Re: These guys seem to get that it's *all* about being able to run that legacy software
"Given that I doubt the mainframe register sets map very well onto the x*^ architecture this is a stunning achievement."
Amen to that, for the MCP machines they are emulating a machine where each word (I think 48bits) has a descriptor of n bits describing the current state of the word, as in - uninitialized, integer, instruction part, floating point part, characters, ... but at least the basic unit is 8bits and the arithmetic twos complement.
For the OS 2200 machines they are emulating the architecture of the 1100 series, which started life as an IBM 7094 alike, not an exact copy but similar architecture, 36bit words, one's complement arithmetic (for you non-computer people that means there is both positive and negative zero and all that entails) and given the natural word is 36bits the natural "byte" is 9bits. The emulated memory map must be a real mess.