[simh] RSTS processor identification

Paul Koning paulkoning at comcast.net
Mon Mar 8 08:58:24 CST 2021

> On Mar 5, 2021, at 9:02 PM, Johnny Billquist <bqt at softjar.se> wrote:
> On 2021-03-06 02:33, Paul Koning wrote:
> ...
>> The explanation I heard for the slow J-11 clock is that the original J-11 spec called for it to operate at 20 MHz.  When Harris failed to deliver and the max useable clock speed ended up to be 18 MHz, most designs had no trouble.  But the Pro support chips were designed to run synchronous with the CPU clock and for various other reasons needed a clock frequency that's a multiple of 10 MHz, so when 20 MHz was ruled out that left 10 MHz as the only alternative.
> I do think it sounds weird that the support chips would require a clock that is a multiple of 10 MHz. But I wouldn't know for sure.
> Somewhere else I read/heard that they didn't work reliable above 10 MHz, but for the F11 that was ok. When the -380 came, they just reused those support chips.

The 380 has an entirely different core design.  Instead of lots of discrete support chips including a pile of screwball Intel chips, it uses a pair of gate arrays that incorporate all those functions.  Or more precisely, the subset that the OS actually needs.  This is really obvious when you compare the 350 and 380 documentation for the interrupt controllers -- the 350 uses Intel chips, the 380 only implements a tiny subset of what those chips do.

I'm guessing here, but a possible reason for the 10 MHz issue is if the support chips use that clock, and use a synchronous design for the clock boundary crossing rather than an asynchronous design.  It's entirely possible to design a chip that has an outside interface with an unrelated clock frequency, but it's harder to do and harder to get right.


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