[Simh] VAX unaligned

[Bob Supnik]

As I wrote, the unaligned logic on the VAX is not quite right. Right now, the simulator fetches the 2 longwords (64b) that surround the unaligned object. That's fine for memory, but in IO space, it doesn't work. One suggestion was to force longword alignment of the addresses being used, but that doesn't work either - the 780 aborts on unaligned references and would now see the requests as aligned.

CVAX does byte-masked transactions that touch exactly the bytes that are needed, and the Qbus adapter changes those transactions into appropriate word transactions (on reads) and byte/word transactions on writes. However, this behavior may or may not be right for other models (it's certainly wrong for 780). Accordingly, I believe that handling of unaligned access to IO space will have to be broken out to model-specific code.

/Bob Supnik