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

Allison ajp166 at bellatlantic.net
Mon Apr 18 09:05:36 CDT 2005


>Subject: Re: PDP11/55 sells on Ebay for 5K$ - was it really the fastest 11?
>   From: "Heinz Wolter" <h.wolter at sympatico.ca>
>   Date: Mon, 18 Apr 2005 01:00:13 -0400
>     To: "Lyle Bickley" <lbickley at bickleywest.com>, <cctalk at classiccmp.org>
>
>snipe
>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~
>faster?
>> > 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.
>McNamara),
>> 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
>pdf...
>So the 55 is roughly 14% faster than 70 - probably due to main memory
>timing.
>This might not be the case with the PEP70, though it's not stock.
>Interesting
>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
>speed?
>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,
>making
>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!
>h

Sorry, while for those metric the 55 was a tad faster, for IO the 11/70 was 
massively faster.  At that time to do large arrays of data you needed lots 
of fast IO to disks as you could only works with part of an array at any 
time due too addressing limitations of the PDP11.  

When you measure systems, measure the system not just the cpu.

Allison




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