PDP11/55 sells on Ebay for 5K$ - was it really the fastest 11?

Heinz Wolter h.wolter at sympatico.ca
Mon Apr 18 00:00:13 CDT 2005

Lyle wrote:
> >  I expected  an 11/70 to be instruction for instruction faster - but
> > add all the cache and complexity - maybe the 55 with bipolar ~was~
> > How about compared to a 11/70 with a PEP70 instead of factory memory and
> > cache?
> > Isn't a 55 just a 45 with bipolar instead of core? or was that a 50?
> Read "Computer Engineering" (Gorden Bell, J. Craig Mudge, John E.
> Copyright 1978 by Digital Equipment Corporation
> Page 408
> --------
> Model   Basic Instructions Floating Point
>         Inst. per second*   Inst. per second
> -------------------------------------------
> 11/70   36                 671
> 11/55   41                 725
> * Relative to 11/03
> No brainer - the 11/55 wins hands down.

sigh - I haven't memorized the DEC sacred texts yet;) no hardcopy, only
So the 55 is roughly 14% faster than 70 - probably due to main memory
This might not be the case with the PEP70, though it's not stock.
that the FP is only 8% faster - given that they are likely similarly
microded and
memory would not be such an issue working out of the FP registers.  Clock
Then again...  if one put 2 MW of 300ns ram on a 70.. well the 55 was only
18 bit address ;)
Anybody know the microclock frequency of both models? Variable timing/phase?
This variable microclock trick was later used in Vax/750 hot wiring kits..
On a similar vein, wasn't the 11/34c clock slowed to allow for cache timing,
it slower than a 34A? (for uncached accesses) Is is this a possible reason
for the 70's
poor performance? Running code withing the cache should have beaten the 55!

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