>IIRC, some extremely core IRS systems are actually written in IBM 360 Assembly.

Yeah, this language is called PL/X and its specifically not for use outside the company. Its a language between Assembly and C and gives the programmer a huge amount of control over the hardware.

Going from PL/X to Java won't work as well as the JVM runtime has overheads that might not be acceptable for 31bit zSeries with its 2gb address space. Also compared to PL/X you're giving up a huge amount of control.