skip = m[a] != m[b]

with Python's native indexing of the mem[] list, which could be an array().