🚀 go-pugleaf

In article <bruce-AA539B....@copper.ipg.tsnz.net>, Bruce
Hoult <br...@hoult.org> writes
>
>[1] manufacturers of microcoded machines frequently made low end models 
>with limited hardware resources and narrow microcode instructions, and 
>higher end models with more hardware and wider microcode instructions.

That reminds me of my days developing a scheduling system for small jobs
in a scientific computing service.  The ICL 1907 had the same fixed
point hardware as the 1906, but had floating point hardware where the
1906 used an interrupt to kernel code.  One morning, I found that my
test run (after some attempt at an improvement of my code) actually took
about 15% longer in clock time, though the computer-measured CPU time
was the same or slightly less.  I reran the unaltered version and got
the same result.  At this point I asked the maintenance engineers what
they had changed.  At first they denied changing anything, but then
realised that I knew they must have changed something, and then said
that they had found that the floating point unit gave wrong answers for
some arguments to one of its instructions (either FIX or FLOAT), so they
had bypassed it and inserted the software emulation from the 1906 to
perform it, in the confident assumption that a) this instruction was
rarely used, b) no one knew what the clock time for execution of a
program was and c) the CPU time spent in the kernel was not reported to
the logging system.  Unfortunately for them, a) we were running Extended
Mercury Autocode, which used this instruction heavily, so I had an EMA
compilation and execution in my test set; b) the kernel call took about
500 microseconds; and c) I was interested in reducing disk access, card
reading and line printing times, so I used a stop watch in my tests.
(Also I didn't trust their logging system).

The main thing I learnt from that was the deviousness of manufacturers'
staff.

-- 
Ken Moore
k...@mooremusic.org.uk
Web site: http://www.mooremusic.org.uk/
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