while the iSeries (think AS/400) has a shorter history many programs compiled to earlier versions run on later versions due to the separation of hardware layer

while the iSeries (think AS/400) has a shorter history many programs compiled to earlier versions run on later versions due to the separation of hardware layer from operating system layer. The system can recompile existing programs without human usable/retrievable source due to the method use to encode them. there is code from the eighties running in slightly updated form on our z and even the i series systems have twenty year old code. both systems have web facing extensions that directly map to older code (as in, green screen 3270/5250 that has no change to its code) pretty much seamlessly. new code is often employed as necessary but the idea that stuff that worked well that is a decade or more old still works today is what amazes me about the various IBM platforms