> On Feb 28, 2018, at 1:10 PM, David Bridgham via cctalk
> <[email protected]> wrote:
>
>
>> Imagine our chagrin when days of trying to correct the
>> problem led to the conclusion that the diagnostic was incorrect.
>
> I may have a situation like this in working on my FPGA PDP-10. The
> Processor Reference Manuals seem quite clear that the rotate
> instructions take E mod 256. One of the manuals I've found even adds
> that they never move more than 255 positions. And yet the diagnostics I
> have clearly want ROT AC,-256 to move 256 positions to the right, not
> 0. Not having a real PDP-10 to compare against, I don't know which is
> right.
In general, manuals are only a rough approximation of reality. I remember an
old joke that "PDP-11/x is compatible with PDP-11/y if and only if x == y".
And sure enough, if you look at the models appendix of the PDP-11 Architecture
Handbook you will see cleary that this is true. More precisely, it is if you
ignore cases where two model numbers were assigned to the same thing, such as
11/35 and 11/40.
With the VAX, this got cleaned up to a significant extent, and ditto with
Alpha. In both cases, an internal validator tool was created to verify that,
at least from the point of view of instruction execution, a new machine worked
the same as an existing reference machine. But this seems to be quite an
unusual notion in the history of computer hardware development generally. Even
when standard specifications exist that appear to spell out how an architecture
is supposed to work, the reality is that two implementations will in general do
it differently. That is particularly likely to happen in cases of "no one will
do this" -- like shifts by more than the word size, or other oddball stuff.
And sometimes CPU designers do stuff that's just plain nuts, like the CDC 6600
which has a shift instruction where some of the high order bits must be zero
and some are ignored. Or the way it executes a 30-bit instruction that starts
in the last 15 bits of the instruction word. Both are cases where there is
additional logic involved (or at least extra wires) to do something that
clearly serves no purpose. And these things are definitely not documented in
any user manual, though you can find them if you read the schematics carefully
enough.
paul
