[TUHS] core

Johnny Billquist bqt at update.uu.se
Tue Jun 19 07:13:32 AEST 2018

On 2018-06-18 14:17, Noel Chiappa wrote:
>      > The "separate" bus for the semiconductor memory is just a second Unibus
> Err, no. :-) There is a second UNIBUS, but... its source is a second port on
> the FASTBUS memory, the other port goes straight to the CPU. The other UNIBUS
> comes out of the CPU. It _is_ possible to join the two UNIBI together, but
> on machines which don't do that, the _only_ path from the CPU to the FASTBUS
> memory is via the FASTBUS.

Ah. You and Ron are right. I am confused.

So there were some previous PDP-11 models who did not have their memory 
on the Unibus. The 11/45,50,55 accessed memory from the CPU not through 
the Unibus, but through the fastbus, which was a pure memory bus, as far 
as I understand. You (obviously) could also have memory on the Unibus, 
but that would be slower then.

Ah, and there is a jumper to tell which addresses are served by the 
fastbus, and the rest then go to the Unibus. Thanks, I had missed these 
details before. (To be honest, I have never actually worked on any of 
those machines.)


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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