yet another pdp-11 in fgpa

[Walter F.J. Mueller]

Johnny Billquist <bqt at softjar.se> wrote:
 > Good thinking.
 > But I'm surprised by some numbers here. The J11 at 20 MHz is
 > only slightly faster than an 11/70. In fact, if you can throw
 > the 11/70 into running all from cache, it might even be slightly
 > faster than an 11/9x.
 > Or so I seem to remember from looking at the numbers back when
 > I last was digging into this.
 >
 > Maybe I'm mixing some numbers up here... What I do remember for
 > sure is that the 11/9x machines run at 20 MHz, and that they are
 > not more than maybe 1.2 times the speed of an 11/70 in general.

oops, you are right, I was mixing numbers here and forgot about
a factor of four. Here the revised arguments:

See http://www.village.org/pdp11/faq.pages/prfmnc.html , there
is a table comparing 11/70 with various J11 systems:

                       11/23 11/53 11/73 11/83 11/93
                       ----- ----- ----- ----- -----
   CPU                 F-11  J-11  J-11  J-11  J-11
   Microcycle(ns)      300   267   267   222    222
   Clock (MHz)         ?     15    15    18     18
   Performance         0.2   0.5   0.7   1.2    1.4
   (11/70 = 1)	
   Cache               no    no    yes   yes    no
   Floating-Pt         opt   no    no    yes    yes
   Coprocessor

My mistake was to forget that the J11 needs 4 clock cycles per
microcycle (MC), that's why 18 MHz clock leads to 222 ns MC period.

In the end the J11 (222ns MC) is faster than the 11/70 (150 ns MC)
because it has a better micro architecture (three stage pipeline
vs. instruction prefetch only). The J11 is roughly a factor two
better compared to 11/70 in terms of MC efficiency.

The w11a on the other side does one microcycle per clock cycle. So
one can expect that is roughly a factor 5.5 faster than a 18 MHz J11
(MC rate is 11 times higher (50/4.5), but J11 has a factor two better
MC efficiency).

Let's look at the Dhrystone benchmarks again:
    w11a    11500 lps
    11/53     830 lps
So the w11a is measured to be ~14 faster than a 11/53. The 11/53
has half the performance of a 11/70, so one expects that the w11a
is about a factor 7 faster than a 11/70. Because w11a and 11/70
have a similar micro architecture that should match the MC rate,
and  indeed it does (50/6.7). So the numbers are consistent, finally.

Thanks for pointing that out.

	Walter