S/38 was practically identical to S/36 from the outside, and from the programming environment, but inside, it was revolutionary. Instead of the traditional distinction between "memory" and "storage", there was a single very large persistent address space (48 bits of address). After a few years, the series was renamed AS/400 and expanded upwards.
AS/400 is a virtual machine architecture, and the physical machine underneath has been radically changed a few times, without affecting the programs running in the AS/400 virtual environment. Today, AS/400 servers are multiprocessors (1 - 12 CPUs) using a 64-bit version of the PowerPC microprocessor that has been enhanced with some special instructions to help with the very large address space.