8 Jun
2020
8 Jun
'20
5:24 p.m.
On Jun 8, 2020, at 6:40 PM, Antonio Carlini via cctalk
<cctalk at classiccmp.org> wrote:
On 08/06/2020 21:54, Chuck Guzis via cctalk wrote:
I would have thought a lot more than that, for string instructions with long operands.
Re the STAR thing, that reminds me of a bug I encountered a decade or so ago on a high end
MIPS architecture processor whose manufacturer shall remain nameless. It would prefetch
instructions, issuing memory reads to fill the cache if necessary. So far so good. But
if you hit a backward branch and then a prefetch for an address past that branch
completed, the CPU would consider that access wasted and throw away the data. It
wouldn't put it in the cache -- so the next time through the loop you'd get that
memory reference again, and again, every iteration.
This actually broke things in a cache flush operation because that required the execution
of some intensely hairy code that was required to run without any extra cache misses.
This bug meant that you'd always get a cache miss at the end of the loop. The fix
required some additional very hairy code to hop ahead through address space to get those
cache lines actually into the cache...
paul
Although there are exceptions. I recall that it
was possible, using
large page sizes on the CDC STAR-100 to execute an instruction that
could never get started. The STAR had 512KW (64 bits) of memory and a
large page size was 64KW. A typical vector instruction could require 6
addresses for source, destination and control vectors. Put the starting
address of any of these in last 8 words of a page and the hardware
faulted preemptively for next page. It was kind of funny to watch; the
P-counter for the user never budged, but the pager was sucking up time
like crazy. I think someone eventually devised a check in the pager for
this case, but I'm not certain.
There was a standard VAX quiz question which was something along the lines of
"what's the largest number of page faults can a single (valid) instruction
cause" and the answer was surprisingly large (in the region of 50+ although I can no
longer remember the details.