AS/400 much?

> Its ancestor, System/38, I think was the first to introduce a _conceptual_ 128 bit pointer? From what I've read, that was never actually reified in the bytecode. Or have I misunderstood?

It exists in the bytecode. From the viewpoint of the underlying hardware, a 128-bit pointer is just a memory block containing the virtual address (48-bit for the original CISC IMPI S/38 and AS/400, 64-bit for the RISC AS/400), plus extra bytes containing metadata about the pointer (its type and capabilities.) This article has some gory details on how it works on contemporary RISC AS/400 (aka IBM i): https://www.mcpressonline.com/programming/rpg/a-close-stu...

And, in the System/38 (and CISC AS/400), it was quite similar. Have a look at Figure 1 in that article, and then read PDF page 238 of this System/38 manual – http://bitsavers.org/pdf/ibm/system38/SC21-9037-3_IBM_Sys... – you'll notice it is largely the same structure