Reproducing old machines with newer technology (Re: PDP-12 at the RICM)

Jon Elson elson at
Wed Jul 15 21:02:45 CDT 2015

On 07/15/2015 01:24 PM, Noel Chiappa wrote:
>      > On 7/14/2015 7:36 PM, Jon Elson wrote:
>      > On the system 360 CPUs, they did not use flip-flops like we are used
>      > to, today. They used latches ... Since these were discrete transistor
>      > implementations, a real flip-flop was too expensive, but a latch could
>      > be implemented in about 6 transistors, I think.
>      > The 11/45 used TTL ICs, so real FFs were available in that technology,
>      > although they may have used latches as well.
> This confused me a bit, until I realized that you were using "latch" for what
> I think of as 'SR flip-flop', and "flip-flop" for 'D and JK flip-flops'.
> Guess that shows how long ago I did hardware... :-)
> To be a bit more detailed, on the 360's, were those latches 'simple' SR flops
> (i.e. un-gated), or were they gated?
Well, one would have to dig into the ALDs to be sure.  But, 
the FEMMs have some large drawings that are essentially RTL 
in graphical form, and a lot of description of how it all 
worked.  My understanding is all those registers were 
essentially D latches. So, they got one data input from the 
ALU or a mux, and a latch pulse, and provided a Q output.  
Each of these latches took up at least 4 SLT packages, I'm 
not sure exactly how many for sure.  So, the whole latch was 
composed of something like 4 NOR gates or the equivalent, 
plus one inverter.

(Sorry about being so vague, I read a bunch of IBM FEMMs 
about a year ago when I had some spare time.)


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