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. :)
Ben.
