So looked into this in Wikipedia and it seems like it is "z/Architecture" which is basically a 64-bit extension of the s/390 instruction set which was the evolution of the s/360 which was in a sense the first instruction set (it was the first instruction set intended to be implemented by multiple CPUs). It looks like software for the s/360 should still be able to run unchanged on the modern CPUs but there was some mention that not operating systems.