Mark Pizzolato sent me a note about a code sequence in the original
VAX750 bootstrap that fails on the 750 simulator. The code is doing a
BBC #7,address,10$
where the address is in the RH750 Massbus adapter. The sequence fails
because the simulated RH750 throws an error - the operand access is not
an aligned longword.
BBC and the other bit branches use a byte access operand. The SRM says
that VAX implementations must access the precise byte specified by the
base address and the byte part of the bit offset. There's no leeway in
the wording, and the VAX chips (and the 780) do precisely that.
If the 750 microcode is correctly implemented, then the best explanation
is that the RH750, unlike the RH780, does not in fact check the length
(and possibly not the alignment) of register accesses. This is true of
the respective Unibus adapters: the 780's does lots of checking, the
750's does none.
The 750 simulator is not mine, but I'd like to track this problem down.
The RH750 schematics don't help, because the critical logic is inside
gate arrays. What's needed is an RH750 specification. Does anyone have it?
Thanks,
/Bob Supnik
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