18 Apr
2005
18 Apr
'05
7:05 a.m.
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:
McNamara),
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
> 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.
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