Re: Race in MSCP (ra/rx) driver

[Johnny Billquist <bqt%update.uu.se@localhost>]

A very small nitpick, but anyway...

On 2020-08-26 21:13, Mark Pizzolato - Info Comm wrote:

> Covering that case is absolutely a good idea, but in practice, real hardware
> had microprocessors implementing the nuts and bolts of both the CPU
> interactions and the rest of the job of the MSCP device.  These microprocessors
> essentially always were slower than the processor they were connected to,
> with a good number of microprocessor instructions just to fetch and interpret
> the info in the command that needs to be processed, so even if data access
> had no mechanical delays the interpretive overhead could never be very
> close to 0.

Not entirely true. Many controllers were designed long after the 
processors they were connected to, and made use of way more modern 
hardware that was much faster. Not to mention that some of them even had 
multiple processors to deal with interfacing to the main CPU, and 
dealing with the actual external device.

The DELUA ethernet controller, for example, have a 68000 processor on 
it. Definitely faster than most PDP-11s it might have been attached to. 
Possibly faster than some of the VAXen as well.

The TMSCP controller for the Unibus have an 80186. Similar story there.

The UDA-50 MSCP controller is build out of AMD 2901 bitslices, and I 
would suspect it was pretty fast compared to most CPUs it was attached 
to as well... The list can be made longer...

> An interesting thing to observe is that a VAX780 simulator booting from an
> RM03 simulated device actually boots noticeably faster than the same
> simulator booting from an MSCP device.  The I/O completion delays that
> were needed for the RP device were significantly shorter and involved
> fewer interactions than the MSCP (RQ) case.

I am in a way a little bit surprised. While the whole interface to a 
massbus disk is much simpler than MSCP, it also exposes more of the 
hardware, and I would have expected MSCP to essentially not require any 
delays between request and completion. One of the nice things of the 
whole protocol design actually.

But it would appear you found out otherwise...

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
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