28 Feb
2018
28 Feb
'18
10:55 a.m.
On Feb 28, 2018, at 1:10 PM, David Bridgham via cctalk
<cctalk at classiccmp.org> wrote:
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
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.