How were 32-bit minis built in the 70s/80?

ben bfranchuk at
Sat May 11 23:50:52 CDT 2019

On 5/11/2019 9:28 PM, allison via cctalk wrote:
> On 05/11/2019 09:30 PM, ben via cctalk wrote:
>> On 5/11/2019 6:28 PM, allison via cctalk wrote:
>>> Not all were 74181 based, Thats an early 1972 part and but 1975 it was
>>> already getting old though useful as it evolved to 74S and 74F series.
>>> The 82s100 and 105 series were out there and even by 1980 the AMD 2900C
>>> series was getting long in the tooth. Mask programable gate arrays were
>>> in the 1000 and up gate level by 1980 and growing by doubles every 6
>>> months to a year. Don't got get programmables like PAL/GAL logic.
>>> There was a lot of designs and even inside DEC you might see several
>>> approaches depending on what machine and the specific date.  For example
>>> the 780, 750 and 730 used very different technology.  I will not go into
>>> those that also went the ECL {10K, 100K, 1M families] route.
>> 74181 is FAST, but I disagree with the way most computer architecture is
> TTl in general is slow a ALU based on 181 is hitting the wall at 5mhz
> with 12 or 32 but carry lookahead.
No BUT's

I have my cpu designed for 1976, with NO pipeline and a 6900 memory 
cycle @ .75 us. I suspect about half the speed and half the price
had it been built in that era compared to a pdp 11.

>> designed. You have a fast micro code cycle, that is out of sync with
>> main memory, that tries to emulate a Harvard? Memory model.
>> It looks fast only on paper or demo programs sadly.
>> The few schematics I have seen (PDP 8/11) have 74H logic hidden
>> inside so you can't say they are pure TTL logic.
> Yes, they are mostly TTL and the typical 8efm use MSI ttl such as
> 7481, a bunch of them.
> I'm likely one of the few that took a 8E and ran semiconductor ram then
> pushed the clock up to the breaking point and you get to about 4x and
> you start getting timing errors and critical path delays that mess with
> the logic.  However at 4X you doing a lot and decently fast but you
> needs a faster generation of logic.
>>   A cpu instruction has 4 parts in general
>>   a) getting the instruction and literal data from memory
>>   b) calculating the the effective address
>>   c) fetching the data from memory  c) ouputing data
>>   d) using the data d) saving to memory.

> Many of those things can be done in parallel.
Or pipe lining, I don't mind tricks being used to
speed up a system,but knowing how slow a instruction
is, or what side effects can be very important.

> The name for that is system overhead and PDP-8 had little and what it
> did have was written in assembler for speed and compact code as it was
> also space constrained.

I don't know, I suspect 3-4 users would bog down a 8 time sharing.
mind you time sharing meant back then meant 4 people editing files
not like to day, where 3 or 4 windows are running with 30 back ground tasks.

It was a marvel how the machines worked with so little core.

> Allison, have the shirt.
I have the paper tape. :)

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