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

Paul Koning paulkoning at
Mon May 13 11:35:13 CDT 2019

> On May 13, 2019, at 11:31 AM, Ethan Dicks <ethan.dicks at> wrote:
> On Mon, May 13, 2019 at 8:20 AM Paul Koning via cctalk
> <cctalk at> wrote:
> ...
>> On the subject of custom chips:  DEC used gate arrays a lot.  For example there is the Pro 380 in which much of the discrete chip logic from the Pro 350 has been absorbed into one or two gate arrays, with all the unnecessary flexibility of the original chips omitted.
> What sort of flexibility was omitted?  I have both models and the
> board layout difference is obvious (there's so much room on the Pro380
> that it has a huge RAM field right on the mainboard instead of on two
> daughter cards (plus any on the CTI bus).

The 350 uses Intel chips for various functions, for example an interrupt controller chip (from the original PC, I think?) that has a bunch of mode choices.  Some of them are typical Intel bad ideas, like edge triggered interrupts.  Also, vectors are programmable.

In the DEC software one choice was used and the others were not needed; for example, interrupts are level triggered because that's the only right way to do it.  So in the 380, the gate array implements an interrupt controller that's like the used settings of the 350 chips, but omitting all the other modes that aren't used.

As a result, emulating a 380 is quite a lot easier than emulating a 350, unless you make it a "380 style subset of the 350".


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